Solar water heater performance is often presented as a graph, or set of three performance variables. Values may be provided based on gross area, aperture area or absorber area. In Europe, aperture or absorber is often used, in the US, gross area is often used. It doesn't really matter which values is used, as long as you use the correct value. ie. Don't use absorber area when using performance values based on gross area.

To adjust from one to the other, multiply by the size difference. ie. If absorber area = 0.6m2 & gross area = 1.1m2 then (1.1/0.6 = 1.83), so multiply the performance factorsby 1.83 to convert from gross to absorber.

The three performance variables for the Erjinsolar collector a provided by the SPF testing laboratory in Switzerland (SPF report C632LPEN) are as follows (for metric calculations, based on absorber area):

Conversion Factor: h0 = 0.717
Loss Coefficient: a1 = 1.52 W/(m2K)
Loss Coefficient: a2 = 0.0085 W/(m2K2)

As well as the three performance variables shown above, insolation level (G) in Watts/m2, ambient temperatures (Ta) and average manifold temperature (Tm) must be know. These values give the value x, also sometimes presented as T*m, used in the formula below.

(other slightly different forms of this formula are used, but provide the same result) How to use the formula?

Based on the ambient temperature, average manifold temperature and insolation level,
firstly calculate the value for X.

Eg. At 2:40pm; ambient temperature of 25oC (77oF); average water temp [(Tinlet+Texit)/2] of 50oC (122oF); insolation level of 800Watts/m2 (252Btu/ft2).

x = (50-25)/800 = 0.03125

Now enter all the values into the formula:

h(x) = 0.717 - (1.52*0.03125) - (0.0085*800*0.031252)

h(x) = 0.717 - 0.0475 - 0.0066 = 0.663

The solar conversion efficiency for that specific point in time and set of environmental conditions is 66.3%. That is: 66.3% of the energy provided by the sun is actually used to heat the water.

Based on the assumption that those three environmental factors (G, Tm and Ta) are stable for a period of one hour, then 800 x 0.663 = 530.4 Watts of energy per m2 of absorber area will be used to heat the water (168Btu/ft2). 530.4Watts is equivalent to 456kcal, which could heat 100L of water by 4.56oC (20 Gallons by 10.9oF)

Below is a graph showing the performance curves for the Erjinsolar collector at three different insolation levels, from 0 to 80oC delta-t. In most cases the delta-t values will be in the range of 20-50oC, with higher values present for high temperature heating such a for absorption cooling applications, or during very cold weather. Except for when the delta-t is zero, conversion efficiency is dependent on solar insolation levels, with higher insolation yielding greater levels of solar conversion.