NI's Heatwave Turned Roofs Into Power Stations: The Numbers

Northern Ireland just had a proper heatwave. Temperatures hit 30.8°C at Castlederg on 25 June, equalling the hottest June day NI has ever recorded, the longest days of the year were in play, and for once the sky stayed blue for the best part of a week. If you have solar panels, you will have watched your inverter app do something it rarely gets to do here. And if you do not, you may be wondering what you missed.

So here is the honest answer, in kWh and pounds. Over the week ending 28 June 2026, a typical 4kW system in Belfast generated about 135.9 kWh, worth roughly £45.54 at current Power NI rates. A larger 6kW system made around 204 kWh, worth about £68.31. That is up 18% on the week before, and it is the kind of week that makes the case for NI solar better than any sales pitch can.

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 22 Jun	Mixed	9.6	£3.21	14.4	£4.82
Tue 23 Jun	Brilliant	22.0	£7.38	33.1	£11.07
Wed 24 Jun	Brilliant	20.7	£6.92	31.0	£10.38
Thu 25 Jun	Brilliant	24.9	£8.34	37.4	£12.52
Fri 26 Jun	Brilliant	20.2	£6.78	30.3	£10.16
Sat 27 Jun	Brilliant	16.6	£5.57	24.9	£8.35
Sun 28 Jun	Brilliant	21.9	£7.34	32.9	£11.01
Total		135.9	£45.54	204.0	£68.31

A Genuine Heatwave, By NI Standards

This was not a borderline warm spell. On 25 June, Castlederg in Co. Tyrone reached 30.8°C, provisionally equalling Northern Ireland’s warmest June day on record, a mark set back on 30 June 1976. The Met Office had an amber extreme heat warning in force across the region into Saturday 27 June, and the nights were unusual too: Murlough held 19.1°C overnight, NI’s warmest June night on record.

For context, the Met Office uses a 25°C threshold to define a heatwave in Northern Ireland, lower than the rest of the UK because we so rarely get there. This week we cleared it comfortably. Among the seven towns we model on Solar Today, Armagh peaked at 28.1°C, Newry at 27.9°C and Belfast at 27.2°C, with five of the seven above 25°C. Castlederg, which set the record, is not one of the towns we track, so the true regional peak ran a couple of degrees hotter than our town figures.

What mattered for solar, though, was not the headline temperature. It was the daylight. We are days past the summer solstice, so panels across NI had close to the longest possible window to work in, and the clear skies meant they used most of it. Belfast logged 92.5 hours of sunshine across the week and averaged 64% cloud cover, which by local standards is a remarkably bright run.

Why The Heat Itself Did Not Do The Work

Here is the part that surprises people: solar panels do not actually like heat. Once a panel climbs above about 25°C, its efficiency dips slightly, by roughly 0.3 to 0.4% for every degree warmer. So on the hottest afternoons, the panels were giving up a sliver of performance to the temperature.

The reason output still jumped is that the gain from long, clear, sunny days dwarfs that small loss. Tellingly, 25 June was both the hottest day of the spell and the best day for generation in Belfast, which shows the sunshine, not the heat, was doing the heavy lifting. This is also why NI is a perfectly sensible place for solar despite the cooler climate: our mild summers keep panels nearer their ideal operating temperature than a baking Mediterranean roof, even if we get fewer bright days overall. Our guide on how weather affects solar panel efficiency goes into the detail.

What £45 Of Free Electricity Looks Like

A single week is abstract until you put it against a bill. A typical NI household uses around 8 to 10 kWh of electricity a day, so a normal weekly bill lands somewhere around £19 to £24. This week a Belfast 4kW system generated £45.54 of electricity, comfortably more than a household’s entire weekly usage, with the surplus either stored in a battery or sold back to the grid through an export tariff.

The standout was Thursday 25 June, the brightest day of the week. A 4kW system made 24.9 kWh that day alone, worth £8.34, and a 6kW system produced 37.4 kWh, worth £12.52. To put the week in everyday terms, 204 kWh from a 6kW system is enough to drive a typical electric car around 800 miles, give or take. Whether you capture that value depends on using the power as it is made, storing it in a home battery, or exporting it, but the generation is real.

What A Heatwave Week Should (And Should Not) Tell You

It is tempting to look at a week like this and decide solar is a no-brainer, just as it is tempting to look at a grey week in January and write it off. Both readings are wrong. A heatwave is the top of the range, not the average. The honest case for NI solar rests on the full year, where bright summer weeks like this one balance the quiet stretch from November to February.

If you want to know what your own roof would do across a whole year rather than one hot week, our solar savings calculator gives a personal estimate based on your home, roof orientation, system size, and current electricity bill. It runs on the same NI weather modelling behind these figures, so the numbers stay grounded in local conditions. And if a week like this has you wondering about timing, our guide on the best time to install solar panels works through why waiting for sunny weather is usually the wrong way to decide.

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.