Stop guessing and start calculating—we’ve made sizing a solar battery simple
Standard solar batteries are 10 kWh, but battery sizes and usable watts vary.
To size a battery for solar, know how much energy you use, what your panels produce, and how much backup you need.
Factors like battery depth of discharge, temperature, and overall costs will help you choose.
Using the wrong battery size can lead to power shortages or higher costs.
Have a solar pro help you find and install the right batteries for your solar panel system.
Learning how to size a battery for solar power systems isn’t just a math problem—it’s the key to lowering energy costs and keeping your off-grid dreams alive. Whether you’re powering a cabin in the woods or aiming for more energy independence at home, sizing your batteries is where the magic happens. Let’s look at the factors, crunch the numbers, and get your solar system humming.
The standard size for a solar battery is 10 kilowatt-hours (kWh). This size is best for homeowners who want solar to lessen their dependence on the public power grid and cut energy costs. If you have a larger home or greater energy needs than average, face frequent power outages or grid shortages, or aim for true energy independence with a stand-alone solar power system, you’ll want a bigger battery or more than one.
You may also see solar batteries size by amp-hours (Ah) and volts (V). To find the watts, simply multiply amp-hours by volts.
There are different types of solar batteries and capacities, and each meets different energy needs. Here’s a breakdown of the most common types of solar batteries, sizes, and what they’re best for.
| Solar Battery Type | Common Sizes (kWh) | Best for: |
|---|---|---|
| Lead acid | 1–10 | Budget-conscious homeowners |
| Off-grid systems | ||
| Low cycle demand | ||
| Lithium ion | 5–20 | Daily cycling Small spaces |
| Minimal maintenance | ||
| Nickel based | 1–10 | Extreme climates |
| Long-term durability | ||
| Flow | 10–100 | Large solar systems |
| Frequent cycling |
Lead-acid batteries are made of lead plates in an electrolyte solution of sulfuric acid. Variants include flooded lead acid (FLA) and sealed lead acid (SLA), like absorbent glass mat (AGM) and gel batteries.
These batteries have the lowest up-front cost but a shorter life span (about 3 to 5 years), lower energy density, and higher maintenance requirements than other battery types. Because of their reliability, they’re best for off-grid systems, but they last longest when they aren’t cycled deeply or frequently.
Lithium-ion solar batteries are the most popular choice. They’re made of lithium-based cathodes and have high energy density and efficient cycling. They come in different chemistries, including lithium-ion phosphate (LiFePO4) and lithium nickel manganese cobalt oxide (NMC).
They’re compact, don’t need routine maintenance, and are ideal for regular use. These batteries also last a long time (10 to 15 years) and thousands of charge cycles. However, they’re more costly than other types.
Nickel-based batteries, including nickel-cadmium (Ni-Cd) and nickel-iron (Ni-Fe), are less common in residential solar systems. They’re durable, can last over 20 years, and perform well in extreme temperatures. However, they’re heavy and have lower efficiency. For these reasons, they’re best for custom applications.
Flow solar batteries use liquid electrolytes stored in external tanks to generate energy through a reversible chemical reaction in a central cell stack. There are two popular types: vanadium redox and zinc-bromine.
They’re easy to expand by increasing the electrolyte tank size and have a nearly unlimited life cycle. That said, they cost more, have a larger footprint, and are more complex than other types of solar batteries. These appeal to homeowners with high energy needs and are suited for commercial applications.
A sunny day is great, but a correctly sized battery makes the night just as bright. Watt-hour by watt-hour, an accurate calculation will save you hours of frustration later. So, let’s discuss the factors that play into battery sizing.
The biggest factor in battery size is how much energy your home uses every day. You can get an estimate by looking at your last few electricity bills. Take the monthly use and divide it by the number of days in the billing period. You’ll get a more accurate number if you do this with bills over several months or an entire year.
Another way is to make a list of every energy-using device in your home and write down their power ratings in watts. Multiply each device’s wattage by the number of hours you use it every day to get the daily energy consumption in watt-hours. Then, add the values for all devices to determine your home’s total daily energy usage. This will help you figure out the battery capacity you need. For example, if your home uses 10 kWh per day, you’ll need a battery with a capacity of at least 10 kWh for a single day’s use.
However, you usually can’t use 100% of a battery’s capacity because most have a depth of discharge range to protect the battery’s life span and performance, and conditions like temperature and battery age can shrink the usable capacity (we dive into those details below). So ask a solar panel pro to help you estimate the best solar battery capacity for your system to be sure you get it right.
The amount of energy your solar panels generate matters, too. If you live in a sunny location and your panels get a lot of sun during peak hours (midday when the sun is at its strongest and highest point), they’ll generate more energy than panels in less sunny or shaded locations. Since solar panels are less efficient when solar batteries are full, and you don’t want to waste all that energy, size your battery proportionally to the amount of energy your panels harvest.
Solar batteries work by storing excess energy from your solar panels during the day so you can use it when the sun isn’t shining. Autonomy days, or backup days, are the number of days solar systems will run on batteries alone without any added power from the sun or the grid. If you plan to use your solar panels for backup power, you’ll need fewer backup days than if you intend to go entirely off-grid.
Depth of discharge (DOD) is the percentage of a solar battery’s capacity that you can use before it needs to be recharged. For example, batteries with 50% DOD can only be discharged halfway before you need to recharge them. This protects the batteries from discharging or charging too much, which can damage the cells.
Batteries with a higher DOD let you use more of their capacity but require larger battery banks for a long life span. Choose a battery with a DOD that balances use and durability.
Rate of discharge is the amount of current you can draw from a solar battery. A higher discharge rate allows you to access more energy quickly, which you’ll want during periods of peak energy demand.
While you can tailor solar power to meet peak energy needs, choose a solar battery sized to handle the maximum anticipated daily energy consumption. This way, you'll have enough storage without excessive costs.
Batteries discharge faster at higher temperatures and slower at lower temperatures. To get the most out of solar batteries, consider average daily temperatures when sizing batteries. If you live in a place with extreme temperature variations, adjust the battery size for the highest temperature to maintain consistent performance.
When electric current converts from direct current (DC) to alternating current (AC), it loses voltage. This isn’t a big deal for most residential setups. However, if you have a large-scale or complex solar system, use long extension cords, or have a lot of appliances, factor in voltage loss when choosing a battery.
Each appliance or device connected to a solar battery represents a load. Common examples include lights, kitchen appliances, water pumps, and entertainment systems. The total load needs to be less than the battery's maximum discharge rate. This protects batteries from overdischarge and helps prolong their life.
If your goal is to offset high electricity costs by using solar instead of the electrical grid during peak rate times, size your battery accordingly. That means having a large enough battery to collect and store energy during off-peak times so you can use it during peak times when energy costs are higher. This will save a lot of money over time, so your system will pay for itself faster.
Solar panel systems are a large investment, so carefully add up your costs, including purchasing, installation, and maintenance. Choose the right sized battery and number of solar batteries to power your house. Batteries that are too small won’t store the energy your system needs to be most efficient, and the cost of batteries that are too large will offset your savings.
Too small, and you’re in the dark—too big, and your wallet feels the pinch. Now that you have the information to calculate the right size battery, let’s use it.
To figure, start by multiplying your total daily energy consumption by the number of days of autonomy you need. Then divide that number by your battery’s DOD for a simplified calculation of the battery size you need.
Here’s an example of what that might look like if your household uses 20 kWh of power per day, you want 3 days of autonomy, and your batteries have a DOD of 80%.
20 (Daily energy consumption) X 3 (Days of autonomy) = 60 kWh (Total energy required)
60 kWh (energy required) / .80 (DOD percentage) = 75 kWh
So, for 20 kWh of daily energy consumption and 3 days of autonomy, with a battery that has an DOD of 80%, you need at least a 75 kWh solar battery.
The math isn’t hard, but it’s always best to have a pro double-check your numbers. They’ll have experience calculating a realistic number based on the above factors and can give you information on specific batteries, system efficiency, and more. Contact a local solar panel installer to perform an energy audit and calculate the right battery size for your solar energy storage needs.
Take a look at these additional tips before moving forward:
Decide whether you’re better off investing in solar batteries versus generators when choosing the best backup power solution for your home.
Make sure the voltage of your solar battery matches the system’s voltage. Common options are 12V, 24V, or 48V.
Always round up battery capacity to make sure you have enough energy. It’s better to slightly oversize your battery than risk running out of power.
If a single battery can’t meet your needs, use multiple batteries together in a series (for higher voltage) or in parallel (for more battery capacity).
Using a solar battery that is incorrectly sized for your system can lead to problems. If it’s too small, you can face power shortages and increased reliance on the grid—the exact issues you likely want to prevent by installing solar. Additionally, frequently discharging a too-small battery beyond its optimal depth of discharge (DOD) can shorten its life span and increase replacement costs. A battery that’s too large costs more and creates an inefficient system.
To avoid this, always have a pro weigh in before you buy or install a solar battery. If it’s too late, you have a few options.
You can upgrade or downsize batteries that are the wrong size. You’ll need to invest in a new battery, but you can recoup some of the cost by selling the used battery and making your system more efficient. You can also connect multiple batteries to underperforming systems to meet storage needs without replacing the existing setup. Call a solar energy expert to assess your system and recommend the best fix.
