How Long Will a Power Station Run?
Divide watt-hours by watts and you will overestimate every time. Real runtime allows for the inverter's cut, the cold, and the fact that almost nothing runs flat out.
How long will your battery last?
Generator
5,000 W generator
Needs at least 2,650 W continuous and 3,700 W to start.
Battery power station
21,700 Wh
5 × 5,000 Wh and up (expandable) — needs 2,100 W of inverter and 3,700 W of surge.
A home backup running for 24 hours draws 2,100 W with everything on, peaks at 3,700 W when the sump pump (1/3 hp) starts, and uses 16.1 kWh over the session.
What this assumes
- Running watts assume everything can be switched on at the same time. That is the safe way to size a generator, and it is why the number looks high.
- Starting watts are the running total plus the single largest start-up surge — not every surge added together, because two motors rarely start in the same instant.
- Energy uses each item's duty cycle: a fridge compressor runs about a third of the time, an espresso boiler idles at a fraction of its element rating between shots.
- Battery sizing allows 15% for inverter losses and keeps 15% in reserve so you do not finish the day at empty. In the cooler bands it also allows for the capacity lithium gives up, which is about 85% of rated around freezing and roughly 60% in severe cold.
- Generator sizing never plans to sit above 80% of the continuous rating, which is the same thing as adding 25% to your running total. In the warmer bands it adds roughly 2% per 10 °F, or per 5.5 °C, above 77 °F (25 °C) for the output an engine loses in hot air.
- Fuel burn is modelled from published inverter-generator run times and is accurate to roughly ±20% in the real world.
Why simple arithmetic always flatters the battery
The formula everyone starts with is capacity divided by watts. A 1,000 watt-hour battery and a 100 watt load gives ten hours. It is the right shape and the wrong answer, for two reasons that pull in opposite directions.
The first makes runtime shorter. A battery stores direct current and your appliances want alternating current, so an inverter sits in between taking its cut. Around 15% of the rated capacity is lost in that conversion, which is why this site multiplies by 0.85 before dividing. Cold takes another bite on top: pick the temperature band you will be working in and a pack delivers about 85% of its rating around freezing and roughly 60% in severe cold.
The second makes runtime far longer, and it is the one nearly every online calculator ignores. Almost no appliance runs flat out. A refrigerator's compressor is on roughly a third of the time. An espresso boiler idles at a fraction of its element rating. A hand-wash heater fires for seconds at a time. Using the nameplate rating as though it were continuous is how people conclude a battery will last four hours when it will genuinely last twelve.
What each battery class actually runs
Every figure below comes from the same engine the calculator uses, with the inverter loss applied and each appliance's own cycling taken into account. A blank cell means that class cannot run the item at all, either because the inverter is too small or because it cannot supply the start-up surge.
| Appliance | Average draw | 300 Wh | 750 Wh | 1,000 Wh | 2,000 Wh | 3,600 Wh | 5,000 Wh |
|---|---|---|---|---|---|---|---|
| Refrigerator (18–22 cu ft) | 55 W | not possible | 12 h | 16 h | 32 h | 2.3 days | 3.3 days |
| Window air conditioner (8,000 BTU) | 500 W | not possible | not possible | 1 h 45 min | 3 h 30 min | 6 h 5 min | 8 h 30 min |
| CPAP machine (no humidifier) | 40 W | 6 h 25 min | 16 h 20 min | 21 h 45 min | 43 h 30 min | 3.2 days | 4.4 days |
| TV (55" LED) | 100 W | 2 h 35 min | 6 h 30 min | 8 h 40 min | 17 h 25 min | 30 h 35 min | 42 h 30 min |
| Microwave (1,000 W cooking) | 120 W | not possible | not possible | 7 h 5 min | 14 h 10 min | 24 h 55 min | 34 h 35 min |
| Espresso machine (1 group, commercial) | 500 W | not possible | not possible | 1 h 45 min | 3 h 25 min | 6 h 5 min | 8 h 25 min |
| Sump pump (1/3 hp) | 65 W | not possible | not possible | 13 h 35 min | 27 h 10 min | 47 h 50 min | 2.8 days |
| RV air conditioner (13,500 BTU) | 980 W | not possible | not possible | not possible | 1 h 45 min | 3 h 5 min | 4 h 20 min |
Two rows are worth dwelling on. The refrigerator runs for a surprisingly long time on a modest battery, because its duty cycle does the work for you. The RV air conditioner does not, on anything, for long: it is a steady kilowatt with a surge on top, and it is the clearest example of a load that wants an engine rather than a battery.
Inverter rating and surge rating are separate limits
A battery has three numbers and all three can stop you. Capacity in watt-hours decides how long. The continuous inverter rating in watts decides whether it can carry your load at all. The surge rating decides whether it can start your load. A 2,000 watt-hour unit with a 2,400 watt inverter will refuse a 3,000 watt griddle instantly, no matter how much energy is in the pack, and it will shut down rather than slow down.
This is why the tool checks all three and tells you which one failed. Use the "Will it fit?" tab with a specific unit and it will also list which items to switch off to bring the load inside its limits.
Getting more out of the battery you own
- Turn off the heated humidifier. A CPAP draws 40 W without it and 90 W with it. Overnight, that is the difference between comfortable margin and waking up to an empty battery.
- Keep the fridge shut. Every opening resets the compressor's work. A full, closed fridge holds temperature for hours, so you can power it in blocks rather than continuously.
- Do not heat with it. Anything with a heating element is a battery's worst case. Cook with gas, heat water on a stove, and save the battery for refrigeration, lights, devices and medical equipment.
- Keep it warm. In winter, a battery inside the vehicle or under a blanket delivers noticeably more than one left in a truck bed overnight.
Where a generator wins instead
Batteries are silent, legal indoors, and need no fuel run. They are also expensive per stored kilowatt-hour, and the gap widens fast above about 3 kilowatt-hours. If your load is measured in kilowatts, or your outage is measured in days, an engine is the cheaper machine by a wide margin. The generator versus power station comparison works through both sides with real prices, and how many watts a refrigerator uses covers the single most common battery question in detail.
Frequently asked questions
How do I calculate power station runtime?
Take the rated watt-hours, multiply by about 0.85 to allow for the inverter converting DC to AC, then divide by the average watts your equipment actually draws. The last part is where most calculators go wrong: they use the nameplate rating instead of the average. A refrigerator rated at 150 watts averages closer to 55, so it runs roughly three times longer than simple arithmetic suggests.
How long will a 1,000 watt-hour station run a refrigerator?
Around fifteen hours for a typical full-size fridge, because the compressor only runs about a third of the time. The same battery running a 1,500 watt space heater lasts under forty minutes. That contrast is the whole lesson: what empties a battery is not how big an appliance is rated, it is how much of the time it is genuinely working.
Why does my battery die faster than the calculator says?
Four usual causes. Cold weather, which can cost fifteen percent or more of usable capacity. An ageing battery that no longer holds its rated watt-hours. Appliances working harder than expected, such as a fridge in a hot room with the door opening constantly. And loads you forgot were connected, because a battery drains through standby draw as well as through the thing you are watching.
Can I run a space heater or an air conditioner off one?
You can, briefly, and it is almost never worth it. Resistive heat is the fastest way to empty a battery: 1,500 watts flat, no cycling, no mercy. Air conditioning is nearly as bad and adds a starting surge on top. If you need heat or cooling for more than an hour or two, an engine or a shore connection is the honest answer.
Does running at a lower load make a battery more efficient?
Slightly, yes. An inverter has a fixed overhead, so at very small loads a larger share of the draw goes to the inverter itself rather than to your appliance. This site uses a flat 85% conversion figure, which is the common rule of thumb across the usual range. Real units vary by a few points either way depending on the load and the model.
Do solar panels change the answer?
They extend it rather than replacing it. A few hundred watts of panels in good sun can offset a light load almost entirely, turning a one day battery into a multi day one. In poor weather, or for a load in the kilowatts, panels contribute a useful fraction and no more. Size the battery for the load first, then treat solar as the thing that stretches it.