Before you begin
What this tool needs
Use the usable capacity from the quote, not the headline size of the battery.
For the charge-cheap strategy, use the rate you would genuinely have paid, not the most expensive published peak rate.
Reading the answer
What your result means
A payback near or beyond the warranty term means the financial case is weak, even if backup has personal value.
The size to aim for follows the electricity your household could actually shift, not the size of the solar array alone.
Boundaries
Assumptions and limitations
Working assumptions
- 90% of what goes in comes back out.
- The battery works on 220 days a year storing solar, 330 charging cheap, or 365 split between the two.
- The electricity a household can shift rises with its size, and caps how much the battery can usefully deliver in a day.
Where to be careful
- The model does not value outage backup or future tariff uncertainty.
- It assumes the household can consistently shift the modelled load.
Common questions
Questions about this tool
Is there a standalone SEAI battery grant?
There is no standalone domestic battery grant in the individual grant scheme. Check SEAI directly if your project uses another programme.
Will a normal battery power the house in a cut?
Only if the installation includes approved backup equipment that can safely disconnect the house from the grid. A battery on its own does not keep the lights on in an outage.