A common mistake is to size solar to the roof instead of to the household. The right system is the one whose output you can mostly use yourself, within what the roof, the grid connection and your budget allow. Here’s how to size it from your own bills, with three worked examples.
Step 1: find your annual and daytime use
Add up a year of electricity bills to get annual use in kWh. Then estimate how much of it happens in daylight: people at home during the day, a home office, daytime washing and EV charging all push this up. If you have a smart meter, the half-hourly data from your supplier or ESB Networks shows it directly. Our 7-day appliance audit is a simple alternative.
Step 2: estimate output per kWp
As a rough guide in Ireland, 1 kWp produces about 900–1,000 kWh a year, depending on orientation, pitch and shading. A south-facing, unshaded roof sits at the upper end; east/west splits and some shading pull it down. Your installer’s estimate for your roof replaces this rule of thumb.
Step 3: convert kWp to panels
Number of panels ≈ system size in kWp × 1,000 ÷ panel wattage
| System size | 400 W panels | 430 W panels | Approx. roof area |
|---|---|---|---|
| 2 kWp | 5 | 5 | 9–10 m² |
| 3 kWp | 8 | 7 | 13–15 m² |
| 4 kWp | 10 | 10 | 17–20 m² |
| 5 kWp | 13 | 12 | 22–25 m² |
Step 4: check the limits
- Roof: usable area after edges, roof windows, vents and chimneys; and its condition — re-roofing under panels later is expensive.
- Grid connection: most homes have a single-phase supply. ESB Networks’ standard “inform and fit” route covers micro-generation up to 6 kVA single-phase (11 kVA three-phase). Larger systems need a different application, and inverters are set to respect the export limit. Your installer handles this.
- Grant: €700 per kWp for the first 2 kWp and €200 per kWp up to 4 kWp — €1,800 maximum. See the SEAI solar grant guide.
- Planning: rooftop panels on houses are generally exempt — check the exceptions in solar planning permission.
Three worked examples
Hypothetical households to show the reasoning — not recommendations for your home.
A couple out at work all day
- Annual use 2,800 kWh, mostly in the evening.
- A 2–3 kWp system (5–7 panels) produces roughly 1,800–3,000 kWh; much of the summer output would be exported.
- Sizing up would mostly add export. Shifting the washing machine and dishwasher to daytime timers raises the value of every panel.
A family with someone at home
- Annual use 4,500 kWh, with a good share in daylight.
- A 4 kWp system (about 10 panels) produces roughly 3,600–4,000 kWh, earns the maximum grant, and much of the output can be used directly.
- This is the most common sweet spot.
A family with an EV and a heat pump
- Annual use 9,000 kWh, with daytime EV charging possible some days.
- 5–6 kWp (12–15 panels) can make sense if the roof allows, even though the grant is the same as at 4 kWp — the extra output has somewhere useful to go.
- Check the connection limit and the inverter size with the installer, and consider a hot water diverter before a battery: export or hot water?
Signs a quote is the wrong size
- It’s sized to fill the roof, with no reference to your bills.
- It assumes you’ll use nearly all the output, while the house is empty during the day.
- It adds a battery without showing how much extra self-use the battery creates.
- It doesn’t state the expected annual generation for your roof.
Use the solar payback calculator to compare two sizes with the same assumptions, and the quote checklist to compare installers like for like.
