Solar Battery Size Calculator
Sized after depth of discharge and round-trip losses — the two things most battery calculators leave out, which is why their answers come out about a third too small.
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In short: Nominal battery capacity is daily kWh divided by (depth of discharge x round-trip efficiency). At 80% DoD and 85% round trip, covering 10 kWh needs about 14.7 kWh of nominal capacity, not 10.
Battery to buy (nominal)
14.7 kWh
Energy you actually get
10.0 kWh
Capacity at 48V
306 Ah
Lost to DoD and losses
4.7 kWh
Assumes 85% round-trip efficiency across cells and inverter. The nominal figure is what a spec sheet calls the battery; the usable figure is what you actually get out of it.
Why the battery is bigger than your usage
A battery never gives back what its label says. Depth of discharge caps how much you may take without shortening its life — 80% for LiFePO4 is the honest working figure, not the 90 to 100% often quoted. Round-trip efficiency takes a further cut on the way in and out, roughly 15% across the cells and the inverter together.
nominal kWh = daily_kWh × days ÷ (depth_of_discharge × round_trip)
amp hours = nominal_kWh × 1000 ÷ battery_voltageMultiply those two together and a battery delivers about two thirds of its nameplate. To cover 10 kWh you therefore need 14.7 kWh on the label, not 10.
Back up essentials, not everything
Whole-home backup for average US consumption needs around 42.4 kWh nominal, which is a serious amount of money sitting in a garage for the few days a year the power is out. A fridge, lights, internet and a few sockets is a fraction of that and covers what actually matters during an outage.
Do not forget the array
A battery is a bucket, not a source. Putting 10 kWh back in a day takes about 2.8 kW of panels at average sun — roughly 7 of them. Sizing the bank and forgetting the array is how a system ends up flat by Wednesday and staying flat.
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Questions
What size battery do I need to run my whole house?
For average US consumption of 28.8 kWh a day, about 42.4 kWh of nominal capacity for a single day — roughly 882 Ah at 48V. That is a large and expensive bank, which is why most people back up essential loads instead and let the rest go dark.
Why is the number bigger than the energy I actually use?
Two reasons, and both are real. You cannot drain a battery to empty without shortening its life, so only about 80% is usable. And you lose roughly 15% to round-trip and inverter inefficiency on the way in and out. Together that means a battery delivers about two-thirds of what its label says, so the label has to be larger than your need.
Is a bigger depth of discharge better?
Manufacturers often quote 90% or even 100% for LiFePO4, and you can use that — it just costs cycle life. We default to 80% because a battery sized on 100% is one that disappoints in year four, when it no longer covers the night it was bought for.
Do I need a battery at all?
If you are grid-tied with net metering and only want to save money, usually no — the grid is a far cheaper battery than a battery. Storage is worth it for genuine outage protection, for time-of-use arbitrage where the spread is wide enough, or for going off-grid. Buying storage purely to improve payback almost always makes payback worse.
What about the array to recharge it?
Putting 10 kWh back in a day needs roughly 2.8 kW of panels at average sun — about 7 of them. A battery with an array too small to refill it is a battery that is flat by Wednesday, which is the most common fault in a first off-grid build.