Solar collector sizing for domestic hot water
Free solar collector calculator for domestic hot water: square metres, store volume and the solar fraction month by month, on ΤΟΤΕΕ 20701-3 climate data.
How many m² of collectors and how much storage for domestic hot water, and the share the sun covers each month — with your own certificate coefficients.
Solar collector sizing for domestic hot water — method and FAQ
How many m² of solar collectors a dwelling needs
The “one square metre per person” rule gives a number, not an answer: it ignores where the building is, at what tilt and orientation the collectors are mounted, how cold the mains water is in January, and what kind of collector is installed. The tool starts from the demand and the location and ends at the area that achieves the coverage you ask for.
Solar radiation is taken on the collector plane, not on the horizontal. Annex B of ΤΟΤΕΕ 20701-3 publishes the mean monthly irradiation on tilted surfaces for 48 ΕΜΥ stations, at 45 and 90 degrees and in five orientations. In Athens a south-facing plane at 45 degrees receives 1,749 kWh per square metre a year, against 1,637 on the horizontal and 1,155 on the vertical — the tilt is worth having, and the standard gives it in print.
With the coefficients from your own certificate
Every EN 12975 or Solar Keymark certificate prints three numbers: the zero-loss coefficient n₀ and the heat loss coefficients a₁ and a₂. The tool takes them as they are and calculates with the product that will be installed, not with a generic type. Anyone without a certificate to hand uses the three generic collectors of ΤΟΤΕΕ 20701-1 Table 5.10 — flat plate, selective and evacuated tube.
The area the coefficients refer to is asked for explicitly and is not pre-selected. On an evacuated tube collector the gross area can be twice the aperture, so the wrong choice doubles or halves every result that comes out of that type.
The method, and what it does not do
Sizing is done with the f-chart method of Klein, Beckman and Duffie — one of the methods named explicitly in §6.3.1 of ΤΟΤΕΕ 20701-1, alongside hourly simulation and EN 15316-4-3. The monthly mains water temperature is taken from Table 2.6 of 20701-1, which is the text that defines the method for domestic hot water.
The method assumes an installation with forced circulation and a heat exchanger. Greek domestic solar systems are as a rule thermosiphon, and on those the answer is indicative — the tool says so on every calculation instead of passing over it. It also says when the area is so large that the method falls outside its range, and when the tilt you asked for is not printed in the standard and was reached by interpolation.
Frequently asked questions
How many square metres of solar collectors do I need for four people?
It depends on the location, the tilt and the type of collector. For four people in Athens, with a selective collector facing south at a tilt of 45 degrees and 60 litres of storage per square metre, three square metres cover 73 % of the annual demand and four cover 83 %. In Thessaloniki the same four-square-metre installation drops to 75 %, and in January the difference is larger. The tool sizes the area for the coverage you set.
What tilt should solar collectors have?
For year-round use, §6.3.1.1 of ΤΟΤΕΕ 20701-1 gives a tilt roughly equal to the site latitude, that is 35 to 41 degrees for Greece, facing south within five degrees. 45 degrees is a printed column of Annex B and gives the largest annual total at all 49 stations — but only among the three tilts the Annex prints (0, 45 and 90 degrees). The true optimum under the latitude rule is lower, around 35 to 41, and the Annex does not publish it; intermediate tilts come out of interpolation and are marked as such. A steeper tilt favours winter, a shallower one summer.
How much storage is needed per square metre of collector?
Common installations sit between 50 and 75 litres per square metre. The volume correction in the method holds from 37.5 to 300 litres per square metre. Outside that range the tool still gives a number, but it computes it as though you were at the nearest limit, and it tells you so: 60 and 150 litres on four square metres give exactly the same percentage, because both are read as 37.5 per square metre. Do not compare them with each other — correct the size. A smaller store means more surplus energy dumped in summer, a larger one means greater storage losses.
Related tools
- Expansion vessel sizing — EN 12828 sizing for heating, solar and DHW — with glycol, pre-charge pressure and a safety-valve cross-check.
- Climate data for Greek locations — Design temperatures, degree days and monthly climate data to ΤΟΤΕΕ 20701-3, for 62 ΕΜΥ stations — with a two-location comparison.
- Heat pump sizing for a dwelling — Heat loss, cooling load and required capacity — always with the operating conditions stated, so quotations become comparable.