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Solar Water Heating - Payback Time and Performance

Payback Time for a 30 Tube Kit

Our 30 tube kit, if fully installed professionally, will cost around £1,800 (about £1,400 if you install it yourself). The amount of time it takes to recoup this amount of outlay will depend on the price of the fuel which normally heats the water. The more expensive that fuel is the more cost-effective these kits are.

The following sequence of graphs gives the estimated payback time for a number of fuels all based on November 2013 figures. Allowance has been made for fuel price inflation.

  • ButanePayback in 3 years
  • ElectricPayback in 3 years
  • PropanePayback in under 4.5 years
  • GasoilPayback in 6 years
  • Wood PelletsPayback in just over 6 years
  • CoalPayback in 7 years
  • Mains GasPayback in under 8 years
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Vacuum Tube Technology for year-round performance.

Our solar collector panels utilize vacuum tube technology and are designed especially for colder climates such as the UK. However please note that a back-up heat source is always required in the UK. The system will work very well on bright sunny days in the winter, however on dull overcast days there will be very little heat contribution (hence the back-up source will have to make up the deficit on these days).

Below is a graph of a typical day's panel performance.  This was plotted from a day in May 2010.  The system in question comprised a 30 tube collector panel with a 250 litre unvented twin coil cylinder.

Panel Performance


The overnight temperatures of the cylinder from the previous day's solar input were 70°C at the top and 50°C at the bottom.
Showers were taken and washing done between 0700 & 0800hrs, this reduced the bottom of the tank temperature to 25°C as cold water is introduced at the bottom.
The panel temperature reaches 33°C soon after 0800, the pump starts and the bottom of the cylinder starts to be heated.
The cylinder heats steadily throughout the day (sunny with broken cloud conditions throughout the day).  Small amounts of water are drawn from the cylinder throughout the day.  Up to 1400 the top of the tank temp falls slightly, this is due to the convection currents occurring as the bottom is heated, which mix the water and hence the top temperature falls.  The average temperature throughout the whole cylinder is however increasing during this period.

By 1400 the entire cylinder has reached 62°C.

By 1900 the cylinder temp is 75°C throughout.

An interesting example of the cold weather performance of evacuated tube collectors is demonstrated by this series of photos.  These were taken on 29th September 2008 when there was a light covering of snow at an installation in Reading UK.  Even though the panel is covered with snow, it was a bright day and the light could penetrate to the tubes.  The panel temperature rose quickly, but even with the panel temperature at 58°C (at 10:00am) the snow stays un-melted on the tubes. This is testament to the efficiency of the highly insulated vacuum tubes.  Note also that the snow remains on the collector panel after the snow has melted on the adjacent roof. 

At the start of the day the tank temperatures were: Top 48°C, Bottom 38°C.

At 17:00 they were: Top 56
°C, Bottom 46°C (210 litre tank).

  • 08:00Panel Temperature 0°C
  • 09:00Panel Temperature 33°C
  • 09:30Panel Temperature 46°C
  • 09:45Panel Temperature 49°C
  • 10:00Panel Temperature 58°C
  • 11:00Panel Temperature 68°C
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By 12:45 the panel temperature had reached 72°C.

By 17:00 the panel temperature had reached 32°C.

Contact us for more information.

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