Amp Hours to Watt Hours Calculator

How to convert amp hours to watt hours

Amp hours (Ah) say how much charge a battery holds. Watt hours (Wh) say how much energy it holds. A battery label that prints only amp hours leaves out the voltage, and the voltage is what turns charge into energy:

• Watt hours = amp hours × volts
• Amp hours = watt hours ÷ volts

A 100 Ah battery at 12 V holds 1,200 Wh. The same 100 Ah in a 48 V bank holds 4,800 Wh, four times as much, because every amp hour is pushed at four times the voltage. Divide watt hours by 1,000 for kilowatt hours (kWh), the unit on an electric bill.

Use the Convert mode when you only want the unit change. The tool prints the step with your numbers in it, so you can check it by hand.

Why a 12 V lithium battery can be 12.8 V

LiFePO4 cells have a nominal voltage of 3.2 V, and four of them in series make 12.8 V. That is why a “12 V 100 Ah” lithium battery is often listed at 1,280 Wh and not 1,200 Wh. The 12.8 V, 25.6 V and 51.2 V chips in the tool fill the volts box with those nominal figures. Use the voltage your battery label prints; the difference between the two is about 7%.

How long will a battery run a load

The Runtime mode answers the question “how long will my battery last?” in four steps:

1. Battery energy: amp hours × volts.
2. Usable energy: battery energy × the share you may use (depth of discharge).
3. Delivered energy: usable energy × the efficiency of the inverter, or 100% for a DC load.
4. Runtime: delivered energy ÷ the load in watts, plus the inverter’s idle draw if you know it.

Pick the battery type to set the usable share. Lead-acid is usually sized to half its rating, LiFePO4 to 80% as a planning figure, and some lithium makers rate 100%. A portable power station is one switch: it sets 100% usable and 85% efficiency overall, which is the factor power-station makers use.

The runtime is rounded down, so the tool never promises more than the arithmetic gives. The idle watts box is the inverter’s own no-load draw from its spec sheet. An inverter that sits switched on all night uses energy even with nothing plugged in, and rivals that skip this field overstate the runtime.

A note on lead-acid and high draw

A lead-acid battery gives less than its rating when you pull the current out fast. The ratings are usually measured over 20 hours, so a draw above the capacity divided by 20 shows the caution line. The tool does not guess how much capacity you lose, because the loss depends on the battery. Treat the runtime for a heavy load on lead-acid as the best case. Lithium batteries are affected far less.

UPS and home battery backup

The Backup unit box at the top of Runtime mode fills the other boxes from a spec sheet, and you can still type over any of them. UPS fills one 1500 VA unit: 1,000 W with two 12 V 9 Ah sealed lead-acid batteries in series (CyberPower CP1500PFCLCD). Use the watts on your label, not the VA. The UPS answer is in minutes and runs high at heavy load, so the maker’s runtime chart wins.

Tesla Powerwall 3 and FranklinWH aPower 2 rate usable energy in kWh at the AC side, so the tool sets 100% usable and 100% efficiency and multiplies by the units. The lamp checks your load against the UPS watts or one home battery’s continuous output. For a Powerwall 2 or another battery, leave the box on Any battery and type its usable kWh.

How big a battery bank do I need

The Bank mode starts from the energy you use in a day:

1. Bank energy: daily watt hours ÷ inverter efficiency × days between charges ÷ the usable share.
2. Bank amp hours: bank energy ÷ system volts.
3. In series: system volts ÷ one battery’s volts, rounded up.
4. In parallel: bank amp hours ÷ one battery’s amp hours, rounded up.
5. Batteries: in series × in parallel.

Batteries always round up, because a bank that is a little too small runs flat early. Series connections add voltage and keep the amp hours of one battery. Parallel connections add amp hours and keep the voltage. A 48 V bank of 12 V 200 Ah batteries needs four in series, and a 600 Ah target takes three of those strings, 12 batteries in all.

Going to a higher system voltage can change the count, because the series and parallel steps round separately. Try 12 V and 24 V and compare. When the parallel count is high, the tool reminds you to check the maker’s limit on parallel batteries.

Worked example

1. Inputs: 200 Ah at 12 V, LiFePO4 (80% usable), 90% efficient, 500 W load.
2. Battery energy = amp-hours × volts = 200 Ah × 12 V = 2,400 Wh
3. Usable energy = battery energy × depth of discharge = 2,400 Wh × 0.8 = 1,920 Wh
4. Delivered energy = usable energy × efficiency = 1,920 Wh × 0.9 = 1,728 Wh
5. Runtime = delivered energy ÷ load = 1,728 Wh ÷ 500 W = 3.4 h, rounded down (3 h 27 min)
6. Battery draw = load ÷ (efficiency × volts) = 500 W ÷ (0.9 × 12 V) = 46.3 A

Questions people ask

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