NI Solar Generation, September 2026: Modelled Output for Seven Towns

September is the first month I have written up on its own. Until now the monthly figures from this site’s daily solar tracker went straight into the 2026 generation record, and that page stays the long view. This one is the month in close-up, for anyone holding their inverter app up against it.

Every figure here is modelled from recorded weather, not read from real panels. The tracker takes the measured daily radiation for each town and works out what a well-sited, south-facing 4kW or 6kW system would have produced from it. The method is set out on Solar Today and in full on the 2026 record.

The data is complete. All 30 September days are in the tracker for all seven towns, and every one is based on historical weather rather than a forecast. I took the figures from the site’s public monthly data file and the 210 daily town pages on 1 October 2026, and the daily pages add up to the monthly file to the decimal.

September 2026 by town

Town	4kW kWh	6kW kWh	4kW per day	Average sunshine hours a day
Belfast	217.6	326.0	7.25	4.77
Newry	217.3	325.7	7.24	4.91
Armagh	202.5	303.9	6.75	4.18
Lisburn	197.6	296.2	6.59	3.96
Derry	194.1	291.6	6.47	3.83
Enniskillen	192.8	289.3	6.43	3.99
Ballymena	177.7	267.4	5.92	3.46

Belfast and Newry finished 0.3 kWh apart, which is a tie in any practical sense. Ballymena was the dullest town in the sample by sunshine hours and came last by about 18 per cent against Belfast.

One thing the month did not do is fade towards the end. In every town the second half of September (16 to 30) produced at least as much as the first half. Belfast made 108.3 kWh on a 4kW system from 1 to 15 September and 109.3 kWh from 16 to 30 September. The shorter days were there, but a run of clearer weather late in the month made up for them.

Best and worst days

Town	Best day	4kW kWh	6kW kWh	Worst day	4kW kWh	6kW kWh
Belfast	Fri 4 Sep	13.3	19.9	Tue 29 Sep	2.3	3.5
Ballymena	Fri 4 Sep	12.0	18.0	Fri 18 Sep	2.0	3.1
Lisburn	Sun 20 Sep	11.4	17.1	Fri 18 Sep	2.0	3.0
Derry	Fri 4 Sep	11.1	16.6	Thu 24 Sep	2.4	3.6
Newry	Sun 20 Sep	10.6	15.9	Fri 18 Sep	2.4	3.7
Armagh	Sun 20 Sep	10.0	15.0	Fri 18 Sep	2.0	3.0
Enniskillen	Sun 20 Sep	9.9	14.9	Fri 18 Sep	1.2	1.9

Friday 18 September was the worst day in five of the seven towns, with no recorded sunshine in any of those five. Enniskillen’s 1.2 kWh that day was the lowest single figure in the month.

The gap between a good and a bad September day was roughly six to one in Belfast, 13.3 kWh against 2.3 kWh. That is the reason a single day’s output tells you very little about whether a system is working. A monthly total does.

How September compared with August

Town	August 4kW kWh	September 4kW kWh	Change
Belfast	290.6	217.6	down 25.1%
Derry	261.5	194.1	down 25.8%
Enniskillen	268.7	192.8	down 28.2%
Armagh	284.1	202.5	down 28.7%
Newry	306.4	217.3	down 29.1%
Lisburn	299.0	197.6	down 33.9%
Ballymena	272.0	177.7	down 34.7%

Every town was down by a quarter to a third. In Belfast, September came in at 44 per cent of July’s 489.1 kWh, and still two and a half times February’s 86.1 kWh. October’s figures will go into the 2026 record at the start of November.

Year to date, 24 January to 30 September 2026

The tracker now holds 250 days for each town.

Town	4kW kWh	6kW kWh
Newry	2,485.7	3,729.1
Belfast	2,445.3	3,669.5
Lisburn	2,373.3	3,561.7
Armagh	2,341.1	3,510.7
Enniskillen	2,243.3	3,365.8
Ballymena	2,242.5	3,363.6
Derry	2,213.7	3,321.2

These are flat-plane figures, because the weather feed measures radiation on a horizontal surface. A tilted south-facing roof collects more over a year, which is why our system sizes guide uses about 3,330 kWh a year for a 4kW system in Belfast. Use the tracker for month-to-month comparison and the system sizes guide for annual planning.

What September’s output was worth

I have used the same assumptions as the site’s solar calculator: 45 per cent of the output used in the house without a battery, valued at 33.125p per kWh (the midpoint of Power NI’s 32.10p to 34.15p domestic unit rates including VAT from 1 July 2026), and the other 55 per cent exported.

September’s exports were paid at Power NI’s 9.64p per kWh, the rate for 1 October 2025 to 30 September 2026. From 1 October 2026 the rate is 11.88p, so I have shown both. Rates are from Power NI’s microgeneration tariff page, checked on 1 October 2026.

Town	4kW, export at 9.64p	4kW, export at 11.88p	6kW, export at 9.64p	6kW, export at 11.88p
Belfast	£43.97	£46.65	£65.88	£69.90
Newry	£43.91	£46.59	£65.82	£69.83
Armagh	£40.92	£43.42	£61.41	£65.16
Lisburn	£39.93	£42.37	£59.86	£63.51
Derry	£39.22	£41.62	£58.93	£62.52
Enniskillen	£38.96	£41.34	£58.46	£62.03
Ballymena	£35.91	£38.10	£54.04	£57.33

Worked example for Belfast on 4kW: 217.6 kWh × 45% = 97.9 kWh used at home, worth £32.44 at 33.125p. The other 119.7 kWh exported is worth £11.54 at 9.64p or £14.22 at 11.88p. The higher export rate adds about £2.68 to a September like this one on a 4kW system.

Most of the value still comes from the units used in the house. In the Belfast example those 97.9 kWh are worth more than twice the 119.7 kWh exported, even at the new rate. What the rate rise means across a full year, and for batteries and hot-water diverters, is in our news piece on Power NI’s 11.88p export rate.

Checking your own system

If your system is south facing and unshaded, and your September total is more than about 15 per cent below the town figure here once you allow for system size, it is worth a look. Soiling, new shading or a string that has dropped out are the usual causes. If your roof faces east or west, expect less than these figures.

If you are still at the stage of deciding whether solar is worth it on your roof, compare quotes from participating NI installers.

Figures checked on 1 October 2026 from comparesolarni.com/solar-today/monthly.csv and the 210 September daily pages on Solar Today. Model: Open-Meteo daily shortwave radiation × system size × performance ratio 0.78. Export rates from Power NI’s microgeneration tariff page, checked 1 October 2026. Import rate midpoint from Power NI domestic tariffs effective 1 July 2026.