Usable capacity vs advertised: why a 2,048 Wh battery delivers about 1,740 Wh at the outlet

The number on the box is the energy in the cells. The number you can plan a night around is smaller, and the gap is not a scam: it is three ordinary losses that every maker knows about and prints in the fine print of its own runtime formula. This page is where those watt-hours go, the formula the makers themselves use, and how every product card on this site turns a sticker into a usable figure you can check.

If watts and watt-hours are still blurring together, the watt-hours guide separates them first.

Three places the watt-hours go

Depth of discharge. The battery management system stops the unit before the cells are empty, because taking lithium iron phosphate to zero shortens its life. Consumer power stations typically allow 90 to 100% of the pack to be drawn; Bluetti’s published formula uses 95%.

Inverter loss. The cells store direct current. Your refrigerator wants alternating current at 120 V, and the inverter that converts one to the other wastes some of the energy as heat. Bluetti’s formula uses 93%; across brands and loads, 85 to 92% is the realistic range, and efficiency falls at very low loads, where the inverter’s own overhead is a large share of what it delivers.

Idle draw. The inverter, display and Wi-Fi radio consume power whether or not anything is plugged in. On a 2 kWh class unit that is commonly 10 to 30 W, which sounds trivial until you multiply by 24 hours and lose a quarter of the pack to a unit that was “just sitting there.”

The formula, in the makers’ own words

Bluetti prints it on its product pages: running time = battery capacity x depth of discharge x efficiency / device wattage, with the worked example 2,048 Wh x 95% x 93% / 1,000 W, about 1.8 hours. That is the same arithmetic as the runtime table on every appliance page here, with two differences. We use a single conservative 85% for the two losses combined, because it holds across brands and leaves room for idle draw and cold weather, and we print the assumptions under the table rather than in a footnote. The sump pump page is the worked example.

How to read the spec strip on our cards

Every ranked product carries an Advertised figure, the sticker, and a highlighted Usable figure beside it. When the maker publishes a usable capacity, the card prints that and labels it “maker figure.” When it does not, which is most of the time, the card prints the sticker multiplied by 85% and labels it “est. 85% inverter.” So a 2,048 Wh unit shows about 1,741 Wh usable, a 1,024 Wh unit about 870 Wh, and that is the number to divide your appliance’s watt-hours into. The sticker still matters for comparing units against each other; the usable figure is for planning a night.

Putting it together for one appliance

The same arithmetic runs the refrigerator and CPAP pages. Take the 1,116 W sump pump from the wattage guide, running a quarter of each hour through an eight-hour storm. That is 279 W average, 2,232 Wh delivered at the outlet. Divide by 0.85 and you need about 2,626 Wh of usable battery. A 2,048 Wh unit gets you most of the way; a 3 kWh class unit or an expansion battery covers the whole night. If you had sized from the sticker alone you would have bought the 2 kWh unit expecting eight hours and got a little over six.

What this does not cover

Cold reduces what a lithium pack will deliver, whichever chemistry it uses (the LiFePO4 versus NMC guide covers the difference), and every maker’s rating is at room temperature; a unit that lives in an unheated garage should be sized with extra margin. Motor loads add a second question, whether the inverter can start the appliance at all, which is the subject of running watts versus surge watts. And expansion batteries add capacity but not output: a bigger pack behind the same inverter runs the same appliances for longer, not larger ones.