Showing the working
How we calculate this
Every figure on this site is modelled from a remote assessment, so it is what we'd expect rather than a promise. Your quote is where we confirm the exact system for your roof, using your orientation, your pitch, your shading and your own twelve months of usage.
The worked example we use
Where this site shows a saving, a payback or a CO₂ figure without your own numbers in it, this is the home behind it.
| The house | A 3-bed semi-detached home |
|---|---|
| The bill | About €200 a month, roughly 6,900 units of electricity a year |
| The system | 3.5 kWp of panels, about 8 panels, plus a 10 kWh BYD battery |
| The roof | South-facing and unshaded |
| Tariff assumed | 35c a unit |
| Yield assumed | 900 units per kWp a year, so 3,150 units a year, all used in the house because of the battery |
| Saving we'd expect | About €92 a month, about €1,100 a year |
| Cost before the grant | €8,500 |
| SEAI grant | €1,700 on the panels: €700 × 2 kWp, then €200 × 1.5 kWp |
| Cost after the grant | €6,800 |
| Payback we'd expect | About 6.2 years |
| CO₂ we'd expect you to avoid | 3,150 units × 0.198 kg, about 0.6 tonnes a year |
A unit of electricity is a kWh, the thing your bill counts. kWp is the size of the system. Every term on this page in plain English
How the savings calculator works
- We start with your bill. You tell us what you pay a month, and we assume the system covers a share of it. The share depends on the house: 45%, 50%, 55%, 60%, 65% for the house types in the table below, in that order.
- We add 15 points for an EV. If you charge an electric car at home, we add 15 percentage points to that share, because you can move the charging to the hours your roof is producing. The total share is capped at 85%.
- We cap it at what the panels can make. The saving never goes above what your own panels produce. We take 900 units per kWp of panels a year, a typical Irish yield, valued at 35c a unit.
- Payback. The cost after the grant divided by the yearly saving.
- CO₂. The units you no longer buy, multiplied by 0.198 kg of CO₂ a unit.
- Backup hours. The size of the battery divided by your average hourly use, worked out from your bill.
The system and the grant behind each house type
| House | Share of bill | Panels | Battery | Cost | SEAI grant | After grant |
|---|---|---|---|---|---|---|
| 1-bed apartment | 45% | 2 kWp | 5 kWh | €4,500 | €1,400 | €3,100 |
| 2-bed house | 50% | 2.6 kWp | 7.5 kWh | €6,000 | €1,520 | €4,480 |
| 3-bed semi-detached | 55% | 3.5 kWp | 10 kWh | €8,500 | €1,700 | €6,800 |
| 4-bed detached | 60% | 4.5 kWp | 12.8 kWh | €11,000 | €1,800 | €9,200 |
| 5-bed or larger | 65% | 6 kWp | 15 kWh | €14,000 | €1,800 | €12,200 |
The cost is the full system, panels and battery together. The grant applies to the panels only. A battery added to panels you already own carries no SEAI grant.
The SEAI Solar Electricity Grant
- €700 per kWp for the first 2 kWp of panels.
- €200 per kWp from 2 kWp up to 4 kWp.
- A maximum of €1,800, which a 4 kWp system reaches.
- Solar panels only. The grant does not cover the battery.
- One grant per MPRN, your meter's number, printed on your bill.
- The home was built and occupied before 31 December 2020.
- A BER assessment is needed after the install to draw down the grant.
Figures checked against seai.ie on 11 September 2026. SEAI Solar Electricity Grant
What can move the number
- Which way the roof faces. South produces the most. East and west roofs produce less over the day, though they spread it more evenly across the morning and evening.
- Shading. A chimney, a tree or a neighbouring gable across the panels for part of the day lowers what they make.
- The pitch of the roof. How steeply it slopes changes the angle the panels sit at, and so what they collect.
- Your tariff. We model 35c a unit. Your own rate, and your day and night rates, move the saving with it.
- How much you use during the day. The more of your own electricity you use rather than buy, the more you save. The battery is what carries the daytime production into the evening.
All five are confirmed in your quote, with the assumption printed beside each figure.
Sources
- SEAI Solar Electricity Grant. Grant values and eligibility checked against seai.ie on 11 September 2026. seai.ie
- SEAI conversion factors, electricity consumption. 197.8 g of CO₂ per kWh, the 2025 provisional value, which includes generation plus transmission and distribution losses. We use 0.198 kg per unit for every CO₂ figure on this site. Checked 12 September 2026. SEAI conversion factors
- BYD storage deployed. BYD deployed over 60GWh of energy storage in 2025, around 13% of the global market and more than any other company. Benchmark Mineral Intelligence, 2025 battery energy storage shipment rankings (May 2026), as reported by Energy-Storage.news, 1 July 2026. Energy-Storage.news
Last checked 12 September 2026.
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