Facts & Figures

How much energy is available from the sun?

The following table shows the approximate amount of solar energy available by month, per day, in kWh (kilowatt-hours), for an “average” domestic installation; it can be seen that the amount of available energy in mid-summer is about 10 times that available in mid-winter; hence the main benefit of a solar panel is naturally in the summer months. A solar panel can capture about 90% of the incident solar energy, which is very efficient – in contrast to solar panels to generate electricity (solar PV), which are far more expensive – per square metre – and far less efficient (10% – 20%)

kWh  JAN  FEB  MAR  APR  MAY  JUN JUL  AUG  SEP  OCT  NOV  DEC
40*58mm 1.6 3 5.5 9 12 12 12 10 7.5 4 2.2 1.2

By way of comparison, an immersion heater might be rated at 3kW i.e. the peak (mid-summer) output of the solar panel above is equivalent to an immersion heater being on for around four hours – which is more than enough to heat a hot water cylinder

What size of solar panel should I have for the size of cylinder I need?

This table gives an idea of the number of tubes appropriate for common cylinder sizes, in optimal conditions (see also notes below):

Cylinder size Litres 150 200 300 500
Number of tubes 47mm 30 40 60 100
58mm 20 30 40 70

The above sizes are chosen to ensure that there will be enough hot water during the summer season to meet almost all demand i.e. without using alternative sources of water heating; while a larger panel area would extend the “summer” season, to do so would 1) cost more 2) take up more space on the roof

58 mm tubes have now replaced 47 mm tubes

Note also the following factors:

  • Other things being equal, it is better to have one rather than two panels, or two rather than three etc.
  • Choose a larger panel if the panel tilt or orientation is not optimal, if the panel is at all shaded or if there is a particularly long pipe run
  • A larger system is slightly more efficient than a smaller system, so should be sized accordingly
  • A larger panel will improve performance outside the summer months

How much will it cost to install?

The major component costs are the panels and the hot water cylinder. The cost of cylinders varies considerably, from as little as around €500 for a small copper cylinder to over €1000 for a large stainless steel cylinder. The total cost of a system is usually between €4000 and €5000;  this price is reduced by about €1000 – €1500 if no cylinder replacement is needed. Prices are VAT inclusive.

Under certain circumstances, a cylinder swap can be avoided by using an EasyFit Solar Coil, which uses the immersion heater tapping to provide a dual immersion / solar coil; the solar coil reaches to the lower part of the cylinder, like a normal dual coil cylinder; this is suitable for smaller cylinders (150l or less), and can significantly reduce the cost of a smaller system.

The cost of solar on a new house (or major refurbishment) will usually be less than a retrofit as the incremental cost of a solar compatible cylinder over a normal cylinder is not great and the labour of swapping a cylinder is avoided. For these reasons it is advisable to fit a solar compatible cylinder if replacing or fitting a cylinder for any other reason; this significantly reduces the cost of fitting solar panels at a later time. Note that the SEAI grant is not available for new houses.

How much will it cost to run?

Very little. The only regular cost is the electricity to run the pump, which runs intermittently, when heat energy is being collected, and the controller, which consumes only a few Watts. A 40W pump is used, similar to a central heating pump, which is equivalent to a rather dim (non-CFL) light bulb.

Recently (2018), European Regulations have required the use of Pulse-width Modulation (PWM) water pumps, which will further reduce pump energy consumption. More information, from Resol.

How much will it save me?

A solar water heating system can be expected to cater for at least half the hot water demand of a house. The cost of hot water is normally considered to be about 1/3 of the total cost of heating and hot water; this will of course depend on the actual hot water demand and energy performance of the house. If the house is well insulated, the energy requirement for heating will be less and the hot water will represent a larger proportion of the total cost.

The “return on investment” is difficult to estimate; the usual figure is from around 5 years. An investment in solar is almost entirely an up-front cost, as the running costs are negligible.

Using a central heating boiler to heat water in the summer is actually less efficient than the energy figures might suggest, as a boiler requires energy to heat itself initially and even if the water heating system is de-coupled from the radiators, which is not always the case, heat commonly “leaks” between the water and space heating circuits. Using an immersion avoids these inefficiencies, but uses the most expensive form of energy: electricity.

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