Battery tools

Battery runtime calculator

Enter your battery capacity in kWh or in amp-hours and volts, then the average load you want to carry. The calculator applies depth of discharge and inverter losses before dividing.

How long will a 10 kWh battery run a 690 W load?

A 10 kWh nominal battery at 90% usable depth of discharge holds 9.00 kWh of usable DC energy. After 92% inverter conversion, 8.28 kWh actually reaches AC appliances. Divided by a 690 W average load, that is 12 h 0 min of runtime. The gap between 10 kWh on the label and 8.28 kWh at the outlet is the part most runtime estimates leave out.

Your inputs

How do you know your battery size?

Nominal battery capacity

Average AC load

The average draw across the outage, not the sum of nameplate ratings.

Largest startup surge

Motor loads such as a fridge, well pump or air conditioner. Used for the power check, not for runtime.

Usable depth of discharge

Inverter efficiency

Reserve held back

Additional reserve kept above the battery's usable-depth limit. This is separate from depth of discharge, and is applied after it.

Inverter continuous rating

Optional. 0 leaves the continuous-power check off.

Inverter surge rating

Optional. 0 leaves the startup-surge check off.

Scenario: average load

12 h 0 min

at 690 W average load

10.0 kWh nominal becomes 8.28 kWh delivered to AC appliances

Calculated from your inputs

Nominal energy: 10.0 kWh
Usable at DC: 9.00 kWh
Delivered to AC: 8.28 kWh
Delivered fraction of nominal: 83%

Runtime in hours

12.0 h

Exact quotient before rounding to hours and minutes.

Energy not delivered to AC

1.72 kWh

Depth-of-discharge limit (1.00 kWh), additional reserve (0.000 kWh) and inverter loss (0.720 kWh).

Startup surge entered

None entered

No inverter surge rating entered, so no check is performed — compare it with your inverter manufacturer's surge rating.

Power check

Continuous-power check off — enter an inverter continuous rating to run it.
Startup-surge check off — enter both a startup surge and an inverter surge rating to run it.

These checks compare power ratings only. They say nothing about how long the battery lasts, and a continuous rating does not by itself define what the inverter can supply for a few seconds.

Method

10 kWh × 0.9 usable depth of discharge = 9 kWh available at DC
9 kWh × 0.92 inverter efficiency = 8.28 kWh delivered to AC loads
8.28 kWh ÷ 0.69 kW = 12 hours

Energy capacity and power capability are separate limits. Passing the energy check does not guarantee the inverter can start the load.

Assumptions

Usable depth of discharge 90% of nominal capacity.
Inverter/conversion efficiency 92% on the DC→AC path.
No reserve held back.
Load is treated as constant at the average value; real loads vary.
Temperature effects, ageing and self-consumption of the system are not modelled.

What can change the result?

Your real average load, which is almost always lower than the sum of nameplate ratings
Depth of discharge allowed by the battery management system
Inverter efficiency, which falls at very low loads
Temperature — cold reduces usable capacity, sometimes substantially
Whether solar or a generator recharges the pack during the outage

Runtime by battery size and load

Every cell applies 90% depth of discharge and 92% inverter efficiency. Capacities are generic sizes, not specific products.

Generated by the battery engine, not typed in by hand.
Load profile	Average load	5 kWh	10 kWh	20 kWh
Fridge, freezer, router and lights	250 W	16 h 34 min	33 h 7 min	66 h 14 min
Small home office plus fridge	400 W	10 h 21 min	20 h 42 min	41 h 24 min
Essential circuits, typical mixed home	690 W	6 h 0 min	12 h 0 min	24 h 0 min
Essentials plus a window air conditioner	1200 W	3 h 27 min	6 h 54 min	13 h 48 min

Energy is one limit, power is another

Runtime answers the energy question: how many kilowatt-hours you have and how fast you are spending them. It says nothing about whether the system can supply the load at all. A 20 kWh battery behind a 3 kW continuous-rated inverter cannot sustain a 4 kW load, regardless of how much energy the battery stores — although a short startup surge may still be possible if the inverter's surge rating permits it. Continuous rating and surge rating are two different numbers on the datasheet, and a well pump drawing a 2,800 W starting surge may still trip an inverter rated 3,000 W continuous and 4,000 W surge if other loads are already running.

Enter your largest startup surge above, together with the optional inverter continuous and surge ratings, and the calculator runs the power check separately from the energy check. Leave the ratings blank and the surge is simply carried through for you to compare against your inverter manufacturer's rating.