Calculate the Real Runtime of a Power Station

How the runtime formula works

The maths is deliberately simple because the inputs matter more than the algebra. Usable energy is the rated capacity times one minus the overhead fraction. Runtime falls out by dividing that usable energy by the average power drawn. A 1,000 Wh station with a 15% overhead delivers about 850 Wh; against a 65 W average load that is roughly thirteen hours. Change any one of the three inputs and the answer moves in proportion. The sensitivity is obvious. Halving your load doubles your runtime. A 20% overhead instead of 15% costs you about six percent of the total.

The overhead figure deserves a moment of thought. Running AC appliances through the built-in inverter is the expensive case - 10 to 20% lost as heat. Some devices charge over DC and skip inversion entirely. Think phones on the station's USB ports, or a 12 V fridge on the car-style socket. For an all-DC load you may drop the overhead to 5 to 10%. Conversely, a very light AC load can look worse than the maths suggests. The inverter's standby draw is a fixed cost, and it eats a larger share of a small budget.

• AC loads: assume 10–20% overhead (15% is a safe planning default)
• DC-only loads: 5–10% overhead is often achievable
• Very light loads: inverter standby draw can dominate the loss

Worked example: a 1,000 Wh station on fridge duty

Take a modern inverter fridge with a measured average draw of 65 W. That is not the 150 W nameplate. It is the true daily average logged by a meter - how fridges behave, with long idle stretches between short runs. Feed in 1,000 Wh of capacity, 65 W of load and 15% overhead. Usable energy is 850 Wh. Divide 850 by 65 and you get about 13.1 hours. Add a router at 15 W and the load becomes 80 W, pulling the estimate down to roughly 10.6 hours. That single arithmetic step - reusing the same formula for a different appliance set - is how you compare scenarios before buying anything.

Now flip the exercise. Suppose your target is surviving a nineteen-hour outage with the fridge and router running. You need 80 W × 19 h ÷ 0.85. That is about 1,790 Wh of rated capacity - a station nearly twice the size of the example unit. Asking for required capacity instead of available runtime is often the faster route to a buying choice. It is the same formula rearranged.

What counts as average load - and why nameplate watts mislead

The label on an appliance almost never tells you what it consumes over an hour. A kettle stamped 2,400 W only runs for a few minutes at a time. A fridge compressor rated 150 W may be active for a third of each hour, averaging nearer 50 to 80 W. A desktop PC idles at 40 W and spikes past 400 W under load. The number this tool wants is the time-averaged draw. The only honest way to get it is to measure with a plug-in energy meter over a full cycle. Or build the average from duty: rated watts times the fraction of the hour the device actually runs.

Where you cannot measure, work from typical averages rather than peaks. Plan on 40 to 80 W for a fridge, 10 to 20 W for a router, and 5 to 10 W per LED bulb. A laptop takes 50 to 90 W while charging. Surge matters too. Motor-driven appliances can draw three to seven times their running rating for a second or two. That barely changes your energy budget. It does decide whether the station's inverter can start the device at all. Check the continuous and surge ratings against the appliance before trusting any runtime figure.

Getting a more accurate estimate

Three refinements take this model from rough plan to dependable number. First, measure: a plug-in power meter costs little and logs real consumption, including the standby loads that quietly drain a small station overnight. Second, build a load profile. List every device, its average watts, and the hours per day you actually need it. Sum to a daily watt-hour budget and divide by your usable capacity. That day-based framing matches how outages are lived. Real outages mix heavy evening use with light overnight draws. A single continuous-hours figure fits that worse. Third, derate for age and temperature. Lithium cells lose capacity as they cycle. Cold garages or caravans can cut available capacity by a fifth or more until the cells warm.

It also helps to treat the result as a band rather than a point. If the tool says thirteen hours, plan around eleven to fifteen. The low end covers an older battery, a colder room and an inverter working hard. The high end assumes a fresh battery, a warm room and mostly DC charging. Plans that only work at the optimistic end of the band usually fail during the first real outage. The whole purpose of the estimate is to build in margin on purpose, not to discover it under pressure.

Accuracy, assumptions and limitations

This calculator models energy, not power. It assumes your load sits inside the station's continuous inverter rating. It assumes a healthy battery and constant overhead. Real inverters vary their efficiency with load, often worst at very low draw. It does not model battery degradation, deeply discharged cells, temperatures below freezing, or the manufacturer's hidden reserve that keeps the last few percent unreachable. None of those are flaws in the maths; they are simply reasons to keep a margin rather than plan down to the final watt-hour.

The companion assumption is behaviour: runtime figures only hold if you actually shed non-essential loads. A station that runs a fridge for thirteen hours will run for three if someone boils a kettle twice and charges a games console. Small extras eat the plan fast. Write the priority list into your plan before the outage, not during it, and the estimate becomes a commitment instead of a guess.