How long will a 100Ah battery last?
By Sithum Imarsha Last reviewed
Runtime = usable Wh ÷ watts drawn from the battery. A 100 Ah LiFePO4 battery (1,024 Wh usable) runs an 80 W TV through a small inverter for 10.8 hours, not the 15 hours a simple Ah × volts ÷ watts sum gives.
Your result
Runtime
10.8 hours
1,024 Wh usable ÷ 94.9 W at the battery = 10.8 h
- Usable energy
- 1,024Wh
- At the battery
- 94.9W
- Battery current
- 7.4A
- C-rate
- 0.07C
- Without idle draw
- 11.5h
- Common shortcut
- 15h
The shortcut: volts × Ah ÷ watts, all of the battery, no inverter. It is what many tools show.
Choosing an inverter? Check its size and the current it pulls.
Fix the highlighted field to update.
How we calculated this
Usable energy
Wh = nominal volts × Ah × the share you can use. LiFePO4 is 12.8 V a 12 V block (4 cells of 3.2 V), lead-acid 12 V. Lead-acid also loses capacity at high current (step 4).
12.8 V × 100 Ah × 80% = 1,024 Wh
Power at the battery
A 230 V load goes through an inverter: load ÷ efficiency, plus the inverter's own idle draw. The idle draw runs as long as the inverter is on, even with nothing plugged in. A DC load is rated at the system voltage, the way fridge makers give Ah an hour at 12 V. It draws watts ÷ system volts in amps from any battery, so its runtime is usable Ah ÷ those amps.
80 W ÷ 90% + 6 W idle = 94.9 W
Runtime
Hours = usable Wh ÷ watts at the battery.
1,024 Wh ÷ 94.9 W = 10.8 h
Current and rate effect
Current = watts at the battery ÷ volts. For lead-acid we cut the capacity with Peukert's rule: rated Ah × (Ah ÷ (current × 20 h)) ^ (exponent − 1), never above the rated Ah. LiFePO4 holds its capacity up to 1C, so it gets no cut.
7.4 A, 0.07C, 100% of rated capacity
| Battery | Usable Wh | Hours |
|---|---|---|
| LiFePO4 (lithium) | 1,024 | 10.8 |
| Lead-acid (AGM, gel or flooded) | 535 | 5.6 |
| Shortcut: volts × Ah, all of it, no inverter | 1,200 | 15 |
Each type at its default usable share and rate effect, same capacity, load and inverter. The shortcut row is what you get from volts × Ah ÷ watts.
| Type | Nominal | Usable share | Peukert | Current limit |
|---|---|---|---|---|
| LiFePO4 | 12.8 V (Official source) | 80% (Official source) | 1 (Estimate) | 1C (Official source) |
| Lead-acid | 12 V (Official source) | 50% (Official source) | 1.25 (Official source) | No source |
Victron battery monitors default to a Peukert exponent of 1.25 for lead-acid, and Victron calls it "no more than a rough approximation": at very high current the battery gives even less. Constant-power loads like an inverter lose capacity faster still.
| Discharge time | AGM | GEL |
|---|---|---|
| 20 h | 100% | 100% |
| 10 h | 92% | 87% |
| 5 h | 85% | 80% |
| 3 h | 78% | 73% |
| 1 h | 65% | 61% |
| 30 min | 55% | 51% |
Measured at a constant current (Official source: Victron GEL and AGM datasheet). Our Peukert figure of 1.25 is a little more cautious than this table for AGM.
| Inverter | Continuous 25 / 40 °C | Peak | Battery | Peak efficiency | Idle draw |
|---|---|---|---|---|---|
| Victron Phoenix 250 VA (200 W) | 200 / 175 W | 400 W | 12 / 24 / 48 V | 87% / 88% / 88% | 4.2 W / 5.2 W / 7.9 W |
| Victron Phoenix 375 VA (300 W) | 300 / 260 W | 700 W | 12 / 24 / 48 V | 89% / 89% / 90% | 5.6 W / 6.1 W / 8.5 W |
| Victron Phoenix 500 VA (400 W) | 400 / 350 W | 900 W | 12 / 24 / 48 V | 90% / 90% / 91% | 6 W / 6.5 W / 9 W |
| Victron Phoenix 800 VA (650 W) | 650 / 560 W | 1,500 W | 12 / 24 / 48 V | 90% / 90% / 91% | 6.5 W / 7 W / 9.5 W |
| Victron Phoenix 1200 VA (1,000 W) | 1,000 / 850 W | 2,200 W | 12 / 24 / 48 V | 91% / 91% / 92% | 10 W / 11 W / 12 W |
| Victron Phoenix 1600 VA (1,450 W, 12 V) | 1,450 / 1,200 W | 2,200 W | 12 V | 92% | 12 W |
| Victron MultiPlus-II 3000 VA (2,400 W) | 2,400 / 2,200 W | 5,500 W | 12 / 24 / 48 V | 93% / 94% / 95% | 13 W / 13 W / 11 W |
| Victron MultiPlus-II 5000 VA (4,000 W) | 4,000 / 3,700 W | 9,000 W | 12 / 24 / 48 V | 95% / 96% / 96% | 15 W / 18 W / 18 W |
| Victron MultiPlus-II 8000 VA (6,400 W, 48 V) | 6,400 / 5,500 W | 15,000 W | 48 V | 95% | 29 W |
| Victron MultiPlus-II 10000 VA (8,000 W, 48 V) | 8,000 / 7,000 W | 18,000 W | 48 V | 96% | 38 W |
| Victron MultiPlus-II 15000 VA (12,000 W, 48 V) | 12,000 / 10,000 W | 27,000 W | 48 V | 95% | 55 W |
Efficiency curve not published; we use the maker's peak efficiency plus its no-load draw, which is our estimate. Victron reaches its peak at part load, so a full load does a little worse. Model names are in VA: a 1200 VA Phoenix gives 1,000 W.
| Battery | Discharge limit | Status |
|---|---|---|
| Victron LFP Smart 12,8/100 | 200 A continuous; 1C (100 A) or less recommended | Official source |
| Renogy 12.8 V 100 Ah | 100 A continuous; 300 A for 5 seconds | Official source |
| Enerdrive B-TEC 100 Ah G2 | 100 A for 30 minutes at 25 °C | Official source |
With no limit typed, we warn above 1C (Victron's recommended continuous discharge). Always use your own battery's figure. We do not size cables or fuses: use the maker's installation manual, and a licensed electrician for anything on 230 V.
“Official source” means read in full on the maker’s own datasheet, manual or spec page, or in an official dataset (BOM). “Dated third-party source” means read on a dated third-party page, such as a product test. “Not yet checked” means one non-official source. “Estimate” means our own pick or reasoning: type your own figure.
Sources
- Victron: LiFePO4 Smart battery datasheet
- Victron: Lithium Smart battery manual
- Victron: GEL and AGM batteries datasheet
- Victron: BMV-712 Smart and SmartShunt manual
- Victron: Phoenix inverters VE.Direct datasheet
- Victron: MultiPlus-II datasheet
- Victron: Blue Smart IP22 charger manual
- Renogy: 12.8 V 100 Ah LiFePO4 battery
- Enerdrive: B-TEC 12 V 100 Ah G2 lithium battery
The one formula
Runtime in hours = usable watt hours ÷ watts drawn from the battery.
- Usable Wh = nominal volts × amp hours × the share you can use. A 12.8 V, 100 Ah LiFePO4 battery holds 1,280 Wh. At 80% usable that is 1,024 Wh.
- Watts at the battery: a DC appliance is rated at the system voltage (a 12 V fridge in Ah an hour at 12 V), so it draws watts ÷ system volts (12, 24 or 48 V) in amps from any battery. Its runtime is usable Ah ÷ those amps. For a 230 V appliance it is the load ÷ the inverter’s efficiency, plus the inverter’s own idle draw.
Three things most calculators leave out
- The inverter’s idle draw. An inverter uses power as soon as it is on. Victron’s Phoenix 500 VA uses 6 W on 12 V with nothing plugged in. The Phoenix 1200 VA uses 10 W, which is 240 Wh a day. On a small load that is a big share.
- The usable share. Victron battery monitors use 50% as the default floor for lead-acid. Two lithium makers rate their cycle life at 80% depth of discharge. Using 100% of either is not realistic.
- Lead-acid at high current. Victron AGM batteries give 85% of their rated capacity over 5 hours and 65% over 1 hour. We use Peukert’s rule with Victron’s default exponent of 1.25 to cut the capacity as the current rises.
Voltage: 12 V or 12.8 V
A LiFePO4 battery is four 3.2 V cells, so it is 12.8 V nominal, not 12 V. A lead-acid battery is six 2 V cells, 12 V. For 24 V and 48 V systems the figures double and quadruple. More volts means less current for the same watts, which matters for the next point.
The current check
Current = watts at the battery ÷ volts. Batteries have a continuous current limit. Renogy and Enerdrive limit their 100 Ah LiFePO4 batteries to 100 A. Victron allows 200 A on its 100 Ah model but recommends 1C (100 A) or less because of heat. A 2,000 W kettle through an inverter pulls about 174 A from a 12.8 V battery at 90% efficiency. The calculator warns you when you pass the limit.
What this calculator does not do
- It does not size cables or fuses. Use the battery and inverter maker’s installation manual, and a licensed electrician for anything on 230 V.
- It uses one efficiency figure. Makers publish their peak efficiency and their idle draw, not a curve, so a full load does a little worse than shown.
- It assumes a battery at room temperature. Victron says its LiFePO4 batteries hold 80% of their capacity at 0 °C.
Questions
How do I work out how long a battery will last?
Usable Wh ÷ watts at the battery. Usable Wh is volts × Ah × the share you can use (about 80% for LiFePO4, 50% for lead-acid). Watts at the battery is the load ÷ inverter efficiency, plus the inverter’s idle draw.
Why is my answer shorter than other calculators?
Most skip the inverter’s losses and its own idle draw (4 to 15 W for Victron’s 12 V models, all day), and they use all of the battery. Lead-acid also gives less than its rated Ah at high current. The common shortcut in the result shows the gap.
Does an inverter use power with nothing plugged in?
Yes. Victron’s Phoenix 1200 VA draws 10 W on 12 V with no load. That is 240 Wh a day, about 19 Ah from a 12.8 V battery. Switch it off, or use its ECO mode, when you do not need 230 V.
Is a LiFePO4 battery 12 V or 12.8 V?
12.8 V nominal: four 3.2 V cells (Victron). A 100 Ah LiFePO4 battery holds 1,280 Wh, not 1,200 Wh.
What is the Peukert effect?
A lead-acid battery gives fewer Ah when you draw a high current. Victron AGM batteries give 85% of their rated capacity over 5 hours and 65% over 1 hour. LiFePO4 barely changes up to 1C.
How much current can my battery supply?
Read your battery’s spec. Renogy and Enerdrive limit their 100 Ah LiFePO4 batteries to 100 A (Enerdrive for 30 minutes). Victron recommends 1C or less. Type your limit under Advanced settings.