Battery runtime calculator: how long will it last?
Enter the battery’s amp-hours and voltage, its type, and what you are running. The calculator uses only the usable part of the battery and adds the inverter loss for AC loads.
Enter your measurements
More options: DoD · Inverter eff. · Duty …
- In hours and minutes
- 9 h 13 min
- Usable energy
- 1,024 Wh
- Average draw from the battery
- 111.1 W
- Battery current while running
- 8.7 A
100 Ah × 12.8 V × 80% usable = 1,024 Wh
100 W ÷ 90% inverter = 111.1 W average
1,024 Wh ÷ 111.1 W = 9.2 hours (9 h 13 min)
Quick answers for common sizes
| Example | Answer |
|---|---|
| How long a 100Ah battery runs a fridge | 41 hours — a 12 V compressor fridge (50 W, on half the time) on 100Ah LiFePO4 |
| How long a 100Ah battery lasts with a 1000W inverter | 55 min at full load (12.8 V LiFePO4) |
| How long a 200Ah battery lasts with a 2000W inverter | 55 min at full load (12.8 V LiFePO4) |
| How long a 100Ah battery runs a CPAP | 15.4 hours at 60 W (LiFePO4, via inverter) |
| How long a 100Ah battery lasts with a 200W load | 4.6 hours (LiFePO4, via inverter) |
Runtime at other loads
| Load | Runtime | Battery current while running |
|---|---|---|
| 25 | 36.9 hours | 2.2 A |
| 50 | 18.4 hours | 4.3 A |
| 100 | 9.2 hours | 8.7 A |
| 200 | 4.6 hours | 17.4 A |
| 500 | 1.8 hours | 43.4 A |
| 1000 | 0.9 hours | 86.8 A |
| 2000 | 0.5 hours | 173.6 A |
Runtime is usable watt-hours divided by the watts the load takes from the battery. Worked examples: how long a 100Ah battery runs a fridge, how long a 100Ah battery lasts with a 1000W inverter, how long a 200Ah battery lasts with a 2000W inverter, how long a 100Ah battery runs a CPAP and how long a 100Ah battery lasts with a 200W load.
The formula
usable Wh = Ah × volts × depth of discharge watts from battery = load watts ÷ inverter efficiency (AC loads) hours = usable Wh ÷ (watts from battery × share of time on)
Depth of discharge is the share of the label capacity you plan to use. Victron rates its LiFePO4 batteries for 2,500 cycles at 80% depth of discharge, so 80% is the default for lithium. Trojan recommends discharging lead-acid 20–50% for long life, so 50% is the lead-acid default. Run a lead-acid battery to 80% and you get more hours today and fewer years of battery.
Why real runtime is often shorter
- High current on lead-acid. Lead-acid capacity is quoted at the 20-hour rate. Trojan’s 27TMX gives 105 Ah over 20 hours, 97 Ah over 10 hours and 85 Ah over 5 hours. The calculator applies a Peukert correction for lead-acid, with an exponent worked out from those ratings (about 1.18 flooded, 1.19 AGM from Trojan’s 31-AES); it shows the on-paper figure alongside.
- Cold. Victron’s LiFePO4 manual says capacity drops about 20% at 0 °C and about half at −20 °C; Victron’s lead-acid table gives 80% at −10 °C. Set Coldest battery temperature under More options.
- Inverter idle draw. An inverter uses some power just being switched on. With a small load on a big inverter, that idle draw can be a large share of the total — enter it under More options from the inverter manual’s no-load figure.
- Age. Both chemistries lose capacity as they cycle; batteries are usually considered worn out at 80% of their original capacity.
Runtime for a 100 Ah battery
| Load | 12.8 V LiFePO4 (1,024 Wh usable) | 12 V lead-acid (600 Wh usable) |
|---|---|---|
| 25 W DC | 40 h 58 min | 24 h |
| 50 W DC | 20 h 29 min | 12 h |
| 100 W AC (90% inverter) | 9 h 13 min | 5 h 24 min |
| 300 W AC (90% inverter) | 3 h 4 min | 1 h 48 min |
| 1000 W AC (90% inverter) | 55 min | 32 min |
Duty cycle: fridges, freezers and heaters
Anything on a thermostat runs part of the time. The two figures on this site come from two sources and two kinds of fridge: Victron’s book puts a typical 12 V compressor fridge at about 50 W and gives 25–50% duty cycles (about 25–50 Ah a day from a 12 V battery); Virginia Cooperative Extension’s rule for a household fridge is about a third of the time. Use 50% for a 12 V fridge in warm weather as the cautious figure, a third for a household fridge — or measure a whole day with a meter and enter the average under Share of the time the load is on.
RV absorption fridges: run them on propane off-grid
Many RVs have an absorption fridge that runs on propane or on electric. On electric it uses a heating element rather than a compressor — Champion’s chart lists RV refrigerators at 150–600 running watts — and that heat is needed for hours on end, which flattens a battery bank quickly through an inverter. Off-grid, switch it to propane and the battery only runs its controls; save electric mode for shore power or a generator. This calculator’s fridge figures are for compressor fridges.
What you’ll need
- Battery monitor with shuntMid-range price
Counts amp-hours in and out, so you see true state of charge instead of guessing from voltage — which barely moves on a LiFePO4 battery until it is nearly empty.
See options on Amazon (opens in a new tab) - 12 V 100 Ah LiFePO4 batteryPremium price
About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.
See options on Amazon (opens in a new tab) - Inline DC watt meterBudget price
Sits between a 12 V device and its supply and shows watts, amps and watt-hours used — handy for measuring a 12 V fridge, Starlink Mini or fan.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How long will a 100Ah battery last?
A 12.8 V 100Ah LiFePO4 battery has about 1,024 Wh usable, so it runs a 100 W DC load about 10 hours, or a 100 W AC load through a 90% inverter about 9.2 hours. A 12 V 100Ah lead-acid battery has about 600 Wh usable — a bit over half as long. See how long a 100Ah battery lasts with a 200W load.
How long will a 12V battery run a fridge?
A 12 V compressor fridge averaging 25 W (50 W running half the time) runs about 41 hours on a 100Ah LiFePO4 battery and about 24 hours on a 100Ah lead-acid battery used to 50%. See how long a 100Ah battery runs a fridge.
How long will a 200Ah battery last with a 2000W inverter?
At a full 2,000 W load, about 55 minutes from a 12.8 V 200Ah LiFePO4 bank — and it would be drawing about 174 A. See how long a 200Ah battery lasts with a 2000W inverter.
Does a bigger inverter drain the battery faster?
Only the load you run sets the drain — but a bigger inverter has a higher idle draw, which matters when it is left on with little or nothing plugged in.
Can I use 100% of a LiFePO4 battery?
The battery’s management system will let you go close to empty, but cycle life falls the deeper you go. Victron rates 2,500 cycles at 80% depth of discharge and 5,000 at 50%, which is why sizing here uses 80% by default.
Related guides
Sources
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
- Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
- Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).