How long will a power station actually run your fridge?
Divide usable watt hours by average draw. Usable watt hours are lower than the number on the box, because turning stored DC into mains AC costs energy. Average draw is lower than the appliance's running wattage whenever it cycles, as a fridge compressor does. Both corrections are set out below, and both are yours to change.
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Why the box number is not the number you get
A power station's capacity is measured at the battery. Everything you plug into a mains socket is fed through an inverter, and that conversion is not free. Manufacturers rarely publish an efficiency figure, so this calculator starts at 85% for AC loads and lets you move it. Running a laptop or a phone off the USB-C ports skips the inverter entirely, which is why the DC ports stretch a battery considerably further than the sockets do.
If you want a feel for the size of this correction: on a unit in the two kilowatt hour class, the difference between assuming 95% and assuming 80% is several hours of fridge time.
The compressor cycle, and why it dominates fridge estimates
A fridge does not draw its running wattage continuously. The compressor runs until the cabinet reaches temperature, stops, and starts again when it drifts. Across a day, a reasonably full fridge in a normal room spends roughly a third of its time actually drawing power, which is why the duty slider starts at 35% for that appliance.
This single assumption moves fridge estimates more than anything else in the arithmetic. A fridge treated as a continuous 120 W load looks like it will flatten a 1,000 Wh unit in about seven hours. The same fridge at a one third duty cycle runs for the better part of a day. Neither number is wrong; they are answers to different questions, and the honest thing is to show which question you asked.
Two things push the real duty cycle up: a warm room, and a door that keeps opening. During an outage, keep it shut.
What this arithmetic leaves out
The result is an order of magnitude, not a guarantee. Three things in particular will move it.
- Start-up surge. A compressor or a pump pulls several times its running wattage for a fraction of a second at start. A unit whose continuous output only just covers the running figure may trip anyway. This is what surge ratings exist for.
- Cold. Lithium iron phosphate cells deliver less capacity in the cold, and most units refuse to charge below freezing at all. A winter outage is the worst case for both.
- Age. Cycle ratings are quoted to a percentage of original capacity, commonly 80%, sometimes 70%. A unit at the end of its rated life is meeting its specification while delivering noticeably less than it did new.
Before committing to a purchase on the strength of a number here, read the rating plate on the appliance you actually own. The wattages offered in the menu are common starting points, not measurements of your kit.
Common questions
Why does the calculator not just use the manufacturer's own runtime chart?
Because those charts rarely state their assumptions. A maker quoting fridge hours has picked an efficiency and a duty cycle to get there, and without knowing which, the figure cannot be compared against a rival's. Showing both numbers on screen is the whole point of this tool.
Should I run my appliance from the AC sockets or the USB ports?
Whatever can run from DC should. Phones, tablets, headlamps and most laptops charge from USB-C without waking the inverter, which removes the conversion loss and the inverter's own idle consumption. Reserve the sockets for things that genuinely need mains power.
Does leaving the unit switched on cost anything?
Yes. An inverter left enabled with nothing plugged in still draws a few watts to stay ready. Over a long outage that idle draw is not trivial, which is why many units let you turn the AC side off independently and why some switch it off automatically after a period with no load.
Can I trust the wattages in the appliance menu?
Treat them as starting points. Appliance power varies widely by size, age and efficiency class, and a chest freezer from a decade ago has little in common with a current one. The plate on the back of your appliance gives the figure that applies to you.