How this calculator works
- The tool estimates how much energy is genuinely available to move through each battery size on a typical day.
- Capacity that cannot regularly be filled and emptied produces less incremental value, even though it increases purchase cost.
- The additional annual saving from the larger battery is compared with its additional installed price.
What can change the result
- Daily surplus solar or cheap-rate energy
- Evening and peak-period demand
- Price premium for the larger battery
- Usable capacity and round-trip efficiency
How to read the result
The key output is the value of the extra capacity. If the larger battery rarely uses those additional kWh, a smaller unit can have a stronger return even if its headline capacity looks less attractive.
A useful way to stress-test the answer is to change the most uncertain input by 10 to 20% and see whether your decision still looks sensible.
Worked example
A 10 kWh battery is not automatically twice as valuable as a 5 kWh battery if your household only has 7 kWh/day available to shift.
Frequently asked questions
What does energy available to shift mean?
The typical cheap-rate or surplus solar energy you can realistically move each day.
Does this include power limits?
No.