FigureDesk

Solar Battery Size Calculator

Estimate the battery capacity needed to cover your daily energy use for a chosen number of autonomy days, from the depth of discharge and efficiency you provide.

Your numbers

How many days with no charging the bank should cover

V
%

From your battery's datasheet — there is no default.

%

From your inverter's datasheet — there is no default.

Result

Required battery capacity

92.59 Ah

Daily energy
2,000 Wh
Required usable storage
2,000 Wh
Required nominal storage
4444.44 Wh
System voltage
48 V
DoD assumption
50%
Efficiency assumption
90%

How it works

Multiply daily energy use by the number of days you want the battery to cover with no recharging — that's the usable energy you need. Since you can't discharge a battery all the way, and some energy is lost in conversion, divide by depth of discharge and efficiency to get the nominal (nameplate) capacity you actually need to buy. Divide by system voltage to convert that to amp-hours.

required usable Wh = daily energy Wh × days of autonomy
required nominal Wh = required usable Wh / (DoD / 100) / (efficiency / 100)
required Ah = required nominal Wh / system voltage

Worked example

A home using 2,000 Wh/day, wanting 2 days of autonomy, on a 48 V system with 60% usable depth of discharge and 90% round-trip/inverter efficiency:

  • Required usable storage: 4000 Wh
  • Required nominal storage: 7407 Wh
  • Required battery capacity: 154 Ah at 48 V

Assumptions and limitations

  • Depth of discharge and efficiency are values you enter, not looked-up defaults — FigureDesk does not guess these from a battery chemistry, since a wrong guess is worse than asking. Check your battery and inverter/charge-controller datasheets.
  • No temperature derating — this assumes standard operating conditions. Cold weather reduces usable capacity for some chemistries.
  • No ageing allowance — batteries lose capacity over their service life; many installers add 10-20% oversize for this, which you can apply by adjusting your DoD or daily energy input.
  • No chemistry selector and no automatic battery recommendation — the math is identical regardless of chemistry once you know the actual DoD and efficiency; only the numbers change.
  • This does not check whether your charge controller, panel array, or wiring can actually recharge a battery this size within your available sun hours.

FAQ

Why isn't there a battery chemistry selector?

Depth of discharge and efficiency vary by chemistry, but FigureDesk doesn't have a sourced, citable default for each one yet — rather than show a number that might be wrong for your specific battery, this calculator asks for your battery's actual usable DoD directly. The sizing math is identical for every chemistry once you know that number.

How many days of autonomy should I plan for?

It depends on your climate and how critical the load is. Off-grid systems in cloudy climates often plan for 2-3 days; grid-tied backup systems covering short outages might use 1 day or less. This is a design choice, not a calculated value.

What's the difference between usable and nominal storage?

Usable storage is the energy you actually need available. Nominal (nameplate) storage is the battery capacity you need to buy — larger, because you can't discharge a battery to 0% and some energy is lost in conversion.

Related calculators

FigureDesk results are advisory and do not replace a licensed electrician, a qualified engineer, or the applicable local electrical code. Verify any electrical work against your local code and with a qualified professional before relying on it.