Battery Outage Coverage Hours

Estimate how long your battery backup can shoulder critical circuits. Provide usable storage in kWh and the load you intend to carry to see outage hours, equivalent days, and how much energy remains after state-of-charge reserves and round-trip losses.

Battery capacity available for discharge after BMS limits.
Average power draw you must support during an outage.
Optional. Defaults to 10% to protect battery health.
Optional. Defaults to 92% to cover conversion losses.

Preliminary engineering aid. Validate runtimes with manufacturer data, protection settings, and detailed load studies before relying on the system for life safety.

Examples

  • 120 kWh storage, 6 kW load, 10% reserve, 92% efficiency ⇒ Backup runtime at 6.00 kW: 16.56 hours (0.69 days) • Effective energy after reserves/losses: 99.36 kWh • Max continuous load for 24-hour coverage: 4.14 kW • Available load for a 4-hour event: 24.84 kW.
  • 80 kWh backup, 4.5 kW load, default 10% reserve and 92% efficiency ⇒ Backup runtime at 4.50 kW: 16.37 hours (0.68 days) • Effective energy after reserves/losses: 66.24 kWh • Max continuous load for 24-hour coverage: 2.76 kW • Available load for a 4-hour event: 16.56 kW.

FAQ

Does this include generator charging?

No. It assumes the battery discharges without recharge. If you plan to backfeed with a generator or PV, rerun the math with the additional energy added to usable storage.

What if my critical load fluctuates?

Use the average kW you expect during the outage window. For peaky loads, run a worst-case scenario so you know the minimum runtime to expect.

How should I pick the reserve percentage?

Residential systems often hold 5–15% SOC in reserve to protect cycle life. Commercial microgrids may increase reserves if regulatory programs require emergency headroom.

Can I translate the hours into amp-hours?

Multiply the effective kWh by 1,000 and divide by system voltage to get amp-hours if you need to size DC components.

Additional Information

  • Usable storage should exclude the battery's built-in reserve—this calculator applies an extra health buffer if you want one.
  • Round-trip efficiency covers inverter, cabling, and DC/DC losses; lithium systems typically range 88–96%.
  • Comparing the 4-hour and 24-hour load outputs shows how aggressively you can dispatch the battery for short vs. sustained outages.
  • If you parallel multiple packs, add their usable kWh together before running the calculation.

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