Solar battery size calculator
Find the storage that runs your loads at night or on cloudy days without draining the batteries too deep.
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How to use it
- Enter daily load, days of autonomy, system voltage, allowed depth of discharge, inverter efficiency and capacity of one battery.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Capacity (Ah) = daily load (Wh) × backup days ÷ (system voltage × depth of discharge × inverter efficiency). Series = system V ÷ battery V; parallel = round up (capacity ÷ battery Ah).
Example
Inputs: Daily load = 3,000 Wh, Days of autonomy = 1, System voltage = 24 V, Allowed depth of discharge = 80 %, Inverter efficiency = 90 %. Result: Capacity needed: 173.6 Ah; Nominal bank energy: 4.17 kWh; Total batteries: 4.
Limits and caveats
- 80% depth of discharge suits most lithium batteries; lead-acid is usually kept to 50%.
- Heat losses and battery ageing are not included.
Questions about this tool
How many batteries for 3,000 Wh a day?
On 24 V with 80% discharge and a 90% inverter you need about 174 Ah, which is four 12 V 100 Ah batteries.
What is the difference between series and parallel?
Series raises voltage at the same capacity; parallel raises capacity at the same voltage.
Why not drain the battery fully?
Repeated deep discharge shortens battery life, especially for lead-acid.