Rated vs Usable Capacity — Why Your Power Station Gives Back Less Than the Box Says

Three things take a share, in roughly this order of size.

The inverter. Your battery stores direct current. Your appliances want alternating current at 120 V. Converting between them costs energy, mostly as heat — which is why the fan runs. A good inverter in its efficient band gives back around 90%; a cheap one, or one running far below its rated output, gives back considerably less.

The reserve. Lithium iron phosphate cells are damaged by being fully emptied. Every manufacturer therefore holds back a slice at the bottom and calls the pack "empty" before it truly is. You never see this energy and you would not want to.

Self-consumption. The screen, the controller, the Bluetooth radio and the cooling fan all draw power while the unit is on. Over a long, slow discharge this adds up — and proportionally it hurts small loads far more than large ones.

These are my own measurements, taken on one method: charged to full, then a constant 300 W load until the unit shut itself off, with a meter at the AC outlet. Because every unit saw the same load, these percentages are directly comparable with each other.

Two patterns are worth pulling out of that table.

Bigger units keep more. The 4,096 Wh EcoFlow Delta Pro 3 returned 92.1%; the 288 Wh Anker SOLIX C300 returned 80.6%. The inverter's fixed overhead is the same either way, so it is a much smaller share of a big pack. If you are choosing between one large unit and two small ones, this is a real argument for the large one.

Same rating, different delivery. The Bluetti Elite 100 V2, Anker SOLIX C1000 Gen 2 and EcoFlow Delta 3 are all rated 1,024 Wh. On my bench they delivered 892, 884 and 862 Wh. Thirty watt-hours is about forty minutes of a camp fridge — and the rating gave you no way at all to know which was which.

If I've tested the unit, use my measured figure. If I haven't, the honest approach is to assume the middle of the range and accept that you are estimating:

Usable watt-hours ≈ rated watt-hours × 0.85

Then divide by your load to get hours. A 1,024 Wh unit becomes roughly 870 Wh, which runs a 45 W camp fridge for about 19 hours — close to the 862 Wh I actually measured from the EcoFlow Delta 3.

Two adjustments matter:

• Small loads: if you are drawing under about 50 W, use 0.80 rather than 0.85. Self-consumption is punishing at that end.
• Old packs: a battery rated to 80% capacity after 4,000 cycles will, eventually, hold 80%. If you are sizing for a system you intend to keep a decade, size against the end-of-life figure, not the new one.

Rated-versus-real is not confined to capacity. Recharge times are claimed too, and they are also measurable.

The clearest case on this site is the Jackery Explorer 2000 v2, which claims a 1 hour 45 minute recharge and was independently timed at 2 hours 30 — a 43% overrun.[2][3] The EcoFlow Delta Pro 3's peak charge rate was measured at about 1,600 W against an advertised 1,800 W.[4]

Others hold up well. The Anker SOLIX C1000 Gen 2 was measured at 56 minutes against a 49-minute UltraFast claim, and the Jackery Explorer 1000 v2's measured 1 h 35 matched its own specification.[5][6]

There is no requirement to publish it, no standard method that would make published figures comparable, and a strong commercial reason not to. A manufacturer that honestly advertised "1,024 Wh rated, about 884 Wh usable" would sit on a shelf beside a competitor advertising "1,024 Wh" and look worse while being identical.

This is a coordination problem rather than a conspiracy, and it is exactly the sort of thing independent testing is for.

Next: how to size a power station turns your actual appliances into a watt-hour number, and the full ranking applies all of this to ten units.