What does depth of discharge mean?

It's the fraction of the battery's rated capacity you actually use before recharging. An 80% setting means you draw 80% and leave 20% in reserve. Lithium (LiFePO4) tolerates 80–100%; flooded lead-acid lasts far longer if you keep it above 50%.

How long will a 100 Ah 12V battery run a 100 W load?

About 9.6 hours at 80% depth of discharge (100 × 12 × 0.8 ÷ 100). With an inverter at ~90% efficiency, closer to 8.5 hours. At 50% depth of discharge, only about 6 hours.

Does this include inverter losses?

No — it's an ideal figure. A real AC load runs through an inverter that's 85–92% efficient, and its own idle draw eats into the total. Multiply the result by your inverter efficiency for a working estimate.

Why does battery voltage matter?

Amp-hours alone aren't energy. Multiplying by voltage gives watt-hours, which is what you compare against the load's watts. A 100 Ah battery at 24 V runs a given load twice as long as one at 12 V.

What if my load isn't constant?

Calculate each phase separately and add the runtimes, or use the average power. A fridge that pulls 120 W for 20 minutes each hour averages about 40 W; use that.

Does a higher discharge rate reduce runtime?

For lead-acid, yes, noticeably — Peukert's law means a hard, fast discharge yields fewer usable amp-hours. Lithium is much flatter, delivering close to rated capacity across a wide range of discharge rates.

Battery Runtime Calculator

A battery runtime estimate helps you check whether a battery can support a device, backup system, or portable setup for the required duration. The calculation converts the battery’s amp-hour rating into usable watt-hours, then compares that energy with the load’s power demand. It is an estimate, not a guarantee of real-world performance.

How to Calculate Battery Runtime (step by step)

Step One: Calculate the battery’s rated energy

Multiply battery capacity by battery voltage to estimate the battery’s total energy.

Battery energy = battery capacity × battery voltage
Battery energy = 100 Ah × 12 V = 1,200 Wh

Step Two: Apply the usable depth of discharge

Multiply total battery energy by the depth of discharge expressed as a decimal. The default depth of discharge is 80%, which becomes 0.80.

Usable energy = battery energy × (depth of discharge ÷ 100)
Usable energy = 1,200 Wh × (80 ÷ 100) = 960 Wh

Step Three: Divide usable energy by load power

Divide usable watt-hours by the load’s wattage to find runtime in hours.

Runtime = usable energy ÷ load power
Runtime = 960 Wh ÷ 100 W = 9.6 h

Step Four: Convert hours to days

Divide the runtime in hours by 24 to express the result in days.

Runtime in days = runtime in hours ÷ 24
Runtime in days = 9.6 h ÷ 24 = 0.40 d

Battery Runtime Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

The runtime formula

is battery capacity in amp-hours, is nominal voltage, DoD is the usable depth of discharge as a decimal, and is the load power in watts. The numerator is usable energy in watt-hours; dividing by watts gives hours.

Worked example

The calculator’s defaults are a 100 Ah, 12 V battery, an 80% depth of discharge, and a 100 W load.

Rated energy:

Usable energy:

Runtime:

That’s the ideal figure. Send that 100 W through an inverter at 90% efficiency and you’d get closer to 8.6 hours of real runtime.

Runtime vs load, for a 100 Ah 12V battery at 80% DoD

Load	Ideal runtime	With 90% inverter
25 W	38.4 h	34.6 h
50 W	19.2 h	17.3 h
100 W	9.6 h	8.6 h
300 W	3.2 h	2.9 h
600 W	1.6 h	1.4 h
1,000 W	0.96 h	0.86 h

Runtime is inversely proportional to load — halve the watts and you double the hours.

What pulls real runtime below the estimate

Inverter conversion and idle draw. 85–92% efficient, plus a constant tax of a few watts just to stay on.
Depth-of-discharge limits. Draining a flooded lead-acid battery to 80% every cycle will wear it out in a year; 50% is the usual limit, which halves the usable energy.
Temperature. A cold battery delivers less; capacity can drop 20–30% near freezing.
Age. Every chemistry fades. A battery at 80% state of health has lost a fifth of its capacity.
Discharge rate. Lead-acid loses usable capacity under heavy load (Peukert’s law); lithium barely notices.
Surge loads. Motors and compressors draw several times their running watts at startup, which the average-power figure doesn’t capture.

Common mistakes

Using 100% depth of discharge for lead-acid. Use 50%.
Ignoring the inverter for AC loads.
Feeding in rated wattage for a cycling load instead of its average.
Forgetting voltage — amp-hours must become watt-hours first.

To convert a battery’s rating between units, see the amp-hours to watt-hours calculator and amp-hours to kWh.

About the formula: Battery energy is estimated by multiplying capacity by voltage, then usable energy is found by applying the depth of discharge. Runtime is usable energy divided by load power.