Turn Any Battery Rating Into a Number You Can Use

Why watt-hours beat amp-hours for comparison

Amp-hours tempt people into bad comparisons. A 100 Ah phone battery sounds the same as a 100 Ah leisure battery, and one is a small brick while the other can run a fridge for a day. The gap is voltage, which amp-hours leave out entirely.

Watt-hours fold voltage back in. Multiply the amp-hours by the pack's nominal voltage and the true energy appears. The 100 Ah at 12 V is 1,200 Wh; the 100 Ah cell at 3.7 V is 370 Wh. Suddenly the sizes make sense and the shopping choice clarifies.

Manufacturers know this, which is why the more honest ones now print watt-hours on power banks. When a brand quotes only milliamp-hours at an unstated voltage, you are being invited to compare numbers that cannot be compared. Convert first, then judge.

There is a second reason amp-hours mislead. The same amp-hour rating means a very different current depending on voltage, and current is what heats cables and blows fuses. A 48 V 100 Ah pack moves the same energy at one quarter of the amps of a 12 V pack, with much thinner wiring. Watt-hours explain the energy; voltage explains the wiring.

• Amp-hours are current times time, and say nothing about voltage.
• Watt-hours are the only unit that fairly compares different pack designs.
• A power bank's 20,000 mAh is 100 Wh at 5 V, not at its 3.7 V cells.

The three conversion steps, in order

Step one is the honest energy figure. Take voltage times amp-hours to get watt-hours. For a 12 V, 100 Ah pack that is 1,200 Wh. Multiply amp-hours by 1,000 to see the same capacity quoted in milliamp-hours for phone comparisons.

Step two removes the part you cannot use. No battery gives its full rated energy. You apply a usable depth of discharge, then an efficiency factor for inversion and wiring. The 1,200 Wh becomes a smaller number that reflects what reaches your appliance.

Step three answers the runtime question. Divide the usable watt-hours by your load. At 80 W, a 12 V 100 Ah lithium pack with 90% usable and 15% loss runs roughly eleven and a half hours. That number, not the label, is what a night of darkness depends on.

Keep one habit while converting: carry the same unit all the way through. Do not multiply amp-hours by a milliamp-hour figure, and never mix a 5 V power-bank claim with a 3.7 V cell rating in a single calculation. The converter enforces consistency by asking for one voltage and treating everything downstream in watt-hours.

When a result feels impossible, recheck the voltage entry. A phone battery typed in at 5 V instead of its real 3.7 V cell voltage inflates the energy figure by a third, and a 48 V rack battery typed at 12 V shrinks it fourfold. Voltage errors are the most common cause of a conversion that looks wrong.

Depth of discharge: the number chemistry decides

Usable capacity is a chemistry decision, not a marketing one. A lead-acid battery pushed below half regularly sulfates and dies young. Lithium cells tolerate far deeper drains, and modern LiFePO4 packs often quote 80 to 90% usable without a longevity penalty.

The tradeoff is price and weight per usable watt-hour. A 100 Ah lead-acid battery drained to 50% gives half the service of a 100 Ah lithium drained to 90%. It also weighs more doing it. The converter makes that visible by multiplying usable percentage into the answer.

One caution on the usable figure. Manufacturers often publish the depth that hits a specific cycle life, and real packs fade over years. Setting your usable slightly lower than the datasheet is a cheap way to keep a margin that survives an ageing battery.

Temperature moves the usable number too. A cold garage or a winter camping night can shave several percentage points off effective capacity, on top of the chemistry limit. Since one fixed percentage cannot describe summer and January alike, owners who plan year-round usually set the figure low.

Reading the result for real decisions

The runtime line is the working output. It tells you what a pack does, not merely what it is. Pair it with your actual load, and the tool answers how long a router stays alive. It also shows how many nights a small station carries a CPAP before you must refill from a wall or the sun.

The full-energy lines still earn their place. Watt-hours per volt matters when you build or combine packs. Series strings raise voltage and keep amp-hours; parallel strings raise amp-hours and keep voltage. The converter handles both by working from the voltage you enter.

For DC loads, set losses near zero. A 12 V fridge on a 12 V battery skips the inverter fight that mains loads pay. This is the single easiest gain in off-grid power, and it is why many owners wire critical 12 V devices straight to the battery.

The conversion also runs backwards. If a night of medical-device duty needs 600 Wh, divide by your pack voltage to find the minimum amp-hour purchase. That flips the converter from a measuring tool into a shopping tool, sizing a battery to a job instead of judging one you already own.

Accuracy, assumptions and limitations

Nominal voltage is an average, not a constant. A 12 V lead-acid pack swings from 13 down to 11 as it discharges, and amp-hour ratings are usually given at a slow discharge rate. At high current, real capacity shrinks, an effect the converter does not model.

Rated capacity assumes the manufacturer's test conditions. Cold temperature, age and a high discharge rate each reduce the honest figure. The usable percentage you enter is therefore a planning choice, not a physical law. Erring low is cheap insurance on a battery you depend on.

Treat the runtime as a planning band with a floor, not a promise. Real nights differ from real datasheets. What the conversion does reliably is put every battery on the same watt-hour footing, which is where fair comparison begins.

One question stays outside this tool entirely. Watt-hour totals say nothing about maximum output. A pack can hold ample energy yet fail to deliver the surge a compressor wants at start-up. For that gate check, use the can-it-run checker here. This converter answers how long, never whether it can.