Battery Runtime Calculator

What it is for

• On the go: will the power bank last a day with phone, earbuds or a small USB fan?
• Camping and garden: how long will a cool box, lights or a laptop run from the leisure battery in a campervan or from a portable power station?
• Buying decisions: what capacity do you need for a given runtime – and is the bigger battery worth the extra money?

mAh or Wh?

Batteries carry two kinds of rating. mAh and Ah describe an amount of charge: how long a given current can flow. Wh describes energy. The two are linked by the voltage, because one watt is one volt times one ampere:

Wh = Ah × V Wh = mAh × V ÷ 1,000

For power banks, the mAh figure refers to the nominal voltage of the cells inside, 3.7 V for most of them. A 10,000 mAh power bank therefore stores about 10,000 × 3.7 ÷ 1,000 = 37 Wh. At the output it converts the voltage, and energy is lost in the process – it never delivers the full capacity printed on it.

That is why you always compare batteries with different voltages in Wh. 100 Ah on a 12.8 V battery is 1,280 Wh – a completely different order of magnitude from 100 Ah at 3.7 V, if such a thing existed.

How the tool calculates

1. Energy: E = capacity × nominal voltage (directly if given in Wh)
2. Usable: E × depth of discharge
3. What the battery has to supply: load ÷ efficiency + self-consumption
4. Runtime = usable energy ÷ that power

The other way round, it works back from the required runtime to the energy needed and – with the nominal voltage – to Ah and mAh.

The assumptions and where the presets come from

Three values in the tool are assumptions. They depend on the battery, the device and how gently you want to treat the battery. The presets set starting values based on manufacturers’ figures – change them if you know better for your device.

• Power bank: 3.7 V and 85 % efficiency – according to Anker, power banks usually deliver 85 to 90 % of their stated capacity; the preset uses the lower end. Depth of discharge 100 %, because those losses are already in the efficiency.
• LiFePO4 12.8 V with inverter: depth of discharge 80 % – Victron rates its LiFePO4 batteries at 2,500 cycles at 80 % depth of discharge (5,000 at 50 %). Efficiency 90 % and self-consumption 6 W: Victron gives a maximum efficiency of 87 to 92 % for its Phoenix inverters, depending on the model, and 6 W idle draw for the 12/500.
• Lead-acid AGM 12 V direct: depth of discharge 50 % – Victron rates its AGM batteries at 600 cycles at 50 % discharge but only 400 at 80 %. Efficiency 100 %, because a 12 V device is connected straight to the battery.

Worked examples

Power bank running a USB fan

Power bank 10,000 mAh, 3.7 V, a fan drawing 5 W (example value), preset “Power bank” – this is what the calculator opens with:

1. Energy: 10,000 mAh × 3.7 V ÷ 1,000 = 37 Wh
2. The power bank has to draw 5 W ÷ 0.85 ≈ 5.88 W from its cells.
3. Runtime: 37 Wh ÷ 5.88 W ≈ 6.3 hours (6 h 17 min)

LiFePO4 battery with inverter running a laptop

LiFePO4 12.8 V with 100 Ah (1,280 Wh), a laptop charger drawing 60 W (example value) through an inverter, preset “LiFePO4”:

1. Usable: 1,280 Wh × 80 % = 1,024 Wh
2. The battery supplies 60 W ÷ 0.9 + 6 W ≈ 72.67 W.
3. Runtime: 1,024 Wh ÷ 72.67 W ≈ 14.1 hours

The other way round: for 10 hours with the same assumptions you need 72.67 W × 10 h ÷ 0.8 ≈ 908 Wh, so about 71 Ah at 12.8 V.

Lead-acid or LiFePO4?

The two battery types cope with deep discharge very differently. Victron’s figures show the difference: an AGM battery manages 400 cycles at 80 % discharge, a LiFePO4 battery 2,500. With lead-acid it therefore makes sense to count on less usable capacity if the battery is to last.

There is more with lead-acid: the nominal capacity applies to a discharge over 20 hours. If the battery is drained in one hour, an AGM battery delivers only 65 % of it according to Victron – and at constant power, such as on an inverter, the usable capacity drops even faster. For large loads on lead-acid the tool therefore comes out too high.

Cold

Batteries deliver less in the cold. Victron rates its 100 Ah LiFePO4 battery at 100 Ah at 25 °C, 80 Ah at 0 °C and only 50 Ah at −20 °C. In the example above, 14.1 hours would become about 11.3 at 0 °C. According to the datasheet these batteries may only be charged between +5 °C and +50 °C. If you want to allow for cold, enter a correspondingly lower capacity.

What to look for when buying

• Power bank: look for the Wh rating or work it out from mAh and cell voltage – it is the only way to compare different models.
• LiFePO4 battery: nominal voltage (12.8 V), capacity in Ah and, in the datasheet, the number of cycles at which depth of discharge and the temperature range for charging.
• Inverter: besides continuous power, efficiency and idle draw matter – the idle draw runs the whole time, even when hardly anything is plugged in. Pick one made for the mains voltage of your country.

Limits of this tool

• It is an approximation. Real runtimes vary with temperature, battery age, load and state of charge.
• A converter’s efficiency is not fixed but depends on the load. The tool uses a single value.
• Loads that vary (a compressor cool box, a laptop under load) need their average power – measured, not from the rating plate.
• How deeply you can really discharge a battery is set by its electronics (BMS) and the manufacturer. Follow the datasheet.

Privacy: the calculator runs entirely in your browser. What you enter is not sent to xmoo.app or anyone else.

Sources

• Watt = volt × ampere, hour as unit of time: BIPM, SI Brochure (9th edition)
• Conversion Wh = mAh × V ÷ 1,000, nominal voltage 3.7 V for “most” power banks, example 10,000 mAh = 37 Wh, delivered capacity “usually only 85–90 %”: Anker, Wh in mAh umrechnen (German, retrieved 23 October 2026)
• LiFePO4 12.8 V: 100 Ah = 1,280 Wh; 100/80/50 Ah at 25/0/−20 °C; 2,500/3,000/5,000 cycles at 80/70/50 % depth of discharge; charging +5 to +50 °C: Victron Energy, datasheet 12.8 V lithium iron phosphate batteries Smart
• AGM 12 V: 400/600/1,500 cycles at 80/50/30 % discharge, nominal capacity at a 20-hour discharge, 65 % at one hour: Victron Energy, datasheet GEL and AGM batteries
• Inverter: max. efficiency 87–92 %, idle draw of the 12/500: 6 W: Victron Energy, datasheet Phoenix Inverter VE.Direct 250–1200 VA

Page last updated: 31 October 2026