What Wh means and how to calculate how long your device will run

Watts (W) measure how much energy a device draws in an instant — it's the rate at which it spends electricity. Watt-hours (Wh) measure accumulated energy: how much electricity is stored in a battery, or how much was consumed over a period of time.

A 45W fan draws 45 watts while it's on, no matter how long it's been running. A 768Wh station has 768 watt-hours stored in total — it's a fixed amount of energy, like the fuel in a tank.

The formula to estimate how many hours a battery lasts with a device connected is: Hours = (Capacity in Wh x efficiency) / Consumption in W. Efficiency isn't 100% because the inverter (the circuit that converts the battery's current into usable current) loses energy as heat — in practice it runs 80% to 90%, depending on the equipment and its condition.

Example: a 768Wh station powering a 45W fan, at 85% efficiency, lasts approximately (768 x 0.85) / 45 = 14.5 hours. That's the same calculation this site's runtime calculator uses, but against the full catalog instead of a single number.

A second example, with a more demanding device: the EcoFlow DELTA 3 has 1024Wh. A home refrigerator draws around 150W while running, but it's cyclic — the compressor isn't on the whole time — so actual consumption over a night is lower than that nominal figure. At 85% efficiency, and assuming the compressor runs half the time (a reasonable assumption for a well-sealed refrigerator), the available energy (1024 x 0.85 = 870Wh) covers well more than a full 12-hour night — this site's calculator does that math with the device's actual declared cycle, not a rough average.

A common mistake is confusing Wh with Ah (amp-hours). Many small batteries — power banks, car batteries — state their capacity in Ah, which depends on the system's voltage: a 100Ah battery at 12V has 1200Wh (100 x 12), but that same 100Ah figure at 3.7V (a typical single lithium cell's voltage) is only 370Wh. If you're comparing two products and one states Ah and the other Wh, convert both to the same unit first — never compare the raw number.

An inverter's real efficiency isn't a fixed number: it drops with heat (a real issue in hot climates), with long or low-quality cables, and with battery age. The 80%-90% range this site uses is conservative on purpose — we'd rather the calculator underestimate than promise more hours than a real device will deliver.

A common mistake is calculating runtime with a single device when several will actually be connected at once. A router (10W) plus three LED bulbs (9W each, 27W) plus a charging laptop (65W) add up to 102W simultaneously, not 10W — on a 768Wh station at 85% efficiency, that's (768 x 0.85) / 102 = 6.4 hours, less than half the 14.5 hours in the fan-alone example. The formula itself doesn't change, but you need to add up the real consumption of everything that will be plugged in at the same time, not just the main device considered in isolation.

At the large end of the catalog, the EcoFlow DELTA Pro (3600Wh) powering that same 102W combo delivers (3600 x 0.85) / 102 ≈ 30 hours — more than a full day with a router, lights and a charging laptop, with no recharge. The difference between 6.4 and 30 hours against the same load is, literally, the difference between 768Wh and 3600Wh of capacity: the formula is the same in both cases, the only thing that changes is the numerator, which is why the same math works for comparing any pair of real capacities in the catalog against the exact same combo of devices.