The arithmetic printed on every portable power box is capacity divided by appliance wattage. It's simple, it's the number brands quote, and it's wrong in a consistent direction: optimistic.
These calculators apply the corrections that the box leaves out, so the figure you plan around is closer to what you'll actually get.
Why the number on the box is optimistic
Three corrections separate the advertised figure from the real one, and they compound.
You never get the full capacity. Turning stored DC into 120V AC costs energy, and most manufacturers reserve some capacity at the bottom to protect the cells. The usable figure is meaningfully below the printed one, and the gap widens on cheap inverters.
Appliances rarely draw their nameplate wattage. A refrigerator's label states what the compressor pulls while running, but a fridge spends most of its time idle between cycles. Treating the label as continuous draw is the single most common error in this category, and it produces runtime estimates several times too pessimistic.
Startup surge isn't in the runtime maths at all. Motors — fridges, pumps, compressors — spike hard for a fraction of a second when they start. That spike doesn't meaningfully affect runtime, but it decides whether the appliance runs at all, which no runtime figure will warn you about.
What these calculators can and can't tell you
They're good at the shape of the answer: whether you're in the right capacity band, whether a unit gets you through a night, whether doubling capacity is worth the weight.
They can't account for how cold it is, how full your fridge is, how often the door opens, or how your particular unit's inverter behaves under light load. Treat every output as an estimate with perhaps twenty percent of slack in it, and size up if the answer lands uncomfortably close to your requirement.
They also assume you know your appliance's real draw. If you're guessing at that, a plug-in energy meter will teach you more in a week than any calculator.
Which calculator you want
If you're asking how long one appliance will run on a given capacity — a fridge overnight, a CPAP machine, a laptop through a workday — that's the power station runtime calculator. It handles the single-appliance case, with losses and duty cycle built in.
If your question is instead about several loads running at once through a household outage, the maths is different and lives in the blackout runtime calculator on the home backup side.
Once you have a number, the buying guide turns it into a capacity band, my comparisons narrow the field, and the reviews cover individual units.
