How long will a power station run a fridge?

How the runtime is worked out

Three steps. First, the energy you can use: the capacity in watt-hours (Wh) printed on the station, times the battery’s health. Second, what each device takes from the battery in a day: its average watts × the hours it runs. Anything on an AC outlet goes through the station’s inverter, which loses some energy as heat, so its share is divided by the inverter efficiency; anything on a USB or 12 V outlet skips the inverter, and we count it at face value (we have no published figure for losses at the DC ports), so treat DC results as a best case. Third, runtime = usable Wh ÷ (daily draw ÷ 24). The result is how long a full charge lasts if the day keeps going the way you described it.

If your battery is rated in milliamp-hours rather than watt-hours, convert it first with Lightnest Market’s power bank capacity calculator.

The fridge: why its running watts mislead

A fridge’s compressor switches on and off to hold the temperature, so multiplying its running watts by 24 hours overstates its use. The calculator offers two ways round that. If its energy label or spec sheet gives a yearly figure in kWh, enter that: dividing by the 8,760 hours in a year gives the average draw, compressor cycles included. A fridge using 300 kWh a year averages 34.2 W, or 822 Wh a day. Otherwise enter its running watts and the share of the time you think it runs. We don’t suggest a share, because we found no official figure for it, and it changes with the room’s temperature and how often the door is opened.

The station also has to start the fridge. On Honda’s generator chart an Energy Star fridge or freezer needs 1,200 W for a moment to start, against 132 to 192 W running, so check the station’s surge (peak) rating as well as its continuous one. Jackery, for example, rates its Explorer 1000 v2 at 1,500 W continuous and 3,000 W surge.

Inverter efficiency and battery health

Victron Energy’s datasheet for its Phoenix inverters gives a best (maximum) efficiency of 87 to 92% across its 12 V models, and a zero-load draw of 4.2 to 12 W: the inverter uses that much just being switched on. At light loads that idle draw weighs heavily: 6 W of idle on top of a 30 W load is 36 W from the battery, about 83%. That is why the calculator starts at 85%, a little below the datasheet’s best. Change it if your station’s maker gives a figure, and switch the AC outlets off when nothing needs them.

Batteries also hold less as they age. Jackery rates the Explorer 1000 v2 for 4,000 cycles to 70% or more of its capacity; EcoFlow rates its DELTA 3 for 4,000 cycles to 80% or more. If your station is well used, lower the battery health.

A worked example

The calculator opens with a 1,000 Wh station at full health and 85% efficiency. The fridge (300 kWh a year) takes 822 Wh a day at the plug, 967 Wh from the battery. The router, 20 W all day, takes 565 Wh from the battery; the TV, 120 W for 3 hours, 424 Wh; an LED lamp, 10 W for 6 hours, 71 Wh; phones on USB, 40 Wh. That is 2,066 Wh a day, an average of 86 W, so the full 1,000 Wh lasts 11 hours 37 minutes: 23 minutes short of the 12 hours set as the target. Switching off the TV, the only device not marked essential, brings the daily draw down to 1,642 Wh and the runtime up to about 14 hours 37 minutes.

Refilling it: solar and the car

Solar depends on your panel, the weather and the season, so the calculator uses your own numbers: panel watts × hours of good sun × the share that reaches the battery. The 68% starting figure comes from Jackery’s own charging time: it quotes 7.5 hours to charge the Explorer 1000 v2, sold as a 1,024 Wh station (its spec sheet gives the battery as 1,070 Wh), from its 200 W SolarSaga panel. That is 68 to 71% of the panel’s rated output over those hours, depending on which capacity you take. Expect much less on a short, grey winter day. Jackery quotes 12 hours to charge the same station from a 12 V car adapter, about 85 to 89 W on average. Only charge from the car while you are driving or parked outside: FEMA says never to leave a vehicle running inside a garage, even with the door open.

Medical devices

If someone at home relies on a CPAP machine, an oxygen concentrator or other powered medical equipment, don’t plan from an estimate like this one. Follow the device maker’s guidance on batteries and backup power, and talk to your medical provider about a power outage plan, as Ready.gov advises. In the UK, join your energy supplier’s and network operator’s Priority Services Register: Ofgem lists people who use medical equipment that needs a power supply among those who can join, and the help includes advance notice of planned power cuts where possible. In the US, ask your electric utility whether it runs a medical or life-support customer programme; in California, for example, the Public Utilities Commission’s Medical Baseline programme covers people who rely on life-support equipment.

Common mistakes

• Using the running watts × 24 for a fridge. The compressor isn’t on all the time; use the yearly kWh.
• Forgetting the inverter. Charge phones and laptops from USB-C where they can, and turn the AC side off when it isn’t needed.
• Opening the fridge often. Every opening lets warm air in. Food safety guidance says to keep the doors shut, as the power cut checklist explains.
• Assuming a full charge. Check the charge level and top the station up as soon as a storm is forecast.

Need more than a battery can give, such as a sump pump or a furnace blower? Size a generator with the generator size calculator. To work out what a heater would cost to run when the power is back, see Warm Window’s heating cost calculator.