How to use it
- Enter your daily energy use in Wh (add up watts × hours for each appliance, or take kWh per day from a meter × 1,000).
- Enter the days of autonomy: how long the bank must run with no solar input.
- Choose the system voltage and set the depth of discharge and system efficiency.
- Enter the voltage and Ah of one battery. Read the required capacity, the series and parallel arrangement and the installed capacity.
Formula
- Required energy E = daily Wh × days / (DoD × η)
- Required capacity C = E / system voltage (Ah)
- Batteries per string = system V / battery V
- Parallel strings = C / battery Ah, rounded up
- Installed capacity = system V × battery Ah × strings
Worked example
A cabin uses 2,000 Wh a day and should run 2 days without sun on a 24 V system with lead-acid batteries discharged to 50 % and 85 % efficiency. E = 2,000 × 2 / (0.5 × 0.85) = 4,000 / 0.425 = 9,411.76 Wh (9.41 kWh), and C = 9,411.76 / 24 = 392.2 Ah.
With 12 V 100 Ah batteries you need 24 / 12 = 2 in series and 392.2 / 100 = 3.92, so 4 strings in parallel: 8 batteries, 24 V × 400 Ah = 9.6 kWh installed.
Tips
- Higher voltage, fewer parallel strings. Many installers prefer few parallel strings because they share current unevenly; a 48 V system needs a quarter of the current of a 12 V one.
- Use identical batteries of the same type, age and capacity in one bank.
- Check the charge side too. The solar array and charge controller must be able to refill the bank; size the panels with the solar panel calculator.