Free electrical planning tool
Leisure Battery Size Calculator
Build an appliance energy budget, then estimate the leisure battery capacity needed for your chosen number of days without charging.
Your system
Preset powers are editable illustrations. Use your appliance specifications or measured usage.
Your results
- Daily consumption
- 428 Wh
- Daily consumption at nominal voltage
- 35.67 Ah
- Estimated nominal battery capacity required
- 99.07 Ah
- Equivalent nominal battery energy
- 1.19 kWh
Common nominal capacities for comparison: 100 Ah · 150 Ah · 200 Ah Not a product or suitability recommendation.
Daily appliance consumption
- Fridge378 Wh
- LED lighting50 Wh
Planning estimates, not installation approval.
How to use this calculator
Add each appliance, edit its measured or rated input power and daily operating time, and set a duty cycle for intermittent loads. Choose chemistry, autonomy and efficiency. Enter AC load power with its conversion loss in overall efficiency, or enter measured DC input with that loss excluded — do not count it twice.
Calculation method
Daily Wh = sum of watts × hours × quantity × duty cycle. Daily Ah = Wh ÷ nominal voltage. Required nominal Ah = daily Wh × autonomy days ÷ (voltage × usable depth of discharge × system efficiency).
What the results mean
Daily Ah is the nominal-voltage energy equivalent before losses. The capacity estimate includes your chosen loss allowance and usable capacity. Comparison sizes are nominal capacities, not product recommendations.
Worked example
A 600Wh daily budget at 12V is 50Ah. For two days, 80% usable capacity and 90% system efficiency, the estimate is 138.89Ah, equivalent to 1.667kWh of nominal battery energy.
Common mistakes
A fridge rarely draws its rated power continuously: apply a measured duty cycle. Include heater start-up energy and standby loads in the budget. Do not reduce usable capacity twice when using manufacturer-provided usable Ah.
Frequently asked questions
Does a 100Ah lithium battery equal a 100Ah AGM battery?
Their nominal Ah may match at the same voltage, but permitted usable capacity and real discharge performance differ. Use each manufacturer’s specifications.
Why does system efficiency increase the battery estimate?
Energy is lost in conversion and distribution. At 90% efficiency, the battery supplies more energy than reaches the appliances.
Can I compare 12V and 24V batteries by Ah alone?
No. Compare energy in Wh: 100Ah at 24V holds twice the nominal energy of 100Ah at 12V.
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