How to choose the right size battery bank for your solar system.
What is Solar Battery Sizing
Solar battery sizing (otherwise known as battery bank sizing) is one of the most important considerations when choosing the specifics of your solar electric system.
The main objective when sizing a battery bank is to get one that can handle the load coming from your PV panel array and provide enough stored power for your needs without having to regularly discharge to an unhealthy point.
By wiring multiple batteries together in different wiring arrangements you can design a battery bank that's right for your solar power system and thus correctly perform solar battery sizing.
Factors Affecting Battery Bank Sizing
The number of batteries you use in your solar system depends on the following factors:
- The amount of money you have to spend on this solar project. Part of solar battery sizing is insuring you can buy enough solar batteries to handle your power storage needs.
- You must also take into account the number of days you want to be able to go before needing to recharge your batteries. If you need to be able to power certain appliances for a specific number of days at a time without interruption, you'll need more batteries to carry a bigger load. This is determined by the number of batteries you use and how you wire them to affect your battery bank's total amp hours (storage capacity).
- Another factor that affects solar battery sizing is the amount of power you will be needing for all of your appliances. If your appliances require many watts (power), you'll need enough batteries to store the power so you can use those appliances.
- Another factor that affects the size of your battery bank is the amount of volts your solar system produces. If your system produces 48 volts, then you're going to want to have enough batteries in your battery bank to store 48 volts. Actually a little less is better - like a 36 volt system with a 24 volt battery bank, just to be sure your system can charge the battery bank even in the case of a sudden voltage drop. When sizing a battery bank, always size your solar panels bigger than your battery bank to be able to compensate for factors such as voltage drop, power fluctuations and energy loss due to wear on the system.
- To charge a battery, a generating device must apply a higher voltage than already exists within the battery. That's why most PV modules are made for 16-18V peak power point. A voltage drop greater than 5% will reduce this necessary voltage difference, and can reduce charge current to the battery by a much greater percentage. Our general recommendation here is to size for a 2-3% voltage drop. So for a 12 volt battery bank, a 16-18V solar panel should be used to allow for unexpected voltage drop.
- Another important consideration when sizing a battery bank is the storage capacity you will need your battery bank to have. If your area gets less hours of sunlight in the day, you're going to want more batteries so you can store more "amp hours" of power in your reservoir and last out the long night's stretch. When sizing a battery bank, the more amp hours you have the longer your total power reserve would take to deplete.
- When doing solar battery sizing, you must also take into account the rate of discharge you want to have. Remember, the slower your batteries can discharge the more hours you'll get out of them. You can find out a battery's rate of discharge by looking at it and finding the value marked: (C-?). If you see (C-10) then this means the battery takes 10 hours to discharge fully, if it's (C-5) then the battery takes 5 hours to discharge fully.
- Lastly, when sizing a battery bank, you must consider the depth of discharge you want to go to before recharging. (This is decided by your specific power needs / capacity and affects the battery's lifespan).
Basically, the bigger your batteries are and the more batteries you have, the more convenient it is for you and the better it is for your batteries' health. This is due to the fact that with more batteries / storage capacity you will have more power available, plus you will be discharging your battery bank in smaller (more shallow) cycles and thus increasing it's overall lifespan.
Therefore, as a general rule in solar battery sizing, it's always better to have more batteries in your battery bank and only discharge them 30-50% of the way down - than to have less batteries and discharge them more. Use a battery bank sizer calculator that can help automate the process for you.
Determining a Battery's Storage Capacity
An important part of solar battery sizing is determining the storage capacity, so you know how long you can use it for.
Sizing a Battery Bank - Watt Hours
Let's say you go out and buy a battery for your solar system that is 12 volts (push) and 105 amp hours (storage capacity).
You could find out approximately how much energy this battery will store / provide by calculating the watt hours. To do this, just multiply the volts (V) x the amp hours (AH) and divide by 100.
Volts x Amp Hours / 100 = Watt Hours
12V x 105AH = 1260 / 100 = 12.6 Watt Hours
What this means is that you can power a 100 watt appliance for 12.6 hours on a fully charged battery.
Make sure you find out what the specs on your batteries are before buying them. By knowing what to look for and what each spec means, you can insure your solar project's battery bank operates smoothly, efficiently and free of costly "battery bank sizing" mistakes.
Battery Life Expectancy
One thing you want to pay close attention to when solar battery sizing is how long the batteries you buy will last. The life expectancy of a sealed lead acid battery is rated using the number of cycles that battery can perform.
The "number of cycles" refers to the number of times the battery can be charged and discharged before it's dead.
So if your battery is a 3000 cycle battery, this means it can be charged and discharged 3000 times before it dies, that is providing it is consistently charged correctly and not discharged past acceptable levels. Batteries are considered to be at the end of their lifespan when 20% of their original capacity is gone.
Your energy usage in kilowatt-hours (kWh) dictates the size of your system. Panels have a broad range of wattages (275W-360W is common as of early 2019), and other factors like local sun exposure, mount orientation and the presence of a battery bank also play a part.
One of the most common questions our design technicians get is: “How many solar panels do I need?”
The answer is pretty complex, and frankly, most people approach it from the wrong angle when they start to look into solar.
We’re often asked to quote a system to power a 3-bedroom home or support a family of 4. In these situations, it’s impossible to provide an accurate estimate until we know more about the household’s energy needs.
In reality, the best place to start is to evaluate your current energy use based on past electric bills. Past usage data is the best baseline to figure out how many panels you will need.
Lifestyle, climate and panel efficiency all play a role in figuring out the size of your solar system. Here’s the process we use to make an accurate estimate.
How many panels for grid-tied systems?
In order to figure out just how many panels you need, you’ll want to gather up 6 months to a year’s worth of electric bills. If you live in an area with snowy winters or blistering hot summers, look at a larger sample of bills to understand when your usage spikes. Take peak periods into account as you estimate how many panels you’ll need to cover your energy usage.
Some companies provide a 12-month summary of how much electricity you use on every bill. Depending on your utilities provider, you may just need one bill to find an estimate for the year.
Got your paperwork in order? Great – you want to look for how many kilowatt hours (kWh) of electricity you use per year.
Wait, what’s a kilowatt hour?
A kilowatt hour (kWh) is a measurement of energy. If an appliance rated for 1 kilowatt (1000W) runs for an hour, then one kWh of energy has been used.
The energy company measures total energy usage in kilowatt hours. Your total usage in kilowatt hours determines how much you are billed each month.
Example 1: A fridge rated at 250 watts runs for 4 hours per day. 250W x 4 = 1000W, or 1 kW. This fridge uses 1 kWh of energy over the course of a day.
Example 2: An oven is rated at 2000 watts (2 kW). Cooking in this oven for half an hour would consume 1 kWh of power (2kw x 0.5 hours = 1kWh).
Find how many kWh of energy you use per year. That will give you a good jumping off point for estimating your energy needs – but you’re not there yet.
Divide that number by 365 to get your daily energy usage in kWh.
Once you have your daily energy usage, use this formula to estimate your total system size:
Daily Usage (kWh) ÷ Sun-Hours ÷ 0.9 inefficiency factor = Minimum Solar Array Output
Sun-hours refers to how much sun you get each day where you live.
The inefficiency factor simply accounts for circumstances that would make your system run below its optimal output, like shade, extreme temperatures, voltage drop and equipment inefficiencies.
Take your daily usage and divide it by these two numbers to get an estimate of the overall output of your system.
According to the U.S. Energy Information Administration, the average household used 10,766 kWh of electricity annually in 2016. That’s about 29.5 kWh per day.
Let’s say you live in Arizona, which gets 5.5 sun-hours per day.
29.5 kWh per day ÷ 5.5 sun-hours ÷ 0.9 = 5.9595 kW capacity system.
That would give you an approximate system size of 5.96 kW, or 5959W (remember, 1 kilowatt = 1000 watts).
From there, the last step is to divide by the energy rating of each individual panel. Solar panels are graded by how much power they use. The panels you would use in a residential setting typically range from 275 to 350 watts per panel.
Let’s say we want to use 335W panels. Take your system size and divide by the panel wattage to figure out how many solar panels you need in your system:
5959W ÷ 335W = 17.78 panels
Round up the final number, since you can’t buy partial panels. In this scenario, we would need 18 panels rated at 335 watts apiece to cover our energy needs.
We can’t stress this enough: this calculation is a very rough estimate. It should only be used to ballpark system size and make early pricing estimates.
But don’t take this estimate as gospel – there are too many factors that can change the size of your system in practice.
Additional considerations for off-grid systems
Calculating power consumption needs for an off-grid system is a tad more complicated. People who live off-grid need to focus on daily power usage rather than monthly or annual consumption.
You’re not staring down a power bill each month – you’re independent and responsible for covering your own day-to-day power needs. The system needs to be able to produce enough (and store enough) to keep things running smoothly.
Without power bills as a starting point, it’s best to start by listing out your major appliances and estimating how much you use them on a daily basis.
If you’re not sure how much power an appliance uses, follow the appliance electrical consumption table as a guide. You can also check for the EnergyGuide sticker, or use a meter to measure energy consumption if possible.
This form will give an estimated daily usage.
Pay close attention to December and January when you estimate your energy needs. Those months tend to have the highest power usage and the lowest output by your system.
When you live off-grid, you will need a battery system that’s large enough to store enough power for the day and then use solar power to recharge them in a timely manner. It’s common to lean on a backup generator during the winter, when there won’t be enough sunlight to fully power your solar system.
Once you know how much power you use in kWh per day, a solar design technician can determine the minimum battery size needed with a formula that accounts for things like inefficiencies and temperature coefficients.
Here are the basic formulas we use to size off-grid systems:
Minimum Battery capacity (for lead acid batteries):
Daily usage (kWh) x 2 for a 50% discharge depth x 1.2 inefficiency factor = Minimum Battery Capacity
Minimum Solar Array Size
Daily usage (kWh) ÷ Sun-hours ÷ .9 inefficiency factor = Minimum Solar Array Output*
*Ensure solar array meets battery charge requirements, typically around 10 charging amps per 100ah battery capacity.
You may need a larger array or battery bank based on your location, ambient temperature, your usage patterns and other factors. Take a look at our Battery Bank Sizing Calculator to help figure out how many batteries you need to power your system.
Factoring in sun-hours based on your location
“Sun-hours” refers to how much solar energy hits a given area over a certain amount of time.
Your local climate determines how many peak sun-hours you get each day. This number can change drastically based on where you live.
It’s important to keep in mind that the term “sun-hours” doesn’t just refer to the hours of daylight that your area receives. The peak hours occur when the sun is at its highest in the sky, which will change based on the season and how close you are to the equator. In the winter, the average sun-hours in your location could decrease by 25% to 50%.
So how do sun-hours affect the number of panels on your solar system? If you live in an area with less sun-hours, you’ll need more panels to capture what you need to cover 100% of your energy usage. However, if you live in an area that gets 5-6 sun-hours per day, you might be able to get by with a smaller system.
Quality vs. quantity: panel efficiency isn’t everything
Another aspect that affects the size of your system is the efficiency of the solar panels themselves. Most residential panels range from 275W to 360W. If you go with a 275 watt option, you’ll need several more panels to build your array.
When it comes to solar, efficiency isn’t always the most important factor to consider when you build your system. It really depends on what your specific goals are.
For example, if you look at your solar panels primarily as an investment and a quick ROI is your biggest goal, you might be better off with a lower output, lower cost panel.
“If a panel is 50% more efficient, but costs 100% more, you’re better off paying for [a larger system] of less efficient panels.”
-Brady Schimpf, technical marketing engineer at Ironridge
Brady Schimpf, technical marketing engineer at Ironridge, he had a few thoughts on this matter. He said one of the most common mistakes he sees people make is that they buy into ultra-efficient, high-quality, and technically advanced system, and it might not be worth it – especially if ROI is your biggest concern.
“While a lot of the proprietary systems like that have really good equipment, it is important to look at the cost per watt,” he explained. He urges consumers to consider this: “How much are you paying for the total wattage/production of the system?
“It doesn’t really matter how efficient a panel is if it costs a lot more. If a panel is 50% more efficient, but costs 100% more, you’re better off paying for [a larger system] of less efficient panels.”
But there are some circumstances where having a smaller yet more efficient solar system makes sense. For example, if your roof space is really limited, you might need a more efficient system to cover your energy needs within that given area.
Considering other variables when designing a solar system
We can’t stress this enough: this breakdown only serves as a very rough estimate and a starting point for planning your system.
It’s great to get you closer to a ballpark figure on the cost of panels. It will help as a benchmark when you measure your roof to see if you can fit a system up there.
But when you get deeper into planning your system, unexpected hurdles always come up and the system size tends to change.
What if you decide to go with 275W panels instead of 350W panels because the cost-per-watt is lower? What if shade covers your system, or your roof doesn’t directly face the sun? What if harsh weather causes your equipment to perform below its rated efficiency? What if you start using more energy than you did in the past?
Yup . . . you’ll need more panels.
Although this breakdown can give you an estimate of how many solar panels you’ll need in your array, at the end of the day it’s just an estimate. There are several other variables that can determine the size of your solar system, but this process is still important as it gives you an idea of what to look for before you seek out quotes on solar systems.
If you’re interested in going solar, the best thing you can do is speak with a solar design technician that can help you find the ideal system for your unique situation. Bring your estimate as a starting point. We’ll go over any potential problems and tweak the design to suit your location and lifestyle.
One thing learned during the coronavirus pandemic was the effectiveness of soap and water — regularly washing our hands was our best defense. While dirty solar panels do benefit from a regular washing to ward off reduced output and efficiency, soap isn’t the answer here.
California-based solar installer and maintenance provider Bland Company never uses a cleaning agent on solar panels, instead relying on deionized water and a rotating-brush system to wash solar panels.
“Soaps can leave a film or residue that not only shades panels like the dirt that was just washed off, but it can also encourage dirt to stick and build up faster,” said Daniel Green, Bland marketing director. “We use deionized water that is applied through our rotating-brush system. This is the best way to leave the solar panels with a spot-free shine that’s as beautiful as the first day they were installed.”
Rather than risk Bland employee safety or potential damage to the solar panels they’re cleaning, the company has been using Sola-Tecs brush cleaners since 2018. A rolling brush is attached to wheels that glide across the solar panels, allowing for an almost-hands-free deep clean. Bland also runs the customer’s water through a demineralizing and ionizing system.
“[Sola-Tecs] is the only cleaning system we use. We’ve found that in combination with our water treatment procedure, it’s the best way to clean solar panels,” Green said. “It requires less water, fewer passes and no harsh chemicals or detergents.”
Premier Solar Cleaning (PSC) in Southern California also finds that using deionized water through water-fed poles and brushes works just fine to clean solar projects.
“If you have ever smelled your hands after washing them with soap, what you smell is the soap left behind, even though you cannot see any soap afterwards,” said Adam Fuller, co-founder of PSC. “If we used soap to clean panels, the very small molecules of soap would leave something behind for dust and dirt to build upon.”
PSC offers a full maintenance suite, including infrared inspections to find damaged panels that need more than just a good wash. Fuller also stays busy doing year-round “pigeon evictions,” cleaning underneath panels and installing critter guards to ward off birds and other animals. He takes pride in giving customers honest answers and showing how production will increase after panels are cleaned.
“The homes and factories nearest the airport and highways gather smog or oil-based pollution, which resist running off with a normal rainfall,” Fuller said. “These make some of the older blue panels appear black at first glance. Dirt on the surface reflects light away from the panels. A clean panel remains cooler, allowing the flow of electrons to move more easily.”
National residential and commercial O&M provider SunSystem Technology also stays away from soap and uses filtered water and a rotating brush to clean panels, but director of marketing Jeff Struhm said the company uses a mixture of diluted vinegar and hydrogen peroxide to aide in scrubbing away dirt and grime.
“When rain happens, soil accumulates at the bottom edge of the solar panel, obstructing the lower PV cell row and hindering the production efficiency,” Struhm said. “It’s like a car getting hosed down — you still need to scrub it or use a soil-releasing mixture in order to avoid scrubbing.”
Sometimes just water isn’t enough, but everyone agrees that soap should never be used. That’s why lubricant manufacturer Polywater released its Solar Panel Wash five years ago. The highly concentrated additive isn’t a filmy soap; instead it enhances water’s ability to clean solar panels, said Charlie Cole, Polywater international VP.
“We’re giving the water the capacity to lift the soil off the panel as opposed to a solvent, which is basically surrounding the dirt particle and taking it off the panel,” Cole said. “[Solar Panel Wash] is safer for the components of the panel. It’s not going to affect anti-reflective films; it’s not going to corrode the aluminum rails. It’s biodegradable, meaning that it’s non-persistent, making it friendly with the environment. It’s not going to affect the groundwater.”
Solar Panel Wash modifies the surface tension of the water, so instead of water beading up, it will form a continuous film across the solar panels to lift dirt and debris. Cole said Solar Panel Wash is especially helpful in arid locations where dust is high and water is scarce, because the wash allows for less water to be used in cleaning.
“A lot of these installations are done in desert regions where the availability of water is really an issue. If we can reduce the use of water, it’s an environmental benefit,” Cole said.
Polywater works with panel manufacturers to get its wash approved for use to not void any warranties. Canadian Solar and other global brands have confirmed Solar Panel Wash is safe for use on their panels. It can be purchased through Polywater’s global network of distributors and from Amazon in North America.
Solar O&M providers are glad the industry is waking up to the need for periodic panel washing.
“At the beginning, panel managers were saying, ‘Don’t use anything but rain water.’ People were taking that to heart, and then three years after the panel installation they started to see a significant degradation of productivity,” Cole said. “For a large installation, if your efficiency goes down by 50%, that really cuts into the economic justification that was made in the first place.”
Even on residential projects, homeowners will benefit from paying some extra attention to the cleanliness of their few solar panels.
“We oftentimes hear customers say that the rain does just fine cleaning their panels, and while the rain does wash some dirt off, it doesn’t truly clean the panels. If the panels are already filthy, the rain just makes it worse by converting dust to thick layers of mud,” Bland’s Green said. “The perfect analogy is to consider your car. If your car hasn’t been washed in six months, does leaving it out in the rain make the dirt better or worse? The same is true with dirty solar panels.”
Just don’t use soap — a little elbow grease and water work fine.
Electric cars or ‘green cars’, as they are popularly known, are leaving the fuel-powered cars far behind since they offer many advantages like low gasoline costs, comparatively low maintenance costs and ‘zero emission’ factor. Lacking any combustion engine, they are operated through batteries and electric motors. However, they, too, require to be looked after. Auto provides the best electric car services.
Let’s talk tubular batteries. While tubular batteries are getting gradually known among inverter users in Nigeria, they are yet to command the respect and popularity they deserve. Knowing that they offer far more value than the popular SMF (sealed maintenance free) battery type, you’ll probably wonder what’s robbing them of their shine.
Knowledge. People haven’t really understood the tubular batteries and what they offer. Users sort of stick with the devil they know. Unfortunately, that is denying them of joy and benefit they could easily appropriate.
So, let’s pick the gauntlet and begin to spread the word about tubular batteries. That’s because battery users really deserve a better lot and urgently need the liberating knowledge. It will save them a lot of money and certainly the agony that often comes with early battery death.
But let’s quickly add a background to all this.
Inverters have grown in usage in Nigeria, as possibly the best power backup solution to manage the continued supply shortage from our public power system. Though the cost of components (mainly the inverter and required batteries) has risen in line with the general price levels, inverters are still arguably the most viable response to our power outages, at the basic usage levels. Those who use an inverter will certainly attest to the improvement in personal comfort and quality of life that comes with it, not to talk of the huge savings in avoided cost of running generators.
It turns out that batteries are at the heart of any inverter setup. Batteries take the higher chunk of acquisition cost. Batteries can also prove the weak link, when wrong choices are made. While the machine can be repaired if there is a fault, battery failure, most times, is final, with no remedy. When that happens, the burden of replacement cost could be traumatizing.
To truly enjoy your inverter, you need to get the battery component right. So, you wonder: what battery, then? The answer is the essence of this post. So read more.
There are many battery types. But more popular around us, for the use of inverters, are the sealed, maintenance free (SMF) batteries. Inverters came into the Nigeria market on the back of the SMF, so to speak. So, the SMF has dominated. But that has been at a hefty cost. There are some good brands, no doubt, but, generally, their technology is not as rugged as that of tubular batteries. Unfortunately, old habits are hard to kill. So, the Nigerian user has stayed stuck to the old habit of using SMF batteries.
As stated, the cost has been high. That’s because their technology generally does not equip them to withstand the kind of heavy and, in many instances, abusive usage batteries are exposed to in our environment. Here, batteries are undercharged: no public power supply to charge, and many generators may not charge adequately, or are not run long enough, to save on petrol. Yet, the undercharged battery is driven to do more than they ought to. Inadequate charging, long hours of use and resulting deep discharge on a frequent basis are the best combination for killing batteries. When battery quality figures in, you find batteries failing in months. There goes the money.
TUBULAR BATTERIES CAN SAVE THE DAY
Well, our thesis here is that tubular batteries are more resilient, perform much better and deliver more durability. In effect, if habits change and Nigerians use more of quality tubular batteries, less of scarce resources will go into changing batteries, users will derive more mileage from their investment in batteries and inverters will give more joy. Of course, sellers will have more peace, as there will be fewer unsatisfied customers.
SO, WHAT ARE TUBULAR BATTERIES?
Typically, tubular batteries are recognisable as tall and large (often called tall tubular battery), which turns out to be one of their advantages, because they contain more volume of the active material, having more space. But the main distinction is in their internal construction. Tubular battery technology seals the active material in polyester tubes called gauntlets, unlike other batteries that paste it on the surface of the plate. The tubular approach results in minimized corrosion or shedding of the battery plates. For you the user, it all translates to far longer battery life. And more enjoyment!
Tubular batteries are flooded batteries (wet cell), which require water (distilled) topping and therefore have capped vents. Unfortunately, that’s typically what is misunderstood by potential users who think they are cumbersome to maintain and unsafe.
Not at all. One of the features of tubular batteries is that they require low maintenance. The required maintenance is to top with distilled water. The water topping is an infrequent exercise: 4 times in a year (quarterly) is enough. For some tubular battery brands, as low as twice in a year is recommended. Not really any hassle! It’s really difficult to get why anybody will lose sleep over such minor and very occasional activity, especially if that process guarantees a lot of benefit.
As to safety, batteries, like many other devices used in the home or office, have hazard issues that can be properly and safely managed. The cooking gas has some hazard potentials. Electricity has. Many other things around us do. When well-handled, though, you are generally safe. You can do further research, but what we know is that if tubular or other battery type is kept in ventilated location, they are safe in the home. There are no emissions of any scale to cause hazard, from our research. Provided batteries don’t have overcharging issue, there is no cause to worry. If they have (which means an inverter fault) and some hydrogen sulfide is produced due to overcharging, there will be a warning (rotten egg) ordour. If that should happen, simply turn off and ventilate and let the odour clear. Then resolve such charging problem. With normal charging, your battery should be safe for your environment, in a ventilated corner.
Overall, the safety concern is overblown and their use is generally safe. Otherwise their production and distribution will not be approved across the globe.
BENEFITS OF TUBULAR BATTERIES
If you condition your mind for checking and maintaining your battery, four times in a year at most, and you come to terms with the worries about safety, the tubular battery is where to put your money. We will itemize the benefits:
They are Capable of operating at extremes of temperatures (high temperature is the bane of some battery installations)
Tubular batteries are better able to withstand extensive load-sheddings
Tubular batteries are better suited for high cyclic applications
They can support heavy loads and applications, far more than SMF
Though flooded, tubular battery has low loss of water and so, low maintenance
Tubular battery technology achieves faster battery charging
Overall, tubular batteries are more reliable and longer lasting
Yes, tubular batteries are better for demanding usage environments like Nigeria
Now, these are general technology benefits. Certainly all tubular batteries are not equal. In buying, you still need to pick through brands. You still need to try to get the best product your money can buy, knowing that these tubular batteries are made by different manufacturers and some will surely outperform others. But on the whole, they work better.
So, the ball is in your court. If you have agonized about battery durability and performance, muster the courage and try out the tubular type. Not much to lose, because tubular is unlikely to give anything worse than you can get from the SMF. In effect, the odds are in your favour, if you choose to try. That is a healthy condition for experimentation. We encourage you to do it, because we know.
If you want to discover an easy and proven way to save money, enjoy extended power supply and long battery life and experience all the comfort and happiness that comes with regular power in your home or office, make the switch. You’ll love the results and you will be glad you chose to try.