Solar Battery Size Calculator
Work out the kWh of battery storage you need for backup power or overnight use.
Size your solar battery
Your battery size estimate
Battery size for different backup loads
Total capacity and cost for each daily backup load, using your days of autonomy, depth of discharge and inverter efficiency. Your backup load is highlighted.
How to size a solar battery
This free solar battery size calculator works out the kWh storage you need for backup power or overnight use. It is for homeowners planning a solar battery, and it runs entirely in your browser with no sign-up and no data uploaded. Enter your daily use, the loads you want to cover, days of autonomy, depth of discharge and inverter efficiency, then read the usable storage, total capacity, battery count and estimated cost.
How much solar battery storage do I need?
Multiply the electricity you want to cover each day by the number of days of autonomy, then divide by depth of discharge and inverter efficiency to get the total capacity you must buy. For example, covering 10 kWh a day for one day at 90 percent depth of discharge and 90 percent efficiency needs 12.35 kWh of rated battery capacity.
The formula is: total capacity = (backup load x days of autonomy) / (depth of discharge x inverter efficiency), with the percentages written as decimals. Usable storage is simply backup load x days of autonomy, before those two adjustments are applied.
What is depth of discharge and why does it matter?
Depth of discharge is the share of a battery you can use before recharging, and it decides how much rated capacity you must buy. A lithium battery of 10 kWh with 90 percent depth of discharge gives 9 kWh of usable energy, so a lower depth of discharge means a bigger and more expensive battery for the same usable storage.
Most modern lithium batteries allow 90 to 95 percent depth of discharge. Older lead acid batteries allow only about 50 percent, which is why two lead acid batteries are often needed to match one lithium battery of the same rated size.
How many days of autonomy should I plan for?
One day of autonomy suits most grid connected homes, because the battery recharges from solar panels or the grid each day. Choose two or three days if you live off grid or face long outages, but remember that each extra day adds the full daily backup load to the battery size and to the cost.
How much does a solar battery cost in 2026?
Home solar batteries typically cost around 700 dollars per kWh installed in 2026, according to EnergySage marketplace data, so a 13.5 kWh battery costs roughly 9,000 to 10,000 dollars before incentives. Prices vary by brand, installer and region, and the United States federal tax credit can cut the net cost by 30 percent.
This calculator uses 700 dollars per kWh installed as its cost assumption, which includes the battery, inverter and installation. Get quotes from two or three local installers, because labour and brand choice move the final price more than the battery size alone.
Can a battery power my whole home overnight?
A battery can power a whole home overnight if its usable capacity covers the evening and night loads and its power rating handles your largest appliances. Most homes need 10 to 15 kWh of usable storage for overnight use, but running air conditioning or electric heating through the night can double that figure.
Check the power rating as well as the capacity. A battery may hold 10 kWh but only deliver 5 kW at once, which is not enough to run an air conditioner, an electric oven and a water heater at the same time.
Frequently asked questions
What size solar battery do I need for a day without power?
Add up the essential loads you want to run during an outage, usually lights, fridge, freezer, phones, internet and a few outlets. Most homes land between 5 and 15 kWh a day for essentials. Enter that figure as the backup load, with one day of autonomy, to get your size.
Is a 10 kWh battery enough for a house?
A 10 kWh battery is enough for overnight essentials in many homes, such as lights, fridge, devices and a television. It is usually not enough to run air conditioning all night or to cover a full day of normal use, so size the battery from your backup loads rather than from a round number.
What is the difference between usable capacity and total capacity?
Usable capacity is the energy you can actually draw from the battery. Total capacity is the rated size you buy, which is larger because depth of discharge and inverter losses keep some energy out of reach. This calculator shows both figures so you can compare batteries fairly.
How long does a solar battery last?
Most lithium home batteries are warrantied for 10 years or about 6,000 cycles, and many keep working well beyond that at reduced capacity. A battery cycled once a day will usually outlast its warranty, while a battery kept only for rare outages ages more slowly.
Can I add more batteries later?
In most cases yes, if you choose a modular battery system that allows extra units to be added. Check the brand and model before buying, because some systems cap the number of batteries or need a larger inverter to accept more storage.
Do solar batteries work during a power cut?
Yes, if the system includes backup capability and an islanding switch that disconnects the home from the grid. Not every solar battery setup offers backup, so confirm with your installer that the system keeps running when the grid goes down.
Does this calculator store my data?
No. The calculator runs entirely in your browser. Nothing you type is uploaded, stored, or shared.
More solar tools
Planning the panels to go with the battery? Try the Solar System Size Calculator to find the kW your home needs, the How Many Solar Panels Do I Need Calculator for the panel count, the Solar Panel Cost Calculator for the price, and the Solar Payback Period Calculator to see when the system pays for itself.
