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Electrical

Battery Bank Calculator

Battery capacity from load, autonomy and depth of discharge.

Your measurementsEvery assumption editable01

The loads you actually want backed up, not whole-house use.

How long the bank must run without recharging.

How much of the rated capacity you may actually use.

Energy out divided by energy in. Lithium is around 90–95%, lead-acid nearer 80%.

Bank capacityCalculated live02

Result

12.08kWh rated

12.08 kWh nameplate for 10 kWh usable

Energy to cover10 kWh × 1 days
10 kWh
Depth of discharge
90%
Round-trip efficiency
92%
Rated capacity needednameplate, before derating
12.077 kWh
At system voltage48 V bank
252 Ah
Usable amp-hours
208 Ah

Rated capacity and usable capacity are very different numbers, and battery marketing rarely distinguishes them. A lead-acid bank discharged past half its rating dies early, so a 10 kWh lead-acid bank realistically delivers 5 kWh — which is why lithium's higher usable depth often wins on cost per usable kilowatt-hour despite a higher sticker price.

Round-trip efficiency is charged separately from depth of discharge because they are different losses. Depth limits how much you may take out; efficiency describes how much you get back per unit put in. Both have to be accounted for or the bank comes up short.

Size the bank on the loads you genuinely need through an outage rather than whole-house consumption. Backing up a fridge, some lighting and a few circuits is a fraction of the cost of backing up air conditioning and an electric range.

Rated capacity and usable capacity are very different numbers, and battery marketing rarely distinguishes them. This works backwards from the energy you actually need to the nameplate figure you have to buy.

Why use this tool?What it does differently03

Why use this tool?

Rated versus usable

The distinction battery specs blur, and the reason lead-acid needs twice the nameplate.

Two separate losses

Depth of discharge limits what you may take out; efficiency describes what you get back. Both count.

kWh and amp-hours

At your system voltage, since banks are specified either way.

Chemistry presets

Lead-acid through LiFePO4, which differ by nearly a factor of two in usable depth.

How this worksThe method04

How this battery bank calculator works

Usable energy is daily load times days of autonomy. Rated capacity is that divided by both the depth of discharge and the round-trip efficiency, because those are separate losses that compound.

Depth of discharge is the fraction of rated capacity you may use without shortening life. Lead-acid discharged past half its rating dies early, so a 10 kWh lead-acid bank realistically delivers 5 kWh — which is why lithium often wins on cost per usable kilowatt-hour despite the higher sticker price.

Amp-hours follow from dividing energy by system voltage. Higher system voltages need fewer amp-hours for the same energy, which is why larger installations run at 48 V rather than 12.

How to use itStep by step05

How to use it

  1. Step 1: Total the loads to back up

    What you genuinely need through an outage, not whole-house consumption.

  2. Step 2: Choose days of autonomy

    One day for grid backup; three or more for genuine off-grid.

  3. Step 3: Set depth of discharge

    By chemistry. This is the input that moves the answer most.

  4. Step 4: Compare on usable kWh

    Not nameplate. It is the only fair basis for comparing chemistries.

Example usageWorked figures06

Example usage

A day of backup
10 kWh a day at 90% depth and 92% efficiency needs 12.08 kWh rated — about 252 Ah at 48 V.
The same in lead-acid
At 50% depth and 80% efficiency the same 10 kWh needs 25 kWh rated, more than double. That gap is the real cost comparison.
Off-grid autonomy
Three days at the same daily load needs 36.2 kWh rated — which is where off-grid battery budgets get serious.
Frequently asked questionsCommon questions07

Frequently asked questions

What size battery bank do I need?

Daily load times days of autonomy, divided by depth of discharge and round-trip efficiency. Ten kWh a day with one day of autonomy needs about 12 kWh of lithium.

What is depth of discharge?

The share of rated capacity you can use without shortening battery life. Lithium tolerates 80–90%; lead-acid should not go past about 50%.

Why does lead-acid need so much more capacity?

Because you can only use half of it, and it returns less of what you put in. A lead-acid bank needs roughly twice the nameplate capacity of lithium for the same usable energy.

How many amp-hours is that?

Energy in watt-hours divided by system voltage. A 12 kWh bank at 48 V is about 250 Ah; the same bank at 12 V would be 1,000 Ah.

Should I back up the whole house?

Rarely worth it. Backing up a fridge, lighting and a few circuits costs a fraction of covering air conditioning and an electric range.

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