Battery Runtime Calculator: How long will your RV battery last?

A 100 Ah LiFePO4 battery powers a 50 W load for about 20 hours – an AGM battery of the same size only about 12 hours, because it should only be discharged halfway. This calculator gives you the realistic runtime of your house battery for a fridge, diesel heater, laptop or coffee maker.

Enter capacity, battery type and the power of the load. If the appliance runs through an inverter, its efficiency is taken into account; for lead-acid batteries the Peukert effect at high currents as well.

Estimated runtime

20 h 29 min

at 80% usable capacity (LiFePO4 (lithium))

Usable energy: 1,024 Wh
Power from the battery: 50 W
Current from the battery: 3.91 A

Show calculation

1Discharge current: 50 W ÷ 12.8 V = 3.91 A
2Usable energy: 12.8 V × 100 Ah × 80% = 1,024 Wh
3Runtime: 1,024 Wh ÷ 50 W = 20.5 h (20 h 29 min)

How it's calculated

Runtime is the usable energy of the battery divided by the power the load actually draws from the battery.

Usable energy [Wh] = nominal voltage [V] × capacity [Ah] × usable fraction

Example: 12.8 V × 100 Ah × 80% = 1,024 Wh

Runtime [h] = usable energy [Wh] ÷ power drawn from the battery [W]

Example: 1,024 Wh ÷ 50 W = 20.5 h

If the load runs through an inverter, the inverter draws more power from the battery than the appliance uses: power from the battery = appliance power ÷ efficiency. At 88% efficiency, 100 W at the appliance becomes about 114 W at the battery.

Thermostat-controlled appliances (refrigerator, furnace) don't run continuously. The duty cycle converts the rated power into an average: 60 W × 40% = 24 W on average.

Peukert effect in lead-acid batteries

The capacity of AGM, gel and flooded batteries is rated for a 20-hour discharge (C20). If the battery is discharged faster, less capacity can be drawn. The calculator uses Peukert's equation for this, referenced to the C20 current:

C_eff = C20 × (I20 ÷ I)^(k − 1) with I20 = C20 ÷ 20

k ≈ 1.10–1.15 (AGM), 1.15–1.25 (gel/flooded); for LiFePO4 the effect is negligible.

Worked example

A compressor refrigerator rated at 45 W runs about 50% of the time in summer on a 100 Ah LiFePO4 battery of which 80% is to be used.

Inputs

Battery capacity: 100 Ah
Battery type: LiFePO4 (lithium)
Load power: 45 W
Duty cycle: 50 %
Load runs through an inverter (120 V AC): no
System voltage: 12 V
Usable fraction of capacity: 80 %

Result

1 day 21 h

Estimated runtime

Usable energy: 1,024 Wh
Average power from the battery: 22.5 W
Current from the battery: 3.52 A
Equivalent to: 1.9 days

Calculation

Average power: 45 W × 50% duty cycle = 22.5 W
Discharge current: 45 W ÷ 12.8 V = 3.52 A
Usable energy: 12.8 V × 100 Ah × 80% = 1,024 Wh
Runtime: 1,024 Wh ÷ 22.5 W = 45.5 h (1 day 21 h)

The variables explained

Battery capacity (Ah): Rated capacity per the manufacturer. For lead-acid batteries the C20 rating (20-hour discharge) applies.
Battery type: Determines the nominal voltage (LiFePO4 12.8 V, lead-acid 12 V), the recommended usable fraction and the Peukert exponent.
Usable fraction (%): How deeply the battery may be discharged without greatly shortening its life. LiFePO4: 80–90%, lead-acid: about 50%.
Load power (W): Electrical power from the nameplate or a meter. For several appliances enter the sum.
Duty cycle (%): Share of time the appliance actually draws power. Continuous loads: 100%.
Inverter efficiency (%): Ratio of output to input power. Typically 85–92% at rated load.

Common mistakes

Confusing Ah with Wh: 100 Ah at 12.8 V is 1,280 Wh – at 24 V it would be 2,560 Wh. Only watt-hours tell you how much energy is stored.
Planning with the full rated capacity: lead-acid batteries should only be discharged about halfway, otherwise they age quickly.
Forgetting the inverter: losses of 10–15% plus idle consumption (often 5–15 W) noticeably shorten the runtime.
Using rated power instead of the average: a refrigerator or furnace cycles – without a duty cycle the runtime is significantly underestimated.
Ignoring cold: at 32 °F (0 °C) lead-acid batteries deliver noticeably less capacity; LiFePO4 usually must not be charged below freezing at all.

Assumptions and limits

The battery is fully charged at the start and in good condition (no aging considered).
Nominal voltage as the calculation value: 12.8 V (LiFePO4) or 12.0 V (lead-acid); real voltages vary with state of charge and load.
The inverter's idle consumption is not included; it adds up during longer pauses of the load.
Temperature effects, self-discharge, BMS cut-offs and cable losses are not included.
The result is an estimate, not a guarantee. For critical applications (e.g. medical devices) plan a safety margin.

Frequently asked questions

How long does a 100 Ah battery last at 100 watts?

A 100 Ah LiFePO4 battery (12.8 V, 80% usable) provides about 1,024 Wh and powers a 100 W load for roughly 10 hours. A 100 Ah AGM battery (12 V, 50% usable) provides about 600 Wh, i.e. roughly 6 hours – slightly less at high current because of the Peukert effect.

Why does my battery last less than calculated?

Common causes: an aged battery with less remaining capacity, low temperatures, the inverter's idle draw, an underestimated duty cycle or additional loads (water pump, lights, furnace fan) that weren't included.

Can I calculate several loads together?

Yes: add up the average power of all appliances and enter the sum as the load. For a whole day with many appliances, the power consumption calculator is better suited – it determines Wh per day.

Sources and background

Peukert's equation (W. Peukert, 1897), standard formula in battery engineering
Typical depths of discharge and efficiencies per manufacturer data sheets (LiFePO4, AGM, gel batteries; inverters)