Battery tools

How long will a battery run a refrigerator?

Far longer than a nameplate calculation suggests, because a refrigerator compressor runs roughly a third of the time. The startup surge is the part that catches people out.

How long will a 5 kWh battery run a refrigerator?

A typical 150 W refrigerator cycles at about 35% duty, so its average draw is roughly 53 W, not 150 W. A 5 kWh battery delivers 4.14 kWh to AC loads after depth-of-discharge and inverter losses, giving about 3 d 7 h of refrigeration at that average draw. The binding constraint is usually not energy but the roughly 800 W compressor start surge, which the inverter has to supply instantly.

Runtime by battery size

150 W refrigerator at 35% duty = 53 W average; 90% usable depth of discharge, 92% inverter efficiency, no additional reserve held back. The last column adds a 120 W freezer at 40% duty = 48 W average, for 101 W combined. Duty cycle is applied once, in the average load; the battery engine does not model cycling separately. Generated by batteryEngine-1.2.0.
Battery size	Delivered to AC	Fridge only	Fridge + freezer
2 kWh	1.66 kWh	31 h 33 min	16 h 29 min
5 kWh	4.14 kWh	3 d 7 h	41 h 12 min
10 kWh	8.28 kWh	6 d 14 h	3 d 10 h
13.5 kWh	11.2 kWh	8 d 21 h	4 d 15 h
20 kWh	16.6 kWh	13 d 3 h	6 d 21 h

Check surge before you check runtime

A refrigerator compressor draws around 800 W for a fraction of a second when it starts. That is a power requirement, not an energy one — it barely affects the runtime numbers above, but an inverter that cannot supply it will fail to start the fridge no matter how full the battery is.

Adjust for your own refrigerator

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

3 d 6 h

at 53 W average load

5.00 kWh nominal becomes 4.14 kWh delivered to AC appliances

Calculated from your inputs

Nominal energy: 5.00 kWh
Usable at DC: 4.50 kWh
Delivered to AC: 4.14 kWh
Delivered fraction of nominal: 83%

Runtime in hours

78.1 h

Exact quotient before rounding to hours and minutes.

Energy not delivered to AC

0.860 kWh

Depth-of-discharge limit (0.500 kWh), additional reserve (0.000 kWh) and inverter loss (0.360 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

5 kWh × 0.9 usable depth of discharge = 4.5 kWh available at DC
4.5 kWh × 0.92 inverter efficiency = 4.14 kWh delivered to AC loads
4.14 kWh ÷ 0.053 kW = 78.113 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?

Compressor duty cycle, which rises in a warm room and with frequent door openings
Whether you also back up a separate freezer
Depth of discharge and inverter efficiency of your system
Defrost heaters and ice makers, which add periodic high-power bursts
How often the door is opened during the outage

Keeping the fridge cold with less energy

During an outage the cheapest kilowatt-hour is the one you do not spend. A closed refrigerator holds temperature for several hours unaided, and a full one holds it longer than an empty one. Running the fridge on battery only intermittently — a few hours on, a few hours off — can stretch a small pack across a long outage without risking the contents, provided the door stays shut.