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Battery bank backup time: how to estimate it in kWh and hours

· 5 min read ·

Learn to calculate the stored energy, the energy you can actually use and how many hours a battery bank can run your home loads, with a step-by-step example.

The calculation at a glance

“How long will the battery run the house?” It depends on how much energy the bank actually delivers and how much power the loads draw. See the path from a 48 V, 100 Ah bank to the outlets:

  1. 4.8 kWh
    Nominal energy48 V × 100 Ah ÷ 1,000
  2. 4.3 kWh
    Usablelithium with a 90% depth of discharge (DoD)
  3. 3.9 kWh
    Delivered to loadsafter inverter losses (90% efficiency)
3,888 Whenergy delivered÷400 Waverage consumption=≈ 9.7 hbackup time
Enough to get through a night on the essential circuits.

Lithium or lead-acid

No battery should be fully emptied. The depth of discharge (DoD) tells how much of the capacity is used in each cycle. Each chemistry has a recommended depth, and that is where they differ most.

Energy delivered by the same 4.8 kWh bank

Lithium (90% DoD)3.9 kWh
Lead-acid (50% DoD)2.2 kWh

Lithium (LiFePO4)

  • Uses 80–90% of the capacity
  • Stable voltage during discharge
  • Charges fast, weighs less
  • Many more cycles: usually cheaper per kWh delivered

Lead-acid / AGM

  • Cheaper to buy
  • Tolerates simple systems
  • Uses only ~50% of the capacity
  • Loses life if discharged beyond that
  • Needs frequent full charges

Load consumption

Essential circuits of a home (average power)

Refrigerator (daily average)150 W
TV and router120 W
LED lighting80 W
Laptop and chargers50 W
Total: 400 W.

Calculate your backup time

Replace the bank, the chemistry and the loads with your own numbers. The funnel shows how much is lost at each stage.

Interactive

Estimate your backup time

Adjust the bank and the loads: the funnel shows how much energy actually reaches the outlets.

Connected loads

  1. 4.8 kWh
    Nominal energyvoltage × capacity
  2. 4.3 kWh
    Usableafter the depth of discharge
  3. 3.9 kWh
    Delivered to loadsafter inverter losses
400 Wload consumption
9.7 hestimated backup time

Off-grid: think in days

In a stand-alone system, the calculation is in days of autonomy: how many cloudy days in a row the bank must cover without recharging. Example: a country house that uses 3 kWh a day and needs 2 days.

3 kWhper day×2 dayswithout sun=6 kWhusable
≈ 7.4 kWhnominal with lithium (≈ 155 Ah at 48 V)
≈ 13.3 kWhnominal with lead-acid (≈ 280 Ah at 48 V)
With lead-acid the nominal capacity needed is almost twice as large for the same autonomy; volume and weight depend on the models.

How to increase the backup time

  • Cut consumption before enlarging the bank: LEDs, no standby loads, an efficient refrigerator
  • Put the essential loads on their own board (no electric shower, water heater or oven on backup)
  • Move pumps, machines and chargers to daytime
  • Grow the bank in a planned way, following the series and parallel rules

The diagram below shows a 4.8 kWh bank feeding an essential loads board, with the estimate noted on it.

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A 4.8 kWh bank feeding an essential loads board, with the backup time estimate.

Try it: Replace the loads with the ones in your home, note the power of each in the Description field and redo the calculation in the diagram text.

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