A question many people, I'm sure, ask themselves. The best way to find out, is employ a consultant to do the calculations for you, as there are many factors that determine the, finacial, viability that the average "solar seller" won't tell you:

Solar thermal panels should provide most of your hot water from April to September, and make a worthwhile contribution in the months on either side of that period. Outside of that estimates vary depending on who you ask.

The Energy Savings Trust says solar panels will "provide about a third of your hot water needs", the Centre for Alternative Technology puts it at about half and a number of solar panel suppliers promise 60 to 70 per cent of annual hot water usage heated by the sun.

The reality will depend on a variety of factors:

How much interest you take in how the system works and adapt to make the most of the free hot water (ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.

The size of your cylinder makes a difference. Many cylinders only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater.

If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar panels when the heating is turned on.

If you have an electric shower it won’t use your solar hot water.

If your dishwasher and / or washing machine are cold-fill they will still have to heat the water using electricity, and won’t use your solar hot water.

So ultimately, how much hot water do you actually use.

What is your primary heating fuel. Gas is much cheaper than oil, for example, so the savings will vary considerably.

How efficient is your current boiler. A modern condensing boiler running at 90% efficiency, will save less than a non condensing boiler, running at, say 60%, efficiency.

The efficiency of the panels/tubes you propose to use. There can be huge differences here.

Research commissioned by Viridian Solar confirmed the significance of the household’s behaviour on performance. Its panels installed in six housing association properties were monitored by the Building Research Establishment (BRE) over 12 months. Energy savings in heating hot water varied from a lowest level of 26 per cent to a highest of 70 per cent. The average was 50 per cent, which is an average saving of 1,200 kWh of energy per year.

These are just some of the factors that need to be considered. The reality is that solar sellers won't ask these questions, or indeed know what to do with the answers.

In my view, it pays to have someone impartial, who doesn't have to sell the product to get payed, to advise you on your situation. They may even suggest another option, such as solar PV. With ROC's doubling in April, there is every chance that the energy companies may review their current tariffs, and the possibilty of a "feed in tariff", may make PV a more viable option, financially.