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Collector Power Outputs

The power output of any solar hot water panel depends on a number of factors.  The size and efficiency of the panel, the level of insolation (how sunny it is) and the temperature of the water flowing through the collector manifold all play a part.

Our Eco-58-1800-20 panel has been tested in Germany for the EN12975 standard, and the power output determined.  It was tested at three levels of insolation, 400W/m2,  700W/m2 and 1000W/m2, representing cloud, partial cloud and full sun.  It was also tested for three levels of water temperature.  The water temperature is the average of the input and output temperature.  The figure used in the test is the difference between this average water temperature and the ambient (air) temperature, known as tm-ta.  For example, if water entered the manifold at 35C and exited at 45C, and the ambient temperature was 30C, tm-ta would be 10.  The test gave the following figures for output in Watts:



tm-ta 400W/m2 700W/m2 1000W/m2
10 393 718 1045
30 302 627 952
50 198 523 848

Since our other panels use exactly the same technology in glass, coatings, heat transfer methods etc, and differ only in tube lengths, diameters and number, it is reasonable to extrapolate from these results for our other panel/tube sizes.



tm-ta 400W/m2 700W/m2 1000W/m2
10 590 1077 1568
30 453 940 1428
50 297 785 1272

Eco-47-1500-12 (extrapolated result)


tm-ta 400W/m2 700W/m2 1000W/m2
10 159 290 422
30 122 254 385
50 80 211 343

Eco-47-1500-20 (extrapolated result)


tm-ta 400W/m2 700W/m2 1000W/m2
10 265 484 704
30 203 423 642
50 133 352 571

To calculate how much hot water can be provided by these power outputs, recall that water has a specific heat capacity of 4.186kJ/kgK.  A Joule is a measure of energy, and 1 Watt (a measure of power) is equal to 1 Joule per second, or 1Js-1.  The specific heat capacity is a measure of the amount of energy required to raise the temperature of a substance, in this case water, a certain amount.  This figure says it is necessary to supply 4,186 Joules to raise the temperature of 1kg of water (which also happens to be 1 litre) by one degree Kelvin (which also happens to be one degree Centigrade).  Joules are quite small.  One Watt supplied for one hour will supply 3600 Joules of energy.  So the Eco 58-1800-20, on a partially cloudy day with water in the collector on average 30C hotter than ambient where the output is 627W, would, in ten hours, increase the temperature of 200litres of water by (627 x 3600 x 10)/(4186 x 200), or 26.96C.  In practice, the efficiency would be higher at the beginning when the water in the hot water tank was cold, and reduce as the circulating water got hotter.

Note that these calculations ignore losses from feed and return pipes.  These losses will not be great provided the pipes are well insulated with suitable insulating material.

A related question which is often asked is how much energy can I collect per year?  Two unrelated Eco-nomical installations in the West Midlands with Resol monitoring equipment attached to them recorded figures of around 35kWh per tube for 2008, which I would guess was an average to below average year for solar insolation.  See here and here for more details.

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Send mail to SW@eco-nomical.co.uk with questions or comments about this web site.
Last modified: 30-06-10