Show the formula and working
This solar diverter calculator shows how much a solar immersion diverter saves by sending surplus solar power into your hot water cylinder instead of the grid, for homes in the UK, Ireland and Europe. Enter how much you export, how much of it the diverter can use, how you normally heat water, your tariffs and the fitted price, and it shows the net yearly saving and the payback time.
Quick answer: A solar diverter saves the cost of the heat it replaces but loses the export payment for that energy. The net saving is diverted kWh times your heat cost per kWh minus your export rate. Replacing an electric immersion at 26.32p with 12p export, 1,200 kWh diverted saves about £172 a year. Replacing gas at 12p export it loses money.
What Is a Solar Diverter Calculator?
A solar diverter calculator is a tool that compares the value of using surplus solar power to heat water with the value of exporting it.
The formula in plain words: the diverter can only use part of your export, limited by the share it can capture and by how much heat your hot water actually needs. Each diverted kWh replaces heat you would otherwise buy: for gas, that is the gas unit rate divided by the boiler efficiency; for an electric immersion, it is your electricity unit rate. Each diverted kWh also loses your export rate. The difference times the kWh diverted is the net saving, and the fitted price divided by that saving is the simple payback.
How Do You Use the Solar Diverter Calculator?
- Enter how much solar you export in a year.
- Enter the share of that export the diverter can use and your yearly hot water heat need.
- Choose how your water is normally heated.
- Enter your gas rate and boiler efficiency, or your electricity rate for an immersion.
- Enter your export rate and the fitted price of the diverter.
- Read the net yearly saving and the payback time.
Is a Solar Diverter Worth It?
A solar diverter is worth it when the heat it replaces costs more than your export rate pays: almost always if you heat water with an electric immersion, but often not if you heat it with gas and have a good export tariff.
| Normally heated by | Export rate | Heat cost replaced | Net yearly saving | Payback on £500 |
|---|---|---|---|---|
| Electric immersion | 4.1p | 26.32p | £266.64 | 1.9 years |
| Electric immersion | 12p | 26.32p | £171.84 | 2.9 years |
| Electric immersion | 15p | 26.32p | £135.84 | 3.7 years |
| Gas boiler | none | 9.38p | £112.52 | 4.4 years |
| Gas boiler | 4.1p | 9.38p | £63.32 | 7.9 years |
| Gas boiler | 12p | 9.38p | minus £31.48 | never |
The key number is the heat cost per kWh. Gas at 7.97p through an 85% efficient boiler costs about 9.38p per kWh of hot water, so a 12p export rate is worth more than the gas a diverter saves. Before the Smart Export Guarantee, many homes got nothing for exports, which is why diverters became popular. Check today’s rates on the Ofgem energy price cap page and your own export tariff.
How Much Solar Can a Diverter Use?
A diverter can only use surplus solar while the cylinder still has room for heat, so a full tank on a sunny afternoon sends the rest back to the grid.
That is why the calculator caps diverted energy at your yearly hot water need and asks for a share of export the diverter can capture. Summer surplus is large but hot water demand is steady, so much summer export still goes to the grid, while winter surplus is small. Your diverter’s app, after a few months, is the best guide to the real share. The solar export calculator shows how much you export in the first place.
Diverter or Battery: Which Is Better?
A diverter costs far less than a battery and pays back faster when it replaces immersion heating, but a battery saves your full electricity rate on every kWh it delivers.
At 26.32p import and 12p export, each kWh a battery delivers is worth about 12.8p after losses, while each kWh a diverter sends to an immersion is worth 14.3p and each kWh replacing gas is worth minus 2.6p. Compare the two with the solar battery savings calculator.
Worked Example: Gas Boiler Home on a 4.1p Export Tariff
Example: a home heats its water with a gas boiler and has a cylinder with an immersion heater. It exports 2,000 kWh a year on a 4.1p export tariff, and a diverter can capture about 60% of that.
- Diverted: 2,000 x 60% = 1,200 kWh, within the 2,500 kWh hot water need
- Gas heat cost: 7.97p / 0.85 = 9.38p per kWh of hot water
- Heating bill saved: 1,200 x 9.38p = £112.52
- Export income given up: 1,200 x 4.1p = £49.20
- Net saving: £63.32 a year, so a £500 diverter pays back in 7.9 years
If the same home switched to a 12p export tariff, the diverter would lose about £31 a year, so it would be better to turn diversion off and export instead.
Diverter Safety and Hot Water Temperature
A diverter needs a hot water cylinder with an immersion heater, and it must be fitted by a qualified electrician because it controls a mains appliance.
Water stored at lukewarm temperatures can let Legionella bacteria grow, so cylinders should still be heated to a safe temperature regularly. The Health and Safety Executive guidance on Legionella explains the risk. Many diverters include a boost timer so the boiler or immersion tops the tank up if the sun has not done it. Combination boilers have no cylinder, so a diverter does not suit them.
Solar Diverter Calculator FAQs
How much does a solar diverter save a year?
It depends on what it replaces. Diverting 1,200 kWh into an electric immersion saves about £172 a year with a 12p export rate. Replacing gas with the same export rate loses about £31.
Is a solar diverter worth it with gas central heating?
Only if your export rate is lower than your gas heat cost per kWh, about 9.38p at 7.97p gas and 85% efficiency. On a 12p export tariff, exporting pays more than diverting.
Can I use a diverter with a combi boiler?
No. A diverter heats water stored in a cylinder through an immersion heater. Combination boilers heat water on demand and have no cylinder to store solar heat.
Does a diverter work in winter?
It works, but there is little surplus solar in winter, so most of the saving comes from spring to autumn. Your boiler or immersion still heats the water on dull days.
Diverter or battery: which pays back faster?
A diverter usually pays back faster because it costs much less. A battery saves more per kWh but costs thousands, so check both with your own rates.
Do I need an electrician to fit a solar diverter?
Yes. A diverter connects to your consumer unit and immersion heater, so it should be installed by a qualified electrician.
Checked October 2026. Sources: Ofgem energy price cap, GOV.UK Smart Export Guarantee guidance, Health and Safety Executive Legionella guidance.