Battery Runtime Calculator (Ah → Hours)
Enter your battery's amp-hour rating, voltage, and the watt load you're running — this calculator gives you the real runtime, not the optimistic figure on the label. It applies the standard formula used by solar installers, UPS engineers, and RV builders: Runtime (h) = (Ah × V × DoD × η) / W. Depth of discharge (DoD) and inverter efficiency are the two factors most often ignored, and ignoring them leads to undersized systems and unexpected outages.
Battery runtime (hours) = (Ah × V × DoD × η) ÷ W. Example: a 100 Ah / 12 V battery with 80% DoD, 95% efficiency, powering a 100 W load lasts **9.1 hours**. Lead-acid batteries: use DoD ≤ 50%. LiFePO4 batteries: use DoD ≤ 80–90%.
When to use this calculator
- Sizing a solar battery bank to survive the night without recharging
- Calculating UPS runtime for servers during a power outage
- Planning RV or boat electrical systems with realistic autonomy
- Estimating e-bike range from battery pack specs
- Verifying that a lithium vs lead-acid swap improves backup time
Worked Example: 100 Ah / 12 V battery, 100 W load
- Capacity: 100 Ah × 12 V = 1,200 Wh nominal
- Apply DoD 80%: 1,200 × 0.80 = 960 Wh
- Apply efficiency 95%: 960 × 0.95 = 912 Wh usable
- Runtime: 912 Wh ÷ 100 W = 9.12 hours
How it works
2 min readHow Battery Runtime Is Calculated
Battery runtime depends on four factors: amp-hour capacity (Ah), voltage (V), depth of discharge (DoD), and system efficiency (η). The formula:
Runtime (h) = (Ah × V × DoD × η) ÷ W
Multiplying Ah × V converts amp-hours to watt-hours (Wh) — the true measure of energy stored. Dividing by watts gives hours.
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DoD by Battery Technology
| Technology | Recommended Max DoD | Typical Cycles (at that DoD) | Common Use |
|---|---|---|---|
| Sealed lead-acid (AGM/VRLA) | 50% | 300–500 | UPS, alarms |
| Flooded lead-acid | 50–60% | 500–800 | Basic solar off-grid |
| Lithium iron phosphate (LiFePO4) | 80–90% | 2,000–6,000 | Solar, EVs, storage |
| Lithium-ion (NMC/NCA) | 80% | 500–1,500 | Portable, e-bikes |
| Gel (VRLA Gel) | 60–70% | 400–700 | Telecom, critical UPS |
| NiMH | 70–80% | 500–1,000 | Portable electronics |
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Common Battery Runtime Examples
| Battery | Load | DoD | η | Runtime |
|---|---|---|---|---|
| 100 Ah / 12 V (LiFePO4) | 50 W | 80% | 95% | 18.2 h |
| 100 Ah / 12 V (LiFePO4) | 100 W | 80% | 95% | 9.1 h |
| 100 Ah / 12 V (LiFePO4) | 200 W | 80% | 95% | 4.6 h |
| 100 Ah / 12 V (AGM) | 100 W | 50% | 93% | 5.6 h |
| 200 Ah / 24 V (LiFePO4) | 300 W | 85% | 93% | 11.9 h |
| 150 Ah / 48 V (LiFePO4) | 1,500 W | 85% | 95% | 3.6 h |
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Inverter Efficiency Reference
| Inverter Type | Typical Efficiency (η) |
|---|---|
| Pure sine inverter (quality) | 90–96% |
| Modified wave inverter | 85–92% |
| DC-DC converter (buck/boost) | 88–97% |
| Direct DC load (no inverter) | 99–100% |
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Common Mistakes
1. Using 100% DoD on lead-acid: destroys it in a few dozen cycles. Always limit to 50%.
2. Ignoring inverter efficiency: overestimates runtime by 5–15%.
3. Confusing Ah with Wh: 100 Ah at 12 V = 1,200 Wh. The same 100 Ah at 48 V = 4,800 Wh — four times the energy.
4. Peukert effect on fast discharge: lead-acid batteries deliver less capacity when discharged quickly (under 5 hours). The nominal rating assumes a 20-hour discharge rate.
Frequently asked questions
What is the formula for battery runtime?
Runtime (hours) = (Ah × V × DoD × η) ÷ W. Multiply amp-hours by voltage to get watt-hours (Wh), apply the depth of discharge percentage and inverter efficiency, then divide by your load in watts. Example: 100 Ah × 12 V × 0.80 × 0.95 ÷ 100 W = 9.12 hours.
How long will a 100Ah 12V battery last?
It depends on your load. With an 80% DoD LiFePO4 and 95% efficiency: at 50 W → 18.2 h, at 100 W → 9.1 h, at 200 W → 4.6 h. With a 50% DoD AGM lead-acid at 93% efficiency and 100 W load → 5.6 h. Enter your exact values in the calculator above for a precise answer.
What DoD should I use for lead-acid batteries?
50% for AGM and VRLA sealed batteries, 50–60% for flooded lead-acid. Discharging a lead-acid battery beyond 50% causes sulfation on the plates and drastically reduces cycle life — from 500 cycles to under 100 at 80% DoD.
What DoD can I use for LiFePO4 batteries?
80–90% is safe for LiFePO4 (lithium iron phosphate). This is why LiFePO4 stores much more usable energy than lead-acid for the same Ah rating: a 100 Ah LiFePO4 at 85% DoD gives 85 Ah usable vs. only 50 Ah for an equivalent AGM. Combined with 2,000–6,000 cycle life, the cost per kWh cycled is 2–3× lower.
Why can't I use 100% depth of discharge?
Fully discharging a battery permanently damages it. Lead-acid batteries sulfate if fully discharged, losing capacity in a few cycles. Even LiFePO4 degrades faster below 10% state of charge. Manufacturers rate cycle life at recommended DoD — exceeding it voids warranties and shortens the battery life dramatically.
How do I calculate runtime for multiple batteries in series or parallel?
Batteries in series add voltages, keeping Ah the same: 2 × 12 V / 100 Ah = 24 V / 100 Ah (2,400 Wh). In parallel they add Ah, keeping voltage: 2 × 12 V / 100 Ah = 12 V / 200 Ah (2,400 Wh). Either way, total energy (Wh) is the same — use the resulting bank voltage and Ah in the calculator.
What is the Peukert effect and does it matter?
The Peukert effect describes how lead-acid batteries deliver less capacity when discharged quickly. The nominal Ah rating assumes a 20-hour discharge (C/20 rate). At 5-hour discharge (C/5), actual capacity can drop 20–30%. For runtimes above 10 hours, the effect is minimal. LiFePO4 and lithium-ion are largely unaffected.
How does temperature affect battery runtime?
Lead-acid loses ~1% capacity per °C below 25°C, reaching -50% at 0°C. LiFePO4 is more stable but loses ~10–20% at -10°C. In hot climates (above 40°C), heat accelerates internal degradation and can reduce cycle life by 30–50% without adequate ventilation.
How do I measure real power consumption (W) for the calculator?
Use a plug-in watt meter (kill-a-watt type, available for $10–30). The label on appliances shows maximum power, but average real consumption is often 30–60% lower — a refrigerator rated at 150 W typically averages 60–70 W because the compressor cycles. Use measured average wattage for realistic runtime estimates.
What efficiency should I use for a solar inverter?
Quality pure sine inverters (Victron, SMA, Growatt, Deye): 93–97%. Standard pure sine inverters: 90–93%. Modified wave inverters: 85–88%. If your load runs directly from DC (e.g., 12 V LED lighting without an inverter), use 99% — there's no conversion loss.