How to size a battery for your solar system
To size a solar battery, decide what it is for, then match its usable energy in kWh to the evening or outage load it must cover and its power rating in kW to your biggest simultaneous loads. A battery that suits one goal, such as a few hours of backup, is usually the wrong size for another, such as covering a whole winter night.
What should decide the size of a solar battery?
Start with the job. A battery for backup needs to cover the circuits that matter for as long as an outage is likely to last. A battery for self-use needs to hold the surplus your panels make in the day and release it between sunset and the next morning. A battery for time-of-use savings needs to cover the dear hours only.
Those three jobs give different answers for the same house, so write down which one you are buying for before asking for quotes. The solar battery guide explains the first-order trade-offs, and the battery terms guide defines usable capacity and power rating, which this page relies on.
How do you calculate the battery size for backup?
List the circuits you want to keep running and estimate each one's average draw. Add them up, multiply by the hours you want to cover, and divide by the usable share of the battery.
Here is an example with stated assumptions. Essential loads: a fridge (about 0.1 kW on average), lights and electronics (0.3 kW), a gas furnace blower (0.3 kW averaged over the hour) and a router and chargers (0.1 kW). That totals roughly 0.8 kW. To cover 12 hours, the energy needed is 0.8 x 12, or about 9.6 kWh. If the battery's usable capacity is 90 percent of its nameplate, the nameplate size would be about 10.7 kWh. Round up, leave a margin and check that the power rating covers the starting surge of the fridge and furnace.
These numbers are illustrations. An electrician or installer should measure your own loads, and the larger case of backing up everything is discussed in whole house battery backup.
How do you size a battery for daily self-use?
Look at your electricity bills or smart meter data for how much you use between roughly 6 p.m. and 6 a.m. A household that uses 12 kWh overnight would need about that much usable storage to run the house from the battery through the night, if the panels filled it during the day.
The calculation is simple: daily usage in the evening and night hours, divided by the depth of discharge the storage system allows, gives the nameplate size. A lithium battery bank that permits 90 percent use needs about 13.3 kWh to deliver 12 kWh. Home battery calculators online do the same sum, and the result typically depends on how accurate your usage figure is. Systems that stay connected to the grid can top up from it when the battery runs low.
The panels must be able to supply it. In summer a modest array may make enough surplus to fill a 10 kWh battery on a clear day. In December, short days and snow can leave the array with little to spare, so the battery would often be only partly charged. The number of panels guide and solar panels in winter help you judge the generation side.
What about winter, cold and heating loads?
Winter use goes up and solar output goes down, so a battery sized from a July bill will be too small in January. Electric heating is the largest swing. A gas-heated house needs little electricity to keep the furnace blower running, while electric baseboards or a heat pump can need many times more.
Cold also affects the battery's own performance and charging; the cold weather guide covers placement. Size for the worst month you care about, not the average one.
What are the common sizing mistakes?
Buying on nameplate capacity instead of usable capacity is the first. Ignoring the power rating is the second, and it leaves buyers with a battery that cannot start a pump or run two appliances at once. A third is oversizing for rare events, paying for energy that sits unused most years. A fourth is not leaving room to add units later.
Ask the installer to show the load assumptions behind the proposed size, and compare at least two quotes. The quote comparison guide has a checklist. Costs vary with size, region and year, so written quotes decide.
Quick answers
How big a battery do I need for a power outage?
Multiply the average kilowatts of the circuits you want to keep on by the hours you want to cover, then divide by the battery's usable share. A fridge, lights, a furnace blower and a router might average under 1 kW, so about 10 kWh covers roughly half a day.
Is a bigger solar battery always better?
No. A larger battery costs more and may rarely fill if the array is small. Size it for a defined job and leave room to add modules later.
How many kWh does an average home use overnight?
It varies widely with heating type, appliances and season. Check your bills or smart meter data for the hours between early evening and morning to get your own figure.
Do I need a bigger battery in winter?
Often. Winter loads are higher and panels produce less, so a battery sized from summer data may run short. Plan around the coldest, darkest month you need to cover.