ToolForage.
ENERGY CALCULATOR

Battery Runtime Calculator

Enter the battery’s Ah, mAh, Wh or kWh rating and a total watt load, load-side current or device rows. Estimate runtime with usable energy, conversion efficiency and idle draw.

Calculator

Changing units converts the entered rating using your battery pack voltage.
V
Use the voltage of the complete battery pack. It is needed for Ah or mAh; Wh and kWh already describe energy.
Ah
Enter the rating for the complete pack in the selected unit. Use Wh or kWh directly if that is what your power station label gives.
%
Use the percentage of rated energy available before the battery reaches its cutoff. This is separate from conversion losses.
Use output-side power or current. Device rows average each running load by quantity and powered-on share.
W
Total steady load delivered to all connected devices, not the inverter’s maximum power rating.
%
Enter a percentage, such as 90 for 90%. Use 100 for a loss-free estimate; check the converter’s efficiency at your load.
W
Set to zero if already included in the efficiency figure.
A
Load-side current, after any converter. Use net battery-side idle draw in its separate field.
V
This belongs to the load current above; it can differ from the battery pack voltage.
One device type per line; paste three comma- or tab-separated values. Up to 100 rows. Use 100% for measured average watts.
YOUR RESULTS
Estimated runtime8 hours 38 minutes
Runtime in decimal hours
8.64 h
Nominal battery energy
1,200 Wh
Energy available before losses
960 Wh
Energy delivered to load
864 Wh
Battery-side power draw
111.11 W
Average power delivered to devices
100 W

Nominal and load-delivered energy

  • Nominal battery energy1,200 Wh
  • Energy delivered to load864 Wh

Both values are in Wh. Their difference includes unused capacity and conversion losses.

Entered load and average power
Entered load and average power
Device rowRunning power (W)QuantityPowered-on (%)Average load (W)
11001100100
Page guide

Usable battery energy and estimated run time

Nominal watt-hours come from voltage and amp-hours. Available energy removes the share you intend to leave unused. Delivered energy is what reaches the load after conversion and any separately entered idle draw.

For the grid electricity cost of running that load, the Appliance Electricity Cost Calculator also allows a separate standby schedule.

The formula

Nominal Wh = volts × amp-hours. Runtime hours = nominal Wh × usable fraction ÷ (load W ÷ efficiency fraction + idle W). Current entry: load W = load V × load A. Device list: load W = sum of running W × quantity × powered-on fraction.

The inverter draws more power from the battery than it delivers when efficiency is below 100%. Any additional battery-side idle consumption is added to that demand. Dividing the available watt-hours by battery-side watts gives hours of constant-load operation. Milliamp-hours are divided by 1,000 before multiplying by voltage. A Wh rating is used directly; kWh is multiplied by 1,000. The hours-and-minutes display rounds to the nearest minute; the energy calculation keeps full precision. Device percentages form an average load over the runtime; they do not represent surge power. Current mode measures current and voltage at the load side of a converter, so its conversion loss is still applied. Changing the pack voltage while retaining Ah reinterprets the pack energy; switching capacity units preserves the current nominal energy.

Worked example

A 12 V, 100 Ah battery contains a nominal 1,200 Wh. Using 80% leaves 960 Wh available. A 100 W output load through a 90% efficient converter draws about 111.11 W from the battery, giving an estimated 8.64 hours with no additional idle draw. Two listed devices, 50 W at 100% and 100 W at 50%, also average 100 W and give the same 8.64-hour estimate. A 2 A load at 50 V is another equivalent entry.

How to use this calculator

  1. Choose the complete pack’s capacity unit and enter its rating. Ah and mAh also need pack voltage; Wh and kWh already describe energy.
  2. Choose Total watts, Current and voltage, or Device list.
  3. For current entry, use voltage and current at the load side. For a list, enter watts, quantity and powered-on percent for each device type, or paste three columns.
  4. Set usable energy, conversion efficiency and any additional battery-side idle draw. Use 100% powered-on for an already averaged device wattage.
  5. Review live runtime, nominal/available energy and the load table. Reset restores the example.

Voltage and amp-hours belong to the same pack

Use the complete battery pack’s rating. For Ah or mAh, use the voltage and capacity of the same pack configuration. If a power station label already gives Wh or kWh, choose that unit and enter its energy rating directly. Changing units converts the entered rating with your pack voltage, while retaining your load and efficiency settings.

Real loads and battery behavior

Starting surges, changing loads, temperature, age, discharge rate, and cutoff settings can reduce usable runtime. Lead-acid capacity is especially sensitive to discharge conditions. The model is an energy balance, not a chemistry simulation. Check both continuous and surge output ratings separately: a large stored-energy figure does not mean an inverter can start every appliance.

Keep pack voltage separate from load voltage

A converter can supply a different voltage from its battery input. Current mode multiplies the supplied load voltage by the current reaching the load to obtain output watts; efficiency then determines battery-side watts. For example, 2 A at 50 V is a 100 W output load, even when the battery pack is rated 12 V. Do not use battery input current in this mode and apply the converter loss twice.

Assumptions & limitations

What this calculation assumes

  • Load and effective conversion efficiency remain constant.

What to keep in mind

  • Excludes Peukert effects, voltage sag, thermal limits, and dynamic battery management.

Common questions

Should idle draw be counted twice?

No. If the efficiency value already includes inverter self-consumption at this load, enter zero additional idle watts.

Is nominal energy the same as usable energy?

No. The usable share accounts for the portion you choose or are permitted to discharge before conversion losses are applied.

Can I enter watt-hours without knowing battery voltage?

Yes. Choose Wh or kWh and enter the energy rating directly. Voltage is only needed when capacity is in Ah or mAh.

What percentage should I use for an intermittent device?

Use its powered-on share over a representative period. A 100 W device powered for half that time averages 50 W. Use 100% if the watts you entered already average its cycling; do not discount it twice.

Sources & further reading

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