An average price for an installed system (including a new cylinder) for a family of 4 is around £4-5000, but there will be geographical variations, as labour can be cheaper in some parts of the country. Other costs typically include around £10 per year for electricity, and a service every 4-5 years, costing between £70-150.

Working out how much money you’ll save is complex, but we can give you some rule of thumb guidelines. Savings are calculated in a number of ways:

First, the energy that the system delivers – around 1250kWh per year for a family of 4 is typical. Then you have to work out the value of that 1250kWh in displaced fuel. You won’t be displacing the fuel at 100% efficiency. Your boiler is probably operating at around 75% efficiency when heating water, so if 1250kWh is 75% of the fuel you’re displacing, then you’ll actually displace around 1600kWh of fossil fuels per year.

Then, the difference in heat loss between your old hot water tank and your new super-insulated tank could be 750kWh per year. So now we have a total of 2350kWh of displace fuel per year.

Plus – you could double the life of your boiler, as you’re not going to be using it as much – and maybe not at all for 6 months of the year.

Plus – solar hot water systems are sturdy, and lifetimes of 25-35 years are not uncommon. One thing is for sure – over that sort of time period, the price of fossil fuels is going to rise. In the first decade of the 21st century, the average rise in fossil fuel prices was around 6% per year.

You can check the current price per kWh of peak-rate electricity, off-peak electricity, gas and oil on the website of the Energy Saving Trust. They have a calculator to show how much money you could save – although it doesn’t account for the cylinder losses that you won’t have with your new cylinder, so actually, you’ll save more.

Grants & other incentives

Again, see the Energy Saving Trust for the latest grants.