Find the Right Solar Battery Size for Your Home, Energy Use & Solar System.

What Size Solar Battery Do I Need?

(Ireland Sizing Calculator)

Most Irish homes land somewhere between 5 kWh and 15 kWh of usable capacity. Where you sit in that range depends on three things: how much surplus solar you actually export, how much electricity you use between four in the afternoon and midnight, and whether you’re on a time-of-use tariff that lets the battery charge cheaply from the grid.

That last one changes the answer more than anything else. On a flat rate, a battery can only ever store what your panels give it. On a night-rate tariff, it can fill every night regardless of the weather — and the correct size goes up, sometimes substantially.

batteryinstall.ie / sizing tool

How big a battery do you actually need?

Sizing isn't about your roof. It's about how much electricity you use after the sun goes down, and whether you're on a tariff that lets you buy cheap overnight. Answer six questions and we'll show you the range that fits — and where it fills from.

Your house 1 of 2

Annual electricity use On your bill, or on the ESB Networks portal. Not sure? Use the estimate buttons.

kWh / year

When is the house busy? This decides how much of your use falls outside daylight hours.

Existing solar array 6.0 kWp

Your loads 2 of 2

Electric vehicle

Heating

Electricity tariff The single biggest factor in what size makes sense.

Recommended size Solar + night rate

9 – 12kWh usable

Enough to carry your evening and overnight use without importing at peak rate.

Inverter power

5.0 kW

How much the battery can deliver at once — a separate question to capacity. 5 kW is what we fit as standard.

Covers

~92%

of a typical evening and overnight draw.

Indicative sizing. No obligation, no sales call unless you ask for one.

A typical day June

00:0006:0012:0018:0023:00

Charging from cheap grid Charging from solar Discharging to the house

Reading this: bar height is how full the battery is at that hour.

How the sizing model works

We would rather show our working model than ask you to trust a number.

Step 1 ) Evening and overnight load.

Your annual consumption is split across a typical Irish daily profile: roughly 15% overnight, 15% morning, 25% midday, 45% evening. Winter loads run about 25% higher than the annual average, and winter is when a battery is working hardest, so that’s the case we size for. The evening window (16:00–23:00) is where most of the value sits; if you want the battery to carry you through to morning, the window extends to 07:00 and the required capacity rises.

Step 2 ) What solar can actually contribute.

A XX kWp array in Ireland generates roughly 3,400–3,800 kWh a year, but the seasonal spread is brutal: June might deliver 500 kWh, December closer to 90. Without a battery, a typical home self-consumes 30–50% of that. With one, 70–80%. The gap is what you’re buying.

Step 3 ) Round-Trip Losses.

Energy in is not energy out. We apply a 0.92 round-trip efficiency once never twice, which is a common error that quietly inflates savings figures by 8%.

Step 4 ) Usable versus nominal.

Energy in is not energy out. We apply a 0.92 round-trip efficiency once never twice, which is a common error that quietly inflates savings figures by 8%.

Step 4 ) Usable versus nominal.

A 10 kWh battery is not 10 kWh of usable storage. Depth of discharge limits, BMS reserve and inverter cut-offs mean usable capacity is typically 90–95% of nominal on modern LFP systems, less on older chemistries. We size on usable and we tell you both numbers, because the industry has a bad habit of quoting nominal and delivering usable.

Step 5 ) Power, not just capacity.

A big battery with a small inverter can’t cover an oven, a kettle and an immersion at once. Continuous discharge power matters as much as kWh. We recommend a 5 kW minimum inverter on residential retrofits; below that, you hit the ceiling regularly on winter evenings and the battery looks like it isn’t working when actually it’s just being throttled.

If you have your ESB Networks HDF file, upload it.

Half-hourly data from your smart meter overrides every assumption above with your actual consumption, half hour by half hour, for the last two years. You can download it free from the ESB Networks customer portal. It takes the sizing from an estimate to a measurement.

Our Position:

EV charging.

We deliberately leave EV charging out of battery sizing. Charging a car from a house battery is a round trip through two sets of conversion losses to move electricity you could have bought at 6–10c on a night rate anyway.

Charge the car directly on the cheap window; size the battery for the house. Anyone selling you a bigger battery on the basis of EV charging is selling you capacity you won’t earn back.

Heat pumps

Are different, they run on winter evenings and mornings, exactly when the battery should be discharging, so they belong in the sizing. But we count them once. If your annual consumption figure already includes the heat pump, we don’t add it again on top.

Typical sizing outcomes in Ireland

Indicative only, Your own numbers may differ, which is what the calculator is for.

Household	Annual use	Tariff	Typical usable capacity
Two adults, no EV, flat rate	~3,500 kWh	Flat	5–8 kWh
Family of four, gas heating	~4,500 kWh	Smart 3-band	8–12 kWh
Family of four, heat pump	~7,000 kWh	Smart with night boost	12–18 kWh
Family, heat pump + EV, night rate	~9,000 kWh+	EV tariff	15–20 kWh
Rural home, immersion + oil backup	~5,000 kWh	Day/night	10–14 kWh

Why bigger isn’t automatically better

A battery earns money by cycling. Every full charge-and-discharge displaces expensive grid electricity with cheap or free electricity. A battery that cycles 300 times a year earns roughly twice what the same battery earns cycling 150 times.

So oversizing does something counter-intuitive: it *reduces* cycles. The extra capacity never fills in winter and never empties in summer, and you’ve paid for kWh that sit at 50% state of charge doing nothing. The payback period gets longer, not shorter.

The exception is a genuinely large, shiftable load — heat pump, immersion, workshop that can absorb the extra capacity on a winter evening. If you have one, size up. If you don’t, don’t let anyone talk you into it.

The other exception is planned change. If a heat pump or an extension is coming in the next two years, it’s usually cheaper to buy the capacity once than to expand later, expansion depends on the manufacturer still supporting your model and generation, and that’s not guaranteed.

See our expansion page for why that matters.

Frequently Asked Questions

Is a 10 kWh battery enough for an Irish home?

For a typical three- or four-bed with gas or oil heating and no EV, yes — 10 kWh usable will normally carry the evening peak and most of the overnight base load. Add a heat pump and it starts to look tight in January.

How many kWh does an Irish home use in the evening?

On a typical 4,500 kWh annual consumption, roughly 5–6 kWh falls between 16:00 and 23:00, rising to 7 kWh or more on a cold winter evening. That evening figure, not your annual total, is what the battery has to cover.

Should I size for summer or winter?

Winter. Summer is the easy case — long days, plenty of surplus, the battery fills by lunchtime whatever its size. Winter is when the sizing decision is actually tested.

Can I add more capacity later instead?

Sometimes. It depends on your battery model, its generation, its firmware and the manufacturer’s expansion rules — and on those still being supported when you come to do it. Expansion is a real option, but it isn’t a guarantee, and it’s rarely cheaper per kWh than buying it up front.

Do I need a bigger battery if I have an EV?

Usually not. Charge the car on a night rate directly. See above.

What’s the difference between usable and nominal capacity?

Nominal is the headline number. Usable is what you can actually take out. On a modern LFP battery the gap is small, typically 5–10% but always quote and compare on usable.