Before you begin
What affects the battery size you need
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
Understanding battery size and payback
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.
Worked example
The same question, answered end to end
A three-person household with a 4.2 kWp array, pricing a 10 kWh usable battery at €7,000 to do both jobs — store solar and charge cheaply overnight.
What was entered
- Both strategies: store surplus solar, and charge overnight to use at peak.
- A 4.2 kWp array and three people in the household.
- 10 kWh of usable capacity quoted at €7,000 installed.
- 36c day, 20c export, 12c night, 40c peak.
How it is worked out
Household size sets how much electricity there is to shift in an evening, which caps what the battery can usefully deliver in a day.
The array size sets how much surplus exists to store at all — a battery cannot store solar a roof never produced.
Stored solar earns the gap between the 36c you would have paid and the 20c you gave up by not exporting: 16c a unit.
Overnight charging earns the gap between the 40c peak avoided and the 12c paid: 28c a unit.
Both are reduced by the 10% round-trip loss, and applied over 365 split days.
Payback is €7,000 divided by the annual saving, and is read against a roughly ten-year warranty life.
What the tool returns
- Size to aim for
- 8 kWh
- Energy shifted
- 2,876 kWh/year
- Annual saving
- €652/year
- Simple payback
- 10.7 years
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.