Runtime calculator
How long will a power station run your devices?
Add what you want to power, pick a power station or type its capacity, and see a runtime range. The math accounts for inverter loss, standby draw and start-up surge, which is why the answer is shorter than watt-hours divided by watts.
Updated
1. What you want to run
Full-size refrigerator (top freezer)
Cycles on about 26% of the time; typical use is 0.99 kWh a day.
Runtime uses daily energy, not running watts. Turn off the station's ECO/auto-off mode so it doesn't cut power between compressor cycles. Older fridges use more.
2. The power station
The formula
Runtime (hours) = rated Wh × 0.9 ÷ (AC watts ÷ 0.85 + inverter standby watts + DC watts ÷ 0.9)
We show the result as a range, from 15% below to 5% above the estimate, because real runtime moves with temperature, battery age and how often your devices cycle on and off. Fridges and freezers use their typical daily energy from ENERGY STAR data, not the number on the nameplate, since the compressor is off much of the time.
Why start-up surge matters
Motors and compressors briefly pull several times their running watts when they start. A fridge that runs at 150 W can need over 1,000 W for a moment. If the station's surge rating is lower, it will shut off instead of starting the fridge. Boost modes such as X-Boost or Power Lifting don't help here: they lower voltage to run heating appliances and do nothing for motors.
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Free for preparedness blogs, RV and camping sites, CPAP communities and local news. Pick the starting devices, copy the code and paste it into any page. It resizes itself, works on phones and uses the same data and formula as this page. Please keep the credit line.
<iframe src="https://rightsizepower.com/embed/runtime-calculator?devices=fridge-top-freezer,wifi-router-modem,phone-tablet:2,led-bulb:2" data-rsp-embed title="Power station runtime calculator" width="100%" height="900" style="border:0;max-width:760px" loading="lazy"></iframe>
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<script src="https://rightsizepower.com/embed.js" async></script>What people actually ask
How do you calculate power station runtime?
Multiply the rated watt-hours by 0.9 (the share the battery management system lets you use), then divide by the battery-side draw: AC watts divided by 0.85 for inverter loss, plus the inverter's standby draw, plus DC watts divided by 0.9. A 1,024 Wh station running a 50 W AC load with 12 W of standby lasts about 13 hours.
Why is my runtime shorter than capacity divided by watts?
Three losses add up: the battery keeps a reserve, the inverter loses roughly 15% turning DC into AC, and the inverter itself draws power whenever the AC outlets are on. On small loads such as a CPAP or a router, that standby draw can cut runtime by a third or more.
How long will a 1000 Wh power station run a refrigerator?
A typical top-freezer refrigerator uses about 1 kWh a day, so a 1,000 Wh station runs it for roughly 12 to 16 hours. Large French-door models with an ice maker use about twice as much.
Does running devices from DC really help?
Yes. The 12 V and USB-C outlets skip the inverter, so there is no 15% conversion loss and no inverter standby draw. A CPAP on its DC cable runs about twice as long as the same CPAP on its AC adapter.
Does cold weather change runtime?
Yes. LiFePO4 batteries deliver less energy in the cold: plan on about 15% less around 14°F and about 35% less near −4°F. Most units also refuse to charge below 32°F.