Northern Ireland's standout solar day: 30 kWh on Saturday

Saturday 9 May 2026 was the kind of day Northern Ireland solar owners remember. Belfast saw fourteen hours of sunshine, cloud cover dropped to 39 per cent, and a typical 4kW rooftop system poured out 20.5 kWh. On a 6kW array that climbed to 30.7 kWh, worth £10.30 at current Power NI rates.

A standout day on its own does not make the case for solar. What it does is show what the equipment is capable of when the Atlantic gives you a break. For a prospective installer in NI, the standout day is the upper bound, and most weeks contain at least one.

The rest of last week tells the harder half of the story. Six days of mixed cloud, an overcast Friday, and a Belfast weekly total of 85.7 kWh on a 4kW system. The standout day did most of the heavy lifting, and that is true for solar in Northern Ireland more often than not.

Last Week, Day By Day

Here is how the week looked in Belfast, day by day, as a benchmark for any NI roof.

Day	Weather	4kW kWh	4kW value	6kW kWh	6kW value
Mon 4 May	Mixed	10.7	£3.59	16.1	£5.38
Tue 5 May	Mixed	10.4	£3.49	15.6	£5.24
Wed 6 May	Mixed	8.7	£2.93	13.1	£4.39
Thu 7 May	Mixed	10.1	£3.37	15.1	£5.06
Fri 8 May	Mixed	7.3	£2.45	11.0	£3.68
Sat 9 May	Brilliant	20.5	£6.86	30.7	£10.30
Sun 10 May	Brilliant	18.0	£6.03	27.0	£9.05
Total		85.7	£28.72	128.6	£43.10

What a brilliant day looks like in NI numbers

Saturday’s 30.7 kWh from a 6kW system stacks up well against a typical NI household. Power NI’s average annual demand for a three or four bedroom home sits around 3,500 kWh, which works out at roughly 9 to 10 kWh a day. That means Saturday’s 6kW output covered three days of household electricity in a single afternoon. The 4kW figure of 20.5 kWh did the work of two days.

Belfast got there with fourteen hours of sunshine and unusually thin cloud at 39 per cent. By contrast Friday delivered just 0.28 hours of sunshine through 100 per cent overcast, and the same 4kW system produced 7.3 kWh. That is a more than two-fold swing across forty-eight hours, and it is the swing that makes solar planning in NI different from the south coast of England.

You can see the live week unfold on the Solar Today dashboard, which refreshes each morning with new modelled output for seven NI towns. Last week Ballymena topped the table at 97.4 kWh on a 4kW, with Newry at the bottom on 83.1 kWh.

Why the standout day matters more than the weekly total

A standout day tells you two things the headline weekly figure cannot. First, it sets a credible ceiling. If a 4kW system in Belfast can deliver 20 kWh on the right Saturday, you know the equipment is sound and the panels are doing their job. Second, brilliant days tend to be long days. Generation starts shortly after dawn and runs into the evening, which gives a home with no battery plenty of self-consumption windows.

Cloud cover is the real variable in Northern Ireland, not latitude. The solar panel efficiency and weather guide walks through why a thin overcast can still produce around 60 per cent of clear-sky output, while heavy cloud cuts generation by 80 or 90 per cent. Saturday and Friday last week sat at either end of that range, which is exactly what NI springs look like in practice.

What that means for a roof in your postcode

If you are weighing up a system, the question is not whether you will get standout days. You will, several times a month between April and August. The question is how the standout days add up against the mixed and overcast ones. A 4kW Belfast roof produced 85.7 kWh last week and is on track for roughly 3,400 to 3,600 kWh across the year, which matches a typical household’s annual usage, especially if some of it can be shifted into daylight hours.

Our solar savings calculator takes your postcode and current bill and gives you a personalised view of payback and bill savings using the same weather model that drives the daily tracker. It will also show how a battery or an export tariff changes the maths.

How Solar Today Works

Each morning, Solar Today pulls weather data for seven NI towns from Open-Meteo. We model what a typical 4kW and 6kW solar installation would produce that day, applying a 0.78 performance ratio to account for inverter losses, panel temperature, and soiling. Today’s value is a forecast. Once the day closes, it firms up to the observed weather.

The seven towns we currently cover are Belfast, Derry, Armagh, Newry, Enniskillen, Lisburn, and Ballymena. Each gets its own daily estimate based on local cloud cover and irradiance.

A few honest caveats:

• These are modelled estimates, not live readings from physical systems.
• Forecast days update as the weather firms up.
• Your actual generation will vary by roof orientation, system size, panel age, and any shading.

The point is not to replace your inverter app. It is to give NI homeowners a fair, locally relevant benchmark that did not exist anywhere else.

See Your Town

Visit Solar Today to see the latest week for your nearest NI town. We update it every morning, and we publish a recap like this one each Monday.

If you do not have solar yet and this week’s numbers got you curious, our solar savings calculator will give you a personal estimate based on your own home, roof, and current bill.