Battery Runtime Calculator: How Long Will Your Battery Last?

“How long will this battery last?” is one of the most common questions in off-grid, backup power, and portable electronics planning. The answer comes from one formula:

h ≈ usable Wh ÷ load W

Once you have energy in watt-hours and power draw in watts, you have a runtime estimate.

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Wh vs Ah — which do you have?

Battery capacity is labelled in one of two ways:

• Watt-hours (Wh) — energy directly. Common on USB power stations, laptop batteries, and EV packs. Just use it as-is.
• Amp-hours (Ah) at a nominal voltage — common on lead-acid, LiFePO4, and other DC packs (e.g. “12 V 100 Ah”). Convert to Wh first: Wh = Ah × V.

The calculator handles both: enter Wh directly, or enter Ah and voltage and it derives Wh for you.

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The formula

h ≈ Wh_usable ÷ P_load

Where:

• Wh_usable = rated capacity × efficiency ÷ 100
• P_load = average load in watts

If you include an efficiency factor (e.g. 85% for a small inverter), the usable energy is reduced accordingly. Leave efficiency blank to assume 100% — useful for direct DC loads with no conversion stage.

Example: 12 V × 100 Ah = 1,200 Wh. Load 200 W, efficiency 90% → 1,200 × 0.9 ÷ 200 = 5.4 hours.

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What efficiency covers

The efficiency field is a single fudge factor for losses between rated capacity and actual delivered energy. Common sources:

• Inverter losses — a 12 V → 230 V inverter typically runs at 85–93% efficiency under normal load
• BMS cutoff — most lithium packs won’t discharge to true zero; a BMS may cut off at 10–20% remaining
• Depth of discharge limits — lead-acid batteries are typically only cycled to 50% to preserve lifespan
• Temperature — cold temperatures reduce effective capacity, especially in lead-acid chemistries
• State of health — aged cells hold less energy than a new nameplate rating suggests

For a conservative real-world estimate, use 80–85% for an AC load through an inverter, or 90–95% for a direct DC load.

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Quick reference

• 100 Wh, 10 W load, 100% efficiency → 10 h — Small power bank
• 1,200 Wh (12 V × 100 Ah), 150 W, 100% → 8 h — Deep-cycle lead at nameplate
• 2,400 Wh (48 V × 50 Ah), 800 W, 85% → ~2.55 h — Home battery through inverter
• 18.5 Wh (3.7 V × 5 Ah), 2 W, 100% → ~9.25 h — Single 18650-class cell

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Real-world examples

12 V fridge on a battery bank

120 Ah at 12 V → 1,440 Wh. The fridge averages 120 W and you use a small inverter at 90% efficiency:

1,440 × 0.9 ÷ 120 = 10.8 hours of average runtime.

Note: compressor fridges have variable draw — this is an average, not a worst-case.

Laptop from a power station

A 500 Wh power station, laptop averaging 40 W:

500 ÷ 40 = 12.5 hours (minimal losses for direct DC output).

Solar storage overnight buffer

10 kWh usable storage (after your own derate), house draws ~500 W average overnight:

10,000 ÷ 500 = 20 hours — a planning sanity check, not accounting for recharge from PV the next day.

UPS short-duration backup

UPS batteries are typically rated in VA with vendor runtime curves at specific loads. For short, high-rate discharges (under 15 minutes), prefer vendor runtime tables over a simple Wh ÷ W estimate — the Peukert effect makes lead-acid deliver significantly less at high currents.

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Why real runtime is often shorter

The Wh ÷ W formula assumes a constant load and that all rated energy is recoverable. In practice:

• Peukert effect — lead-acid batteries lose effective capacity at high discharge rates. A 100 Ah battery discharged in 1 hour delivers less than 100 Ah.
• Voltage sag — load voltage drops under heavy current, which may trigger cutoffs before all energy is used.
• Temperature — cold reduces capacity, especially below 0 °C.
• Age and cycle count — a battery at 80% state of health holds only 80% of original capacity.

Use the formula for planning estimates and direction-of-magnitude checks. For critical systems, test discharge your actual hardware.

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FAQ

Wh or Ah — which should I enter?

If the label lists Wh (most USB power stations do), use Wh. For packs labelled in Ah at a voltage (most car, marine, and solar batteries), use Ah and voltage.

I only have milliamp-hours (mAh).

Divide by 1,000 to get Ah: a 10,000 mAh cell is 10 Ah. Then Wh = Ah × V — for example 10 Ah × 3.7 V = 37 Wh.

Does this work for AC loads?

Yes. Either enter the DC-side watts the battery must supply (after the inverter draws its cut), or enter the AC load and fold inverter efficiency into the efficiency field.

What if I enter both Wh and Ah × V?

If the Wh field has a valid number, it takes priority. Ah and voltage are ignored until you clear Wh — avoids double-counting when experimenting with both inputs.

How does this relate to Watts to kWh?

The Watts to kWh Calculator goes from power and time to energy. This tool goes the other direction — from stored energy to runtime at a given load.

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Try the Tooladex Battery Runtime Calculator — enter your capacity in Wh (or Ah and voltage), your load, and an optional efficiency to get an instant runtime estimate.