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AmpSize

Battery bank calculator

Calculate battery bank capacity and autonomy for a load.

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Each job fills in example values. Replace them with your values.

Inputs

Worked example with the default inputs. The live calculator replaces it when the page loads.

Loads
2 rows
System voltage
48 V
Depth of discharge
0.5
Inverter efficiency
0.9
Temperature factor
1
Autonomy
1 days

Required capacity

225.93Ah

Energy per day
4,880Wh
Charge per day
112.96Ah

This result is a design aid. Verify it against the current standard.

Calculation steps

  1. Energy per day4,880Wh
  2. Charge per day112.96Ah
  3. Required capacityResult225.93Ah

About this calculator

How this is calculated

The tool sizes a battery bank in ampere-hours for a set of loads. It reads no table, so it cites no clause.

  1. Inputs. You enter each load in watts with its hours of use. You also enter the system voltage and the depth of discharge. Then you enter the inverter efficiency, the temperature factor and the days of autonomy.
  2. Energy per day. For each load, it multiplies the power by the hours of use. It adds the loads.
  3. Charge per day. It divides the daily energy by the system voltage and by the inverter efficiency. The result is the daily charge in ampere-hours.
  4. Required capacity. It multiplies the daily charge by the days of autonomy and the temperature factor. It then divides by the depth of discharge.

The depth of discharge is the share of the battery you allow to be used. A lower value gives a larger bank. The temperature factor is 1 at normal temperature. A value above 1 adds capacity for a cold site.

Worked example

  • A 48 V bank for two loads. The loads are 500 W for 4 h and 120 W for 24 h. The depth of discharge is 0.5 and the inverter efficiency is 0.9. The energy per day is 4,880 Wh. The charge per day is 112.96 Ah. The required capacity is 225.93 Ah.

Limits

The tool gives capacity only. It does not choose a battery chemistry, a charger or a cable. For the cable between the bank and the inverter, use the DC cable size calculator.

The result assumes the loads are the same each day. It does not model charging from solar or the grid. It does not model battery ageing or a changing discharge rate. For a home battery with an inverter and solar, use the home battery tool.

The tool uses one system voltage for the whole bank. A 24 V bank and a 48 V bank hold the same energy but give different amp-hours. Take the depth of discharge from the battery maker. It differs between chemistries.

See also: Battery bank for three days of autonomy, Off-grid battery bank at 48 V and Home batteries and AS/NZS 5139.

Questions

Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.

What does the battery calculator do?

The tool adds the energy of every load you enter, in watts multiplied by hours. It converts that daily energy into ampere hours at the system voltage. It then reports the bank capacity needed for the days of autonomy you ask for.

Which inputs matter most?

The load list and the days of autonomy scale the capacity directly. The depth of discharge divides it, and defaults to 0.5. The inverter efficiency defaults to 0.9. The temperature factor multiplies the result and defaults to 1. The system voltage sets the ampere hours.

What does this tool not do?

It does not size the cables, the charge controller or the charging source. It cites no standard table, because the result is a direct calculation. Use the cable size tool for the DC run. The inputs sit in the page address, so a link reproduces the sizing.

Worked examples

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