Why the runtime on the box won't match your outage

Almost every power station's advertised runtime is calculated the optimistic way: divide the battery's nameplate watt-hours by the load's watts and print the result. Real outages don't work that way. Four separate gaps sit between the number on the box and the hours you actually get — and once you can see them, you can do the honest math yourself in about ten seconds.

Data as of Oct 19, 2026. The worked example below uses the EcoFlow DELTA 2's manufacturer-published specs (1,024 Wh capacity, 1,800 W continuous) from our database.

Gap 1: Nameplate watt-hours are not usable watt-hours

The capacity on the box is the battery pack's total stored energy. Your appliances never see all of it. Anything you plug into an AC outlet is fed through an inverter that converts the battery's DC power to household AC, and that conversion wastes a share of the energy as heat. The station's own electronics take a cut too.

This is not a WattLong opinion — it is in the manufacturers' own paperwork. Bluetti's published runtime formula reads: estimated runtime = total capacity × depth of discharge × inverter efficiency ÷ load. Our calculators apply a single combined real-world factor of 0.85 for AC loads — the same structure, mid-range for inverter efficiency (roughly 0.80–0.90 depending on load). Grade: REPORTED, and the full reasoning is on how we calculate.

What that does to the worked example: the DELTA 2's 1,024 Wh nameplate becomes about 870 usable watt-hours (1,024 × 0.85). Any runtime you calculate from the full 1,024 is inflated by about 18% before anything else goes wrong.

Gap 2: Runtime ads assume the load never rests — or barely exists

The longest runtime figures in marketing are typically quoted against a very small load — a light, a phone charger — where hours are easy to make big. Run the same battery into a serious load and the hours collapse; runtime is always at a stated load, and if the load isn't stated next to the number, the number is decoration.

The opposite mistake is just as common, and it mostly happens with refrigerators. A fridge's compressor cycles on and off; ours draws its 150 W running watts only about 40% of the time in our default planning assumption (INFERRED — it varies with the fridge, the room, and how often the door opens). Assume the compressor runs non-stop and you'd predict the DELTA 2 keeps a fridge cold for 5.8 hours (870.4 ÷ 150). Model the duty cycle honestly, at a 60 W average draw, and the same battery calculates to 14.5 hours (870.4 ÷ 60). The naive nameplate division — 1,024 ÷ 60 — says 17.1 hours. Three methods, three answers: 5.8, 14.5, 17.1. Only the middle one accounts for both the inverter and the cycling, and it is the one our fridge runtime calculator runs.

Gap 3: Boost wattage is not a rating

Spec sheets carry three different watt numbers, and the biggest is the one that gets advertised. Continuous watts are what the inverter can deliver indefinitely. Surge watts are what it can deliver for a split second to start a motor. Boost modes — EcoFlow's X-Boost, Bluetti's Power Lifting, Anker's SurgePad — are neither: they let the station feed certain (mostly resistive) loads above the continuous rating by reducing the output voltage. A heater doesn't care. A motor or a piece of electronics might.

The DELTA 2 makes the pattern concrete: 1,800 W continuous, 2,700 W surge, and a 2,200 W X-Boost figure that sits between them on the marketing page. If you size your loads against the 2,200 W number, you have planned around a voltage-reduction mode as if it were inverter capacity. WattLong stores boost figures in a separate field, labels them, and never merges them into the start check or any runtime math — the same rule is written into our methodology. When you compare spec sheets, compare continuous to continuous.

Gap 4: The battery you have in year three is not the battery you bought

Cycle-life ratings in our 30-model database run from 500 to 6,000 cycles, each quoted to the point where the battery holds 70–80% of its original capacity. A station that has lived a few years of regular use delivers its runtime at that reduced capacity — a 14.5-hour calculation quietly becomes a 12-hour reality. Temperature moves the number too: lithium batteries give up less energy in a cold garage than at room temperature. None of this is a defect; it is why a plan that only just barely works on paper is not a plan. Our calculators offer an explicit 10% reserve toggle rather than hiding a second penalty in the formula — if the outage matters, run the numbers with the reserve on.

The ten-second version

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