Watt-hours made visible
Battery Runtime Calculator
Estimate how long a battery can power a load after inverter loss and depth-of-discharge limits.
The obvious way to estimate runtime is to divide the battery's watt-hours by the load's watts. That calculation is always optimistic, sometimes by close to half, because it silently assumes you can extract every stored watt-hour and deliver it to the load with no losses on the way.
Two deductions apply before the load sees anything. First, depth of discharge: a lead-acid battery conventionally gives up about half its nameplate capacity before cycle life suffers badly, while LiFePO4 gives up most of it. Second, conversion loss: if the load runs on AC, the inverter takes a cut in the middle.
This calculator applies both and shows the intermediate usable-energy figure, so you can see where the capacity went rather than just receiving a smaller number at the end.
The method
Nothing here is hidden. These are the exact expressions the calculator evaluates, in the order it evaluates them.
usable_Wh = nameplate_Wh × depth_of_discharge × inverter_efficiencyruntime_h = usable_Wh / load_Wplanning_h = runtime_h × 0.85The 0.85 is a deliberate margin, not a physical constant. It covers battery ageing, cold-temperature capacity loss, and the fact that real loads exceed their nameplate more often than they fall below it. If your battery is new and warm and the load is precisely known, ignore this row and use the raw runtime.
What each input means
| Input | What it actually is | Typical range |
|---|---|---|
| Battery capacity in watt-hours | Nameplate energy, not amp-hours. Convert with Wh = Ah × nominal volts. A 12 V 100 Ah battery is 1,200 Wh; a 12.8 V 100 Ah LiFePO4 is 1,280 Wh. | 600–5,000 Wh |
| Continuous load in watts | The actual sustained draw, not the device's maximum rating. A laptop charger rated 65 W may idle at 15 W. | 20–500 W |
| Inverter efficiency | Conversion efficiency at your actual load level, which is usually below the peak figure on the datasheet. Set this to 100 for a DC load with no inverter in the path. | 85–93% for AC, 100% for DC |
| Usable depth of discharge | How much of the nameplate you are willing to use each cycle. This is a cycle-life decision, not a hard limit. | 50% lead-acid, 80–100% LiFePO4 |
Worked example
A 12.8 V 100 Ah LiFePO4 battery running a 120 W AC load through an inverter that is 90% efficient at this power level, discharged to 80%.
| Step | Calculation |
|---|---|
| Nameplate energy | 12.8 V × 100 Ah = 1,280 Wh |
| After depth of discharge | 1,280 Wh × 0.80 = 1,024 Wh |
| After inverter loss | 1,024 Wh × 0.90 = 921.6 Wh |
| Runtime | 921.6 Wh / 120 W = 7.68 h |
| Planning target | 7.68 h × 0.85 = 6.5 h |
Result: About 7.7 hours theoretical, 6.5 hours as a planning figure.
The naive calculation — 1,280 Wh divided by 120 W — gives 10.7 hours. The honest answer is roughly 40% lower, and every part of that gap is visible in the steps above.
Assumptions baked into the result
Every calculator makes assumptions. Most do not tell you what they are. These are ours, and if one of them does not describe your system, the answer will be wrong in a direction you can now predict.
- The load is constant. Intermittent and cycling loads behave differently and usually do better than this estimate.
- The battery is near room temperature. Capacity falls substantially in the cold.
- The battery is healthy and reasonably new. Capacity fade with age is not modelled.
- Inverter idle draw is not counted separately. On a large inverter serving a small load this is a significant omission — fold it into the load figure.
- Peukert's effect is not applied. For lead-acid at high discharge rates the real capacity is lower than this estimate; for LiFePO4 the effect is small enough to ignore at typical rates.
What this tool will not tell you
- This is a planning estimate for sizing decisions, not a performance guarantee.
- It does not model battery internal resistance, which causes voltage sag under heavy load and can trip an inverter's low-voltage cutoff well before the calculated energy is used up.
- It does not account for voltage drop in the cable between battery and inverter, which has the same effect.
- For lead-acid at discharge rates above roughly C/5, apply Peukert's effect separately or expect the real runtime to fall short.
Common questions
Why is my real runtime shorter than this even with the margin applied?
Three usual causes, in order of frequency: inverter idle draw not counted in the load figure, voltage drop in undersized battery cable causing early low-voltage cutoff, and a battery that has lost capacity with age. Measure the actual DC current at the battery with a clamp meter and compare it against what you assumed.
Should I set depth of discharge to 100% for LiFePO4?
You can, and several manufacturers rate their batteries that way. But cycle life improves substantially at shallower discharge — Victron's published figures show 2,500 cycles at 80% against 5,000 at 50%. If the battery cycles daily, the shallower setting may be cheaper over the system's life.
What efficiency should I use if I do not know it?
For a decent pure sine inverter running near its sweet spot, 90% is a reasonable planning figure. At very low load fractions efficiency falls sharply, so if you are running 40 W through a 2,000 W inverter, model it much lower and count the idle draw separately.
