Solar System Sizes in Northern Ireland: 3kW to 10kW Compared

What size solar system do I need in Northern Ireland?

Sizing gets decided by two things: the usable roof you have, and how much electricity you actually use while the sun is up. It does not get decided by the biggest number an installer can fit on the rafters. Every kilowatt you fit above what you can use or store gets exported, and NI has no Smart Export Guarantee, so export is paid voluntarily by suppliers at 5p to 8p per kWh against an import price of 32.10p to 34.15p on Power NI’s rates from 1 July 2026.

I built this site, and I run the Solar Today tracker that models daily output for seven NI towns. The numbers below come out of that tracker rather than out of a national rule of thumb, and I have shown the working so you can check it.

System size	Panels (400W to 450W)	Roof area needed	Installed cost in NI, 2026	Annual generation	Annual saving	Simple payback
3kW	7 to 8	14 to 16 m²	£5,000 to £6,500	2,500 kWh	£470	12 years
4kW	9 to 10	18 to 21 m²	£6,000 to £8,000	3,330 kWh	£580	12 years
5kW	11 to 13	23 to 26 m²	£7,000 to £9,500	4,160 kWh	£680	12 years
6kW	14 to 15	28 to 31 m²	£8,000 to £12,000	5,000 kWh	£760	13 years
8kW	18 to 20	37 to 41 m²	£11,000 to £14,000	6,660 kWh	£900	14 years
10kW	23 to 25	46 to 51 m²	£14,000 to £18,000	8,320 kWh	£1,030	15 to 16 years

Generation comes from the tracker’s measured Belfast output, corrected to a south-facing pitched roof. Savings assume self-consumed units are worth 29p and exported units 6p, with self-consumption falling as the array gets larger. Payback uses the mid-point of each cost range and ignores future price rises, which shorten it. All prices carry 0% VAT, which applies to domestic installations until 31 March 2027 and then reverts to 5%.

What the tracker actually measured in Belfast

The tracker pulls daily shortwave radiation from Open-Meteo for each town and converts it to output using energy = radiation × system size × performance ratio, with the performance ratio set at a deliberately conservative 0.78. It values savings at 33.5p per kWh, Power NI’s standard credit rate when the tracker was set up.

Here is Belfast, every measured day from 24 January to 5 September 2026.

Month, 2026	Days measured	4kW output	6kW output	Sunshine hours a day
January (24th to 31st)	8	0 kWh	0 kWh	0.0
February	28	67 kWh	100 kWh	2.4
March	31	213 kWh	320 kWh	5.0
April	30	348 kWh	523 kWh	7.7
May	31	415 kWh	623 kWh	7.0
June	30	369 kWh	554 kWh	6.3
July	31	489 kWh	734 kWh	9.1
August	31	291 kWh	436 kWh	4.5
September (1st to 5th)	5	35 kWh	53 kWh	3.6
Total	225	2,227 kWh	3,342 kWh

Two things there are worth pausing on. June came in below May, at 369 kWh against 415 kWh, because June 2026 was duller in Belfast than May was. And the eight January days record zero, which is a gap in the feed rather than eight genuinely dead days, so the 225-day total is if anything slightly light. The best single day was 21 June: 24.8 kWh on a 4kW array, 37.2 kWh on a 6kW, off 15.55 hours of sunshine.

Scaling 225 days to a full year

Pro-rating 2,227 kWh across 365 days would give 3,612 kWh, which is nonsense, because the 140 days missing from the window are the darkest of the year. Instead I filled the gaps from the tracker’s own measured months.

Period	How it was filled	4kW	6kW
24 Jan to 5 Sep	Measured	2,227 kWh	3,342 kWh
6 to 30 Sep	25 days at the 7.1 kWh a day measured 1 to 5 Sep	177 kWh	266 kWh
October	31 days at March’s measured 6.9 kWh a day	213 kWh	320 kWh
November	30 days at February’s measured 2.4 kWh a day	72 kWh	108 kWh
December	31 days at half February’s rate	37 kWh	56 kWh
1 to 23 January	23 days at half February’s rate	28 kWh	41 kWh
Full year		2,754 kWh	4,133 kWh

October is paired with March and November with February because they sit at matching points either side of midwinter. December and early January get half of February.

That estimate stands up to an independent check. PVGIS, the European Commission’s PV database, puts a 4kWp array on a horizontal plane in Belfast at 3,042 kWh a year using the SARAH3 radiation database averaged over 2005 to 2023, at 14 per cent system losses. Rerun that at the tracker’s more cautious 0.78 performance ratio and it gives 2,759 kWh, against my scaled figure of 2,754 kWh. The two agree to within 0.2 per cent.

Why the headline table shows more than 2,754 kWh

The tracker uses radiation on a horizontal plane, because that is what the weather feed provides, and your panels will not be flat. PVGIS puts the same 4kWp array in Belfast at 3,675 kWh a year tilted 35 degrees and facing south, against 3,042 kWh flat. That is a 21 per cent gain from the pitch alone. Applying that 21 per cent to the tracker’s full-year figures gives 3,330 kWh for a 4kW and 5,000 kWh for a 6kW on a well-pitched south-facing NI roof, which are the numbers in the master table. If your roof faces east or west, take some of that gain back off.

The 4kW versus 6kW gap, and when it turns up

Across those 225 measured days the 6kW array produced about 1,115 kWh more than the 4kW. The useful question is which months that extra arrives in.

Month, 2026	Extra output from 6kW over 4kW, Belfast
February	33 kWh
March	107 kWh
April	175 kWh
May	208 kWh
June	185 kWh
July	245 kWh
August	145 kWh
September (1st to 5th)	18 kWh

May, June, July and August account for 783 kWh of it, which is 70 per cent of the extra output for the year. February and March together account for 140 kWh, or 12 per cent.

The extra capacity shows up almost entirely in the four months when your house needs the least electricity and daylight already outruns demand. On 21 June the tracker had the 6kW array at 37.2 kWh in a single day. A household getting through 10 kWh a day with a 5 kWh battery might absorb 15 of those units. The other 22 exported, and 22 units at 6p is £1.32 where the same 22 units used in the house would have been worth £6.38.

That is the argument for sizing to daylight usage rather than roof capacity, and for a battery before extra panels once you get past about 4kW. Our guide to the best home battery brands in NI covers the sizing.

3kW systems

Seven to eight panels, 14 to 16 square metres, £5,000 to £6,500 installed. About 2,500 kWh a year on a south-facing pitch.

This suits a one or two-bed home, a small terrace, or a household under 2,500 kWh a year. It is also the size you land on when the only decent roof face is small. Scaffolding, the inverter and a day of labour cost roughly the same whether there are seven panels up there or fourteen, so the cost per kW is the worst on this page. If the roof takes 4kW and the usage justifies it, 4kW is better value per pound.

4kW systems

Nine to ten panels, 18 to 21 square metres, £6,000 to £8,000 installed. About 3,330 kWh a year.

This is still the default recommendation across the NI installer network, and it fits the housing stock. A typical three-bed semi has 18 to 22 square metres of usable rear roof, which is exactly a 4kW array at modern panel ratings. There is a dedicated guide for that property type: solar panels for a 3 bed semi in NI.

The other reason 4kW keeps coming up is the grid connection. A 4kW array with a 3.68kW inverter stays inside the free G98/NI route, which is covered further down.

5kW systems

Eleven to thirteen panels, 23 to 26 square metres, £7,000 to £9,500 installed. About 4,160 kWh a year.

This is the step up for a three or four-bed detached where somebody is home during the day, or where an EV charger is going in soon. The cost per kW is better than 4kW, and the payback is about the same. It crosses the 3.68kW inverter threshold, so factor in the NIE Networks application.

6kW systems

Fourteen to fifteen panels, 28 to 31 square metres, £8,000 to £12,000 installed. About 5,000 kWh a year.

Six kilowatts makes sense for a large detached house running an EV, a heat pump or electric hot water, and it makes much better sense with a battery than without one. The price range is the widest on this page because the equipment choices widen at this size, so the spread between quotes is at its largest here.

Across 368 quote requests through this site since March, 235 came from detached houses. That is 64 per cent, and it is why 6kW comes up more often in my inbox than the 4kW default would suggest. It reflects who asks for quotes rather than NI’s housing stock.

Compare quotes from MCS-certified installers across NI, free, no obligation. Start here.

8kW systems

Eighteen to twenty panels, 37 to 41 square metres, £11,000 to £14,000 installed. About 6,660 kWh a year.

You are past what a standard semi or terrace can carry here, and into large detached houses, farmhouses and rural properties with outbuildings. The output exceeds almost any household’s daylight demand, so an 8kW array with no battery and no large controllable load such as EV charging or an immersion diverter will export most of its summer generation. It also needs a G99/NI route, which carries a fee.

10kW systems

Twenty-three to twenty-five panels, 46 to 51 square metres, £14,000 to £18,000 installed. About 8,320 kWh a year.

The top of the domestic range, and the point where the case usually rests on something other than household consumption: a home business, a farm, several EVs, or a heat pump running through the winter. The payback in the table is the longest on the page at 15 to 16 years, and that is with self-consumption at 28 per cent. If the real figure is lower, it stretches further.

How the savings figures are built

Generation is not the saving. What you avoid paying for is the saving, and that depends on how much of your own output you use at the moment it is produced.

Size	Annual generation	Assumed self-consumption	Value at 29p	Export at 6p	Annual saving
3kW	2,500 kWh	55%	£399	£68	£466
4kW	3,330 kWh	50%	£483	£100	£583
5kW	4,160 kWh	45%	£543	£137	£680
6kW	5,000 kWh	40%	£580	£180	£760
8kW	6,660 kWh	33%	£637	£268	£905
10kW	8,320 kWh	28%	£676	£359	£1,035

The self-consumption percentages fall as the array grows because the same household absorbs a smaller share of a bigger output. A battery pushes each of those percentages up by roughly 20 to 30 points, and the saving column moves with them, at a cost of £3,000 to £8,000 up front.

I have used 29p per kWh deliberately, which is below the market. Power NI’s published domestic unit rates from 1 July 2026 run from 32.10p to 34.15p per kWh including VAT depending on tariff and payment method. Recalculate the middle column at your own rate and the paybacks shorten. Export is set at 6p, mid-range for what NI suppliers pay voluntarily, since the Smart Export Guarantee is Great Britain legislation and does not extend here. Our guide to solar export payments in NI has the current rates.

Inverter sizing and the NIE Networks threshold

This is the part of sizing that gets skipped in most guides, and it can change what you are allowed to connect.

The rule is set by NIE Networks and it is about the inverter, not the panels. In their own words: if your new equipment exports a maximum of 3.68kW on single phase, or under 11.04kW on three phase, or under 16 Amps per phase, then G98/NI applies; above this value the G99/NI rules apply. Almost every NI home is single phase, so 3.68kW is the number that matters.

G98/NI is the micro-scale route and it works on fit and inform. There is no application fee for a G98/NI Stage 1, and your installer has 30 days from commissioning to submit the Stage 1 commissioning form. Nobody waits on an approval before the scaffolding goes up.

G99/NI Fast Track is the streamlined route above that threshold. NIE Networks sets it at a Total Installation Capacity of up to 60 Amps per phase with a maximum inverter of 32 Amps, which they state as 13.8kW capacity with a 7.36kW inverter ceiling. Most domestic systems from 5kW to 10kW land here.

G99/NI proper covers anything beyond the Fast Track limits and involves a network study, a connection design and a quotation pack you have 90 days to accept. NIE Networks charges an application fee for every generation application except G98/NI, and for G99/NI and Fast Track that fee depends on system size, set either from Schedule 2 of the Statement of Connection Charges or from the outturn cost of the studies.

Two practical consequences. An installer can fit a larger array behind a smaller export-limited inverter to keep the job inside G98/NI, which is a common way to get more panels on a roof without a chargeable application. And NIE Networks describes the G98/NI boundary in terms of what the equipment exports, while the Fast Track bands are written in terms of Total Installation Capacity, so ask your installer which basis NIE is applying to your job before you assume a 5kW array is a simple notification. Our guide to solar panel inverters explains how export limiting works.

Start from the bill, not the roof

Of the 368 quote requests that have come through this site since March, 246 reported a monthly electricity bill of £100 to £250. At Power NI’s current rates that is roughly 3,400 to 9,000 kWh a year, which is a range wide enough to justify anything from 4kW to 10kW. A bill band is not a system size.

The other pattern in those requests is batteries. 323 of the 368 asked about battery storage alongside panels, and 83 mentioned an EV charger. Both change the sizing answer, because both raise the share of your own generation you can actually use.

Three checks, in order:

1. Pull twelve months of kWh off your bills or your supplier account. Not the pounds, the units.
2. Work out how much of that you use between roughly 9am and 5pm. If the house is empty on weekdays, that share is small and a battery matters more than extra panels.
3. Measure the roof face you would actually use. Length by width, minus chimneys, vents and velux windows, then divide by 2 square metres per panel.

Our guide on how many solar panels you need works through that calculation with examples, and the full cost breakdown for NI covers what each line of a quote should contain.

Then get three quotes and compare the system size each installer proposes, not only the price. If two out of three land on the same size, that is a reasonable signal. If one comes in a great deal larger, ask which of your annual kWh it expects you to use rather than export, and check the answer against the monthly pattern in the tracker table above.