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Home Backup Power

Free Home Backup Power calculator with clear step-by-step results.

About Home Backup Power

Sizes a home backup bank from the load you actually need to keep alive rather than whole-house demand. Energy delivered is grossed up for depth-of-discharge limits and inverter losses to give installed capacity, and the inverter is sized on the greater of continuous load with headroom or half the motor surge.

Formulas

Installed capacity

Bank kWh = load W x hours / (depth of discharge x inverter efficiency) / 1000

Inverter rating

Rating = greater of continuous load x 1.25 and surge / 2

Worked example

With the default values below, Home Backup Power returns Battery bank capacity required of 18.414. Change any input above and every figure here is recalculated in your browser.

Inputs used

Critical load to keep runningminimum 1 W
1,200 W
Hours of backup wantedaccepts 0.5–168 h
12 h
Battery bank voltageminimum 6 V
48 V
Usable depth of dischargeaccepts 20–100 %
85 %
Inverter efficiencyaccepts 40–100 %
92 %
Largest motor surgeminimum 0 W
3,500 W

Results

Battery bank capacity required
18.414 kWh
Bank capacity in amp-hours
383.6 Ah
Energy delivered to the loads
14.400 kWh
Minimum continuous inverter rating
1,750 W

Step by step

  1. 1Values used
  2. 2Installed capacity
  3. 3Inverter rating
  4. 4Battery bank capacity required= 18.414 kWh
  5. 5Bank capacity in amp-hours= 383.6 Ah
  6. 6Energy delivered to the loads= 14.400 kWh
  7. 7Minimum continuous inverter rating= 1,750 W

Assumptions and limits

  • The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Backup power systems involve significant stored energy and interact with the grid supply. Installation, transfer switching and earthing must comply with local electrical code and be carried out by a licensed electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Backup power systems involve significant stored energy and interact with the grid supply. Installation, transfer switching and earthing must comply with local electrical code and be carried out by a licensed electrician.

Frequently Asked Questions

What counts as critical load?

Typically a fridge or freezer, some lighting, networking equipment, phone charging and any medical device. Space heating, electric cooking and tumble dryers are almost never economic to back up.

Why is surge divided by two?

Quality inverters tolerate roughly twice their continuous rating for a few seconds, which covers compressor and pump starting. Cheaper units have less headroom and need the full surge figure.

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Home Backup Power – Calculate Battery bank capacity required