The battery size on a solar quote is a number you can check for yourself before you sign.
SEAI says a good estimate is to look at how much electricity you use in the evenings, from around 4pm onwards. If you have a smart meter, you can download that usage from your ESB Networks account. 5 kWh is the smallest battery the grant will pay for.
Start With Your Evening Usage, Not Your Annual Bill
A battery does not serve your whole year. It serves the hours when the panels have stopped and you are still cooking, washing and watching telly. The annual figure on your bill is the wrong input.
Battery capacity is measured in kilowatt hours, and SEAI says there is “no exact formula for working out what you need”. It adds that your installer will be able to advise you based on your energy needs and circumstances.
That is a method, not a size, and two houses with the same roof can land on different batteries. Our guide to home battery storage in Ireland covers what the battery does once it is in.
How to Read Your Own Evening Usage
If you have a smart meter, the data is already yours. ESB Networks says only you can see it, through your online account.
- What you get. ESB Networks publishes the data as Harmonised Downloadable Files, in csv format. One of the files holds 30-minute readings in kWh. Another gives a daily snapshot of day, night and peak usage.
- How far back. You can view up to two years of electricity use, and ESB Networks says it does not matter if you changed supplier in that period.
- If the meter is new. A newly fitted smart meter establishes its connection over the next 30 days. ESB Networks says the meter is then read automatically and the usage data becomes available.
- If you already export. Microgeneration customers can also view and download the electricity they sent to the grid. Each is reported separately in the file.
Open the 30-minute file in a spreadsheet, add up the half hours from 4pm to bedtime, and do it for a winter week and a summer week. That is the number a quote should be built on. It is also the number that tells you whether the battery you were offered is too small.
Why 5 kWh Is a Grant Floor, Not a Recommendation
There is now an SEAI grant for a home battery. It is a flat rate of €600 for systems of 5 kWh or larger, and SEAI states that batteries smaller than 5 kWh are not eligible to apply.
The home needs an MPRN and must have been built and occupied before 2025. The grant is closed to homes with previous funding for a battery at that MPRN. You have to wait for your Grant Offer from SEAI before the battery goes in, and applications cannot be accepted for installations that have already started. Once approved, you have 8 months to complete the works and submit the Declaration of Works. Solar panels and a battery can go on a single application.
So the grant sets a floor and a deadline, not a size. Full eligibility detail sits on our solar battery grant page.
Rated Capacity Is Not What You Get Out
When the battery gets down to a set level, SEAI says your system will stop taking electricity out of it. SEAI calls that level the minimum state of charge, or the depth of discharge limit. Its example is a 5 kWh battery with a 10% state of charge, which stops drawing from the battery when it has 0.5 kWh left. SEAI says the reason is to protect the battery, because discharging until it is empty can increase wear and tear.
Ask any installer for the usable figure in writing, not the headline one. Then compare that against the evening usage you measured.
What Sets the Upper Limit
There is a ceiling on useful size, and it is money rather than kit.
When the battery is full and the panels are still generating, SEAI says the surplus goes to the grid and your supplier pays you for it through the Clean Export Guarantee. The two prices are very different. SEAI puts export at about €0.18 to €0.20 per unit. It puts the saving on a unit you generate and use yourself at approximately €0.35, based on an average across energy suppliers on 6 October 2026.
That gap is the case for storage, and also its limit: capacity your evenings never empty earns you nothing. Our page on Clean Export Guarantee rates lists what each supplier pays.
A battery with no panels is sized differently again. SEAI says that works, but only if you are on a tariff where electricity is cheaper at some times, such as a day and night rate. There the battery charges overnight and discharges when power costs more. See a home battery without solar and night rate electricity.
Modular Beats Guessing
If the measured number lands between two sizes, SEAI points at modular kit. It says it is worth considering a modular battery, as this gives you the flexibility to increase your storage over time rather than having to replace the existing battery with a bigger one.
Size has a physical side too. SEAI says a typical battery storage system is around 100cm high, 60cm wide and 25cm deep, about the same width as your washing machine. Height increases if you add capacity to a modular system. It also recommends the battery goes outside, including on outbuildings, in an appropriate enclosure, or indoors in a well-ventilated space.
Walk the spot with the installer before you sign, because a taller stack later needs room now.
The Size You Pick Changes the Inverter and the Paperwork
The battery is not the only thing that has to change.
- A retrofit may need a new inverter. SEAI says standard solar PV installations come with a string inverter, and that these “are not compatible with battery storage so you may need a new (hybrid) inverter”. Our guide to solar inverters explains the types.
- The grid application comes first. SEAI says your installer must apply to ESB Networks to connect your battery system before installing it, and that this usually takes at least 4 weeks or 20 working days.
- The inverter carries the grid limit. ESB Networks says single-phase systems must have an inverter rated at 25A, about 6 kVA, and three-phase systems 16A per phase, about 11 kVA. It adds that the inverter limits output to these levels even if additional generation is installed behind it. See the NC6 form and maximum solar system size.
What It Costs, and What the Grant Pays
SEAI's own figure is that having a battery fully installed can cost of the order of €3,500 to €5,500.
Timing changes the tax. SEAI says that if you install the battery at the same time as solar panels there will be 0% VAT on the cost of the battery, and that a standalone installation carries 23% VAT on top.
The two grants work differently as well. The battery grant is a flat €600. The Solar Electricity Grant pays €700 per kWp up to 2 kWp and €200 for each additional kWp up to 4 kWp, to a maximum of €1,800, and it is paid on panel capacity. A bigger battery does not raise it. See the SEAI solar grant and 5 kWh battery prices for the numbers behind a quote.
SEAI advises getting at least three quotes. Bring your own evening kWh figure to each one, ask for the usable capacity rather than the rated capacity, and ask whether the inverter they propose can take more storage later. Then compare the three answers side by side.
Get a solar quote →Frequently Asked Questions
SEAI says there is no exact formula. Its starting point is how much electricity you use in the evenings, from around 4pm onwards, which you can download from your ESB Networks account if you have a smart meter. The minimum capacity for the grant is 5 kWh. Your installer should then advise you based on your energy needs and circumstances.
It depends on what your house draws after 4pm, and 5 kWh is where the grant starts rather than where sizing starts. SEAI pays a flat €600 for a battery of 5 kWh or larger and says smaller batteries are not eligible. SEAI's example is a 5 kWh battery with a 10% state of charge, which stops discharging with 0.5 kWh left.
With modular kit, yes. SEAI says a modular battery gives you the flexibility to increase your storage over time rather than having to replace the existing battery with a bigger one. Height increases as you add capacity, so leave room. Note that the grant is closed to homes with previous funding for a battery at the same MPRN, so a later addition is at your own cost.
The surplus goes to the grid and your supplier pays you for it under the Clean Export Guarantee. SEAI puts that payment at about €0.18 to €0.20 per unit, against approximately €0.35 saved on a unit you use in the house instead. That difference is why storage pays, and why capacity your evenings never use is dead weight.
No. The battery grant is a flat €600 for any system of 5 kWh or larger. The Solar Electricity Grant is separate and is paid on panel capacity: €700 per kWp up to 2 kWp, then €200 for each additional kWp up to 4 kWp, capped at €1,800.
You may, on a retrofit. SEAI says standard solar PV installations come with a string inverter and that these are not compatible with battery storage, so you may need a new hybrid inverter. Your installer also has to apply to ESB Networks before the battery is installed, which SEAI says usually takes at least 4 weeks or 20 working days.