What Size Home Battery Do I Need?
Wondering what size home battery you need for solar backup or a power outage? Our free home battery sizing calculator converts your daily electricity use, backup days and battery depth of discharge into a recommended battery capacity in kWh — plus a rough count of typical 13.5 kWh battery units.
Enter your numbers
Find this on your utility bill — the average U.S. home uses about 29 kWh per day.
100% = whole house. Lower it (e.g. 50%) to back up only critical loads like fridge, lights and Wi-Fi.
DoD is the share of rated capacity you can safely use — an assumption, not a spec. Check your battery manufacturer's guidance.
Your results
Planning estimate. Sizes the battery for your target backup days; actual autonomy also depends on solar recharge, inverter losses and weather.
| Depth of discharge used | 90% |
| Rough count of 13.5 kWh units | — |
Example: 30 kWh/day × 1 day × 100% load needs 30 kWh usable; at 90% DoD that's 30 ÷ 0.90 = 33.33 → 34 kWh rated (rounded up).
How to use this calculator
Start with your daily electricity use in kWh — you'll find it on your utility bill or your utility's app. The average U.S. home lands around 29 kWh per day, but homes with electric heating, an EV, or a pool can be much higher.
Next, decide how many days you want the battery to carry you without the grid, and what share of your home to back up. Backing up only the essentials (fridge, freezer, lights, Wi-Fi) at 50% can cut the required battery size dramatically versus whole-home backup.
Finally, pick your battery chemistry to set the depth of discharge — lithium defaults to 90%. Everything recalculates instantly as you type, so try scenarios: one day versus two days of backup, or 100% versus 50% of loads, and watch how the recommended kWh changes.
How it works
The math is energy bookkeeping in two steps. First, the calculator finds the usable energy you need: usable kWh = daily kWh × backup days × (backed-up % ÷ 100). Then it accounts for the fact that you can't drain a battery to zero: nameplate kWh = usable kWh ÷ depth of discharge. The result is rounded up to the nearest whole kWh, because you can only buy whole capacity.
For the unit count, the recommended nameplate is divided by 13.5 kWh — a typical all-in-one home battery size — and rounded up to whole units. That's informational only; real systems have fixed unit sizes, wiring constraints, and inverter limits, so treat it as a starting point for a conversation with a licensed installer.
Assumptions, stated honestly: the depth-of-discharge values are assumptions, not manufacturer specs — discharging deeper than your battery's guidance shortens its life. The calculator ignores inverter efficiency, self-discharge, capacity loss in cold weather, and the solar array needed to recharge the battery — all of which a professional design accounts for. Results are planning estimates, not engineering advice.
Frequently asked questions
What size home battery do I need for a power outage?
Start with your daily electricity use in kWh, decide how many days you want to ride through without the grid, and choose which loads to back up. This calculator multiplies those together to get the usable energy you need, then divides by the battery's depth of discharge to get the nameplate size to buy. As an example: a home using 30 kWh per day, wanting 1 day of full backup with a lithium battery (90% depth of discharge), needs 30 kWh usable — 34 kWh of rated battery capacity after rounding up.
How many kWh does a typical home battery hold?
Popular all-in-one home batteries hold around 13–14 kWh of usable storage per unit — for example, a Tesla Powerwall 2 or 3 stores about 13.5 kWh. Units can be stacked: this calculator estimates how many 13.5 kWh units your recommended nameplate size equals, rounding up to whole units. Two to three units is a common setup for backing up the essential loads of an average U.S. home for a day.
What is depth of discharge (DoD) and why does it matter for sizing?
Depth of discharge is the share of a battery's rated capacity you can safely use. Most lithium home batteries allow about 90% DoD; lead-acid batteries are typically limited to around 50%. Because you can't use the full nameplate, you must buy more rated capacity than the usable energy you need — the calculator does this for you by dividing usable kWh by the DoD fraction. Always check your battery manufacturer's DoD guidance before buying.
Should I size my battery for the whole house or just critical loads?
For most people, backing up critical loads — fridge, freezer, lights, Wi-Fi, phone chargers, and maybe a medical device — costs far less than whole-home backup and covers what actually matters in an outage. Use the '% of home backed up' field to model this: entering 50% halves the usable energy you need. Whole-home backup is an option if outages are long and frequent, but it typically means several battery units plus a large solar array to recharge them.
Is this calculator different from a generator size calculator?
Yes. A home battery sizing calculator works out how much energy storage (kWh) you need to carry you through an outage — like the size of a fuel tank. A generator size calculator works out how much instantaneous power (watts) a generator must produce to start and run your appliances — like the engine's horsepower. Use this page for battery storage, and our generator size calculator if you're comparing a fuel generator.