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Home Battery System Sizing Calculator

This calculator estimates the usable nameplate energy capacity needed for a home battery to cover a selected number of hours of average daily consumption. It prorates daily kWh to the autonomy interval, increases the requirement for storage losses, and preserves a chosen capacity reserve. The result helps homeowners and renewable-system designers compare battery configurations on a consistent energy basis before checking product-specific module sizes. This is an energy-sizing model, not a power-sizing model. A system with enough kWh may still be unable to start a pump, support a large appliance, or absorb renewable generation if its kW rating is too low. Load timing also matters: daily consumption is treated as evenly distributed across 24 hours. For critical backup designs, build a device-level load profile and apply manufacturer limits, environmental derating, aging allowance, code requirements, and qualified engineering review.

Calculation inputs

Result
—
Calculated result
Load energy for interval—
Energy including losses—
Added reserve capacity—

1. Measure daily energy

Use kWh/day for only the loads the battery is expected to cover.

2. Set autonomy time

Enter how many hours those average loads should be supported without new energy.

3. Choose a capacity reserve

Keep room for operating limits, uncertainty, or future degradation.

4. Enter storage efficiency

Use a consistent efficiency basis for the energy path being sized.

5. Review required kWh

Round up to available battery-module capacity, then separately verify continuous and surge power.

Interval energy = Daily energy × (Autonomy hours ÷ 24)

Required capacity = Interval energy ÷ (Efficiency ÷ 100) ÷ [1 − (Reserve ÷ 100)]

The model assumes demand is evenly distributed during the day and interprets reserve as a portion of installed usable capacity held back.

What the result means

The result is the modeled storage energy capacity needed to serve average demand for the selected autonomy interval.

Results are planning estimates based on the values entered. Confirm equipment limits, site conditions, and project assumptions before making a purchase or investment decision.

Given

18 kWh/day, 12 hours of autonomy, 20% reserve, and 92% efficiency.

Calculation

Interval energy = 18 × 12 ÷ 24 = 9 kWh. Including losses = 9 ÷ 0.92 = 9.78 kWh. Required capacity = 9.78 ÷ 0.80 = 12.23 kWh.

Result

The energy requirement is 12.23 kWh before rounding up to purchasable module sizes and checking kW capability.

Does the result size the inverter too?

No. The result is in kWh. Size inverter and battery power in kW from simultaneous continuous loads and startup surges.

Should daily energy include every household appliance?

Include only the loads intended to remain powered during the autonomy period. Excluding nonessential loads can reduce required capacity substantially.

How is reserve different from efficiency?

Efficiency covers energy lost during storage and delivery. Reserve is capacity deliberately left unused for operating margin or uncertainty.

What if demand is concentrated at night?

The even-load assumption may misstate interval energy. Use an hourly load profile when timing is important.

How should the calculated capacity be converted to a product quantity?

Divide by the usable kWh per compatible module and round up, while respecting the manufacturer's minimum and maximum configuration rules.