Solar PV Hot Water Diverters: How Much Can They Really Save You Per Year?

If you have solar panels, there are times when your roof makes more electricity than the house is using. Without anything else in place, that surplus goes out to the grid, and you get paid a few pence per kWh for it under the Smart Export Guarantee (SEG).

A solar PV diverter sends that surplus into your hot water cylinder instead. It is one of the cheapest add-ons you can fit to a solar system. But how much it saves depends on how you heat your water now and what you are paid for export. Here is the honest picture for homes around Spalding and South Lincolnshire.

What is a solar PV diverter?

A diverter is a small control unit wired between your consumer unit and the immersion heater in your hot water cylinder. Well-known examples include the Marlec iBoost and the myenergi eddi, and there are several others that work the same way.

It has a sensor (a CT clamp) on your incoming supply cable that watches whether you are importing or exporting electricity, second by second.

How it works with your immersion heater

Your panels generate more than the house is using, so power starts to flow out to the grid
The diverter spots the export and sends that spare power to the immersion heater instead
It varies the power to match the surplus. If you only have 800 W spare, the immersion gets about 800 W, not its full 3 kW
When the kettle goes on or a cloud passes over, it backs off straight away so you do not import from the grid
Once the cylinder reaches temperature, the immersion thermostat cuts out and any surplus goes back to export

In effect, your cylinder becomes a heat battery. Spare solar in the afternoon becomes hot water for the evening.

Most diverters also have a boost button or timer, so you can top the water up on a cheap overnight tariff in winter if you want.

Who a diverter suits

A diverter is a good fit if:

You have a hot water cylinder with an immersion heater (or room to fit one)
Your solar system regularly exports a good amount, typically 3 kWp or larger, with the house empty for part of the day
You currently heat water with electricity (immersion, Economy 7, off-grid homes), where the saving is biggest
You are on a low SEG export rate

It is less useful, or not worth it, if:

You have a combi boiler. There is no cylinder to heat, so a standard diverter has nothing to work with
You have a heat pump. A heat pump makes hot water at around three units of heat per unit of electricity. A diverter's immersion makes one unit of heat per unit. Using surplus solar to run the heat pump itself is often better value, so this needs looking at case by case
You are on a high export rate. If you are paid close to what the diverted electricity would save you, the gain is small or even negative (see the example below)
You already have a battery that soaks up most of your surplus. A diverter can still mop up what is left after the battery is full in summer

Diverter vs battery

A battery stores surplus electricity and gives it back as electricity, which is worth the full unit rate (around 26p per kWh under the October to December 2026 price cap). It can power anything in the house and can also charge on cheap overnight rates.

A diverter only makes hot water, but it costs a fraction of the price of a battery. Many homes end up with both: the battery fills first, then the diverter takes what is left.

How much can a diverter save per year?

The figure you save depends on three things:

How many kWh get diverted. This is limited by your surplus and by how much hot water you actually use. A full cylinder cannot take any more
What it replaces. Electric immersion heating costs around 26.32p per kWh. Gas costs about 7.97p per kWh, and a boiler loses some of that, so diverting into a gas-heated home saves far less
What export income you give up. Every kWh diverted is a kWh you are no longer paid for under the SEG. Rates range from a few pence up to around 15p per kWh depending on your supplier

A worked example

Assumptions (clearly simple, for guidance):

A typical 4 kWp system in Lincolnshire, with about 1,200 kWh a year diverted into the cylinder. This is mostly March to October, and is roughly what a normal family cylinder can absorb
Electricity at 26.32p/kWh and gas at 7.97p/kWh (Ofgem price cap averages, 1 October to 31 December 2026)
Gas boiler heating water at 85% efficiency, so 1,200 kWh of hot water would have taken about 1,410 kWh of gas
Lost export valued at either 4p/kWh (a low SEG rate) or 15p/kWh (one of the better SEG rates)
Cylinder heat losses ignored, since the cylinder is kept hot either way

If you heat water with an electric immersion now:

Electricity no longer bought: 1,200 x 26.32p = about £316
On a 4p SEG rate, you lose about £48 of export, so the net saving is about £268 a year
On a 15p SEG rate, you lose about £180 of export, so the net saving is about £136 a year

If you heat water with a gas boiler now:

Gas no longer bought: 1,410 x 7.97p = about £112
On a 4p SEG rate, you lose about £48 of export, so the net saving is about £64 a year
On a 15p SEG rate, you lose about £180 of export, so you could be about £68 a year worse off

The honest range

For most homes, a realistic net saving is somewhere between nothing and about £100 a year with gas, and roughly £130 to £270 a year with electric water heating. Bigger systems, bigger families and lower export rates push you towards the top. Combi boilers, high export rates and small surpluses push you towards the bottom.

That sits in line with independent guidance. The Renewable Energy Consumer Code (RECC) suggests a best case of around £300 a year replacing electricity, or around £90 replacing gas. The Energy and Utilities Alliance estimated £90 to £240 a year. Manufacturers sometimes quote higher figures. Treat those as claims rather than guarantees.

There is also a smaller benefit that is harder to price. In summer, many gas-heated homes can leave the boiler off for hot water for weeks at a time, which saves the boiler short-cycling just to heat a cylinder.

Tips to get the most from a diverter

Check your export rate first. If you are on a high SEG tariff and heat water with gas, a diverter may not be worth it
Use the bottom element if you have two. Heating the whole cylinder lets it soak up more surplus
Set the boiler timer for hot water later in the day, so the sun gets the first chance to heat it
Insulate the cylinder properly so the free heat lasts into the evening
Make sure the immersion is in good condition. An old or failing element will waste the diverter's work

Diverter installation in Spalding and South Lincolnshire

PPM Electrical Ltd fits solar PV diverters to new and existing solar systems across Spalding and South Lincolnshire. We are NAPIT registered and MCS certified (MCS number NAP/68477/24/4), and we will check your cylinder, immersion and export rate first so you know whether a diverter, a battery or both makes sense before you spend anything.

Find out more about our solar PV installations, call 07714 736921, email info@ppmelectricalspalding.co.uk, or get in touch.

Savings depend on your solar system, hot water use, cylinder, tariff and export rate. Figures above are worked examples using Ofgem price cap averages for 1 October to 31 December 2026, not a quote. Sources: Ofgem price cap rates, RECC guidance on add-on equipment, Energy and Utilities Alliance.