battery duration calculator

How this battery life calculator works

A battery duration calculator estimates how long a battery can run a device before reaching a target discharge level. The core idea is simple: convert available battery energy into watt-hours, adjust for real-world losses, and divide by total power draw.

Where usable battery energy depends on your battery capacity, depth of discharge limit, conversion efficiency, and battery health. This gives you a practical runtime estimate rather than an ideal lab value.

Understanding each input

1) Battery capacity (Wh or Ah + V)

You can enter battery capacity in watt-hours (Wh) directly, or in amp-hours (Ah) and voltage (V). If you use Ah and V, the calculator converts it automatically:

Example: a 12V 100Ah battery has roughly 1200Wh of nominal energy.

2) Device load (W)

This is the average power your device consumes while running. If your appliance cycles on and off (like a fridge), use the average measured load over time rather than the peak surge power.

3) Parasitic load

Real systems often have background draws: inverter idle consumption, monitoring electronics, routers, and standby electronics. Even a small 5-10W continuous load can shorten runtime significantly over many hours.

4) Usable depth of discharge (DoD)

Not all battery energy is usually used. Lead-acid users often stop around 50% DoD to preserve cycle life. Lithium systems commonly allow deeper discharge (80-95%), depending on BMS and manufacturer recommendations.

5) System efficiency

Inverters and conversion hardware are never 100% efficient. If your inverter is around 90% efficient at your load level, setting efficiency to 90 gives a more realistic runtime estimate.

6) Battery health factor

As batteries age, usable capacity declines. If your battery now delivers only about 85% of its original capacity, entering 85% gives a more accurate battery runtime estimate.

Worked example

Suppose you have a 12V 100Ah battery, and you want to run a 60W device with 5W inverter standby draw. You set DoD to 80%, efficiency to 90%, and health to 95%.

• Nominal energy: 100 × 12 = 1200Wh
• Usable energy: 1200 × 0.80 × 0.90 × 0.95 = 820.8Wh
• Total load: 60 + 5 = 65W
• Runtime: 820.8 / 65 ≈ 12.63 hours

So, you can expect roughly 12 hours and 38 minutes under those assumptions.

Tips to increase battery duration

• Reduce continuous background loads (inverter idle draw, always-on accessories).
• Use DC devices directly when possible to avoid inverter losses.
• Keep batteries in moderate temperatures; extreme cold and heat reduce usable capacity.
• Right-size cables and components to minimize resistive losses.
• Track real usage with a power meter for better planning.

Common mistakes when estimating runtime

• Using peak load instead of average load (or vice versa) incorrectly.
• Ignoring inverter efficiency and idle power.
• Assuming full nameplate capacity for old batteries.
• Forgetting that high discharge rates can reduce effective capacity in some chemistries.
• Ignoring temperature effects in outdoor and off-grid systems.

Use cases

This calculator is useful for:

• Solar battery bank planning
• RV and camper battery runtime estimation
• UPS backup duration checks
• Portable power station usage forecasts
• Emergency preparedness and outage planning

Final note

Treat results as planning estimates. Real-world battery performance varies with temperature, discharge rate, chemistry (LiFePO4, NMC, AGM, GEL), and system design. For critical systems, add safety margin and test with actual loads.