Solar Panels in an Oil-Heated Home: The Northern Ireland Maths

The numbers this page uses (checked on 23 September 2026)

Input	Figure	Source
Heating oil, 900 litres	£1,007.16 (111.9p a litre), 17 Sep 2026	Consumer Council weekly survey
Heating oil, August	£778.15 (86.5p a litre), 13 Aug 2026	Consumer Council archive
Energy in kerosene	10.3 kWh per litre	UK government fuel properties (10.29 gross)
Oil boiler efficiency	85% (and 70% as a lower case)	My assumption, explained below
Electricity	32.10p to 34.15p per kWh	Power NI domestic tariffs from 1 Jul 2026
Export payment	11.88p per kWh, 1 Oct 2026 to 30 Sep 2027 (checked 1 Oct 2026)	Power NI tariff rates

I reworked the maths on this page on 23 September 2026, after the Consumer Council’s 17 September survey put 900 litres of heating oil at £1,007.16. The answer is less flattering to solar than most sites will tell you: in an oil-heated home, a solar diverter earns about 1p for every unit it diverts at Power NI’s export rate from 1 October 2026, against about 20p in an all-electric home. At August’s oil price it would lose money against exporting.

That gap exists because 61% of Northern Ireland homes heat with oil (NISRA, 2024/25), and nearly every solar guide you will read was written for Great Britain, where hot water comes from electricity or mains gas. Here it usually comes from an oil boiler. That changes what surplus solar power is worth.

For the weekly oil price and how to buy it for less, see heating oil prices in Northern Ireland. This page is about what solar does and does not do once you are on oil.

Step 1: what a useful kWh of oil heat costs

A litre of kerosene holds about 10.3 kWh of energy. Your boiler does not turn all of it into hot water; some goes up the flue and some is lost from the pipes. I use 85% as the main case, which is about what a modern condensing oil boiler in good order achieves. Many NI boilers are older than that, and a boiler firing in summer just to heat a cylinder runs in short bursts and does worse, so I also show 70% as a lower case. I have not measured either figure; they are assumptions, and you can see below how much they move the answer.

At 85%, one litre gives about 8.75 kWh of useful heat. At 70%, about 7.2 kWh.

Consumer Council survey	900 litres	Pence per litre	Useful heat cost at 85%	Useful heat cost at 70%
18 Sep 2025	£472.65	52.5p	6.0p per kWh	7.3p per kWh
2 Jul 2026	£615.23	68.4p	7.8p per kWh	9.5p per kWh
13 Aug 2026	£778.15	86.5p	9.9p per kWh	12.0p per kWh
17 Sep 2026	£1,007.16	111.9p	12.8p per kWh	15.5p per kWh
9 Apr 2026 (2026 peak)	£1,088.71	121.0p	13.8p per kWh	16.8p per kWh

Compare that with electricity at 32.10p to 34.15p. Even at £1,007 for 900 litres, oil heat costs well under half of direct electric heat. That is the fact every oil-home calculation has to start from.

Step 2: what a diverted unit is actually worth

A solar diverter (an Eddi or similar) watches your meter. When the panels make more than the house is using, it sends the surplus into the immersion heater in your hot water cylinder instead of letting it flow out to the grid.

The saving is two things netted off:

• What you gain: the oil you no longer burn to heat that water. An immersion turns electricity into heat inside the cylinder at close to 100%, so one diverted kWh replaces one useful kWh of oil heat.
• What you give up: the export payment on that kWh. Power NI pays 11.88p per kWh from 1 October 2026 to 30 September 2027, up from 9.64p the year before. Northern Ireland has no Smart Export Guarantee; each supplier sets its own rate, and Power NI’s resets each October. I checked the new rate on 1 October 2026.

Oil price (900 litres)	Oil heat saved per diverted kWh (85%)	Minus export given up	Net gain per kWh (85%)	Net gain per kWh (70%)
£615.23 (2 Jul 2026)	7.8p	11.88p	−4.1p	−2.4p
£778.15 (13 Aug 2026)	9.9p	11.88p	−2.0p	0.1p
£1,007.16 (17 Sep 2026)	12.8p	11.88p	0.9p	3.6p
£1,088.71 (9 Apr 2026)	13.8p	11.88p	1.9p	4.9p
All-electric home, Power NI 32.10p	32.10p	11.88p	20.2p	20.2p

Read the first two rows again. At the July and August prices, with a decent boiler, diverting surplus into the cylinder loses money compared with exporting it.

The break-even oil price. With an 85% boiler and an 11.88p export rate, a diverter only beats exporting when oil costs more than about £1.04 a litre, or £935 for 900 litres. With a 70% boiler the line drops to about 86p a litre, or £770 for 900 litres. August sat just above the second line and below the first. September is above both, but only just above the first.

A worked year

How much surplus a diverter actually uses depends on your cylinder, your hot water use and the weather, so I will not pretend to a precise figure. I use 1,000 kWh a year as a round working number because it is easy to scale: if your diverter moves 1,500 kWh, multiply by 1.5.

The timing matters too. My solar tracker averaged the output of a 4kW system across seven NI towns: 3.06 kWh a day in February 2026, 11.01 in April, 14.74 in July, 9.14 in August and 6.57 in the first 22 days of September. A diverter does nearly all its work from April to September. In winter there is little surplus to divert, and that is when the boiler is on for the heating anyway.

Home	Oil at £778.15 (Aug)	Oil at £1,007.16 (17 Sep)
Oil, 85% boiler	about −£20 a year (exporting pays more)	about £9 a year
Oil, 70% boiler	about £1 a year	about £36 a year
All-electric (immersion on Power NI at 32.10p)	about £200 a year	about £200 a year

The all-electric figure does not move with oil, and it is more than twenty times the oil-home figure even at September’s price. When a salesperson quotes you a diverter saving, ask which of these two homes the number was worked out for.

When a diverter still makes sense in an oil home

• Your boiler is old or fires just for hot water in summer. The lower the real efficiency, the more oil each diverted unit saves. At 70% the gain roughly doubles.
• Oil stays above about £935 for 900 litres. Below that, a good boiler heats water more cheaply than the export payment you give up.
• The export rate falls. Every penny off the export rate is a penny added to the value of each diverted unit. If it rises, the opposite: Power NI’s went from 9.64p to 11.88p on 1 October 2026, which took about 2p off every diverted unit.
• You are not being paid for export at all. Then the full oil cost of the heat is your gain.
• You want the boiler off in summer. Some people value not firing the boiler from May to September for its own sake. That is a fair reason; just do not mistake it for a large saving.

It does not make sense if you have a combi boiler and no cylinder, because there is nothing to divert into. How diverters work, and what they need, is in our immersion diverter guide.

What solar is worth in an oil home: the electricity side

None of the above touches the main reason to fit solar, which has nothing to do with oil. Every kWh of solar power you use in the house saves what you would have paid for it: 32.10p to 34.15p per kWh on Power NI’s domestic tariffs from 1 July 2026, and up to 42.01p on the dearest standard tariff in the Consumer Council’s comparison table. That is nearly three times the export rate, so an oil home gets most of its solar value from running the washing machine, dishwasher and kettle in daylight, and from a battery if it has one, not from the diverter.

Of the 394 quote requests this site received between 12 March and 22 September 2026, 262 came from households with electricity bills of £100 to £250 a month. At those bills, the electricity saving is what pays for the panels. If that is your bill, comparing a few installers’ prices is the sensible next step. For the payback maths, see are solar panels worth it in Northern Ireland.

The heat pump crossover

If you want to stop buying oil, the replacement is a heat pump, not solar panels. Whether it saves money to run comes down to two numbers: the oil price and the heat pump’s seasonal efficiency (how many kWh of heat it makes per kWh of electricity over a year, usually called SCOP).

At Power NI’s 32.10p:

Heat pump seasonal efficiency	Cost per kWh of heat	Oil price where they are level (85% boiler)
2.5	12.8p	112p a litre, about £1,012 for 900 litres
3.0	10.7p	94p a litre, about £843 for 900 litres
3.5	9.2p	80p a litre, about £723 for 900 litres

Put in household terms: 900 litres of oil in an 85% boiler gives about 7,880 kWh of heat. A heat pump at an efficiency of 3 would use about 2,630 kWh of electricity to make the same heat, costing about £843 at 32.10p. On 17 September 2026 the oil cost £1,007.16, so the heat pump was about £164 cheaper to run for that heat. On 13 August the oil cost £778.15, and the heat pump was about £65 dearer. The crossover sits right in the range oil has moved through this year.

That comparison leaves out the upfront cost of the heat pump, any radiator changes and servicing on both sides, so it tells you about running costs only. If your oil boiler is near the end of its life, the fair comparison is heat pump against a new oil boiler, not against keeping the old one.

Solar helps a heat pump less than people expect. A heat pump does most of its work from November to March, and my tracker’s 4kW system averaged 3.06 kWh a day in February. Solar covers a good share of a heat pump’s hot water load in summer and very little of its heating load in winter. The full comparison is in solar panels vs heat pumps, and what grant help exists is on our heat pump grants page.

Few people asking this site for quotes are there yet: 37 of those 394 requests mentioned a heat pump.

Help with the cost

No grant in Northern Ireland pays for domestic solar panels. Solar and heat pumps carry 0% VAT until 31 March 2027. The Affordable Warmth Scheme pays for insulation and heating upgrades for households with an income under £23,000; ring NI Energy Advice on 0800 111 4455. The £100 Home Heating Oil Support Scheme, open from 9 September 2026 to 31 March 2027, helps with the oil itself; the details are on our oil price page.

The short version

At £1,007 for 900 litres, oil heat costs about 12.8p per kWh. A diverter in an oil home saves that, minus the 11.88p export payment it gives up, so about 1p per unit. At August’s price it would lose money against exporting. Solar’s value in an oil home is on the electricity bill. And if you want off oil, a heat pump runs cheaper than oil whenever 900 litres costs more than about £843, assuming it reaches a seasonal efficiency of 3.

These tables need rerunning when Power NI publishes its new export rate in October, and whenever the oil price moves far enough to change the answer. The date at the top tells you when I last did.