Solar Water Heater Size for a Family of 4 (and Why Counting Tubes Isn’t Enough)

How much hot water your home uses

The guide from SEAI (the Irish government’s energy agency) takes an average of 40 litres of hot water per person per day, and that figure decides the tank size. It’s an average: a home with long showers will use more, and one where few people are home during the day, less.

SEAI sizes the tank for 1 to 2 days of use: that way the water the sun heats one day is stored for the next, in case it’s cloudy.

How big a collector: area, not tubes

The same guide gives these areas per person:

• Evacuated tubes: 0.7 to 1 m² per person (about 7.5 to 11 sq ft).
• Flat plate: 1 to 1.5 m² per person (about 11 to 16 sq ft).

And it warns: look at the aperture area (the part that catches light) in m², not the number of tubes. One tube can be longer or wider than another, so “18 tubes” from one brand aren’t the same as “18 tubes” from another. The area is on the collector’s datasheet.

Evacuated tubes need less area because, according to SEAI, they give about 20% more per m² than flat plates. That doesn’t make one better than the other: a flat plate can be built into the roof and the difference is made up with more area, while tubes suit tight roofs.

Worked example: a family of 4

1. Hot water per day: 4 × 40 = 160 litres (42 gallons).
2. Tank for 1 to 2 days: 160 to 320 litres. In its own example, SEAI goes for 2 days: 320 litres (85 gallons).
3. Evacuated-tube collector: 4 × 0.7 = 2.8 to 4 × 1 = 4 m² (about 30 to 43 sq ft).
4. Or flat plate: 4 × 1 = 4 to 4 × 1.5 = 6 m² (about 43 to 65 sq ft).

SEAI’s example is a home with the collector facing south, no shade and a 45° tilt. If your roof is shaded or faces another way, you need a bit more area.

How much electricity or gas it saves

Heating water takes a lot of energy. The math: it takes 1 BTU to raise one pound of water by 1 °F, and 1 kWh is 3,412 BTU (both figures from the US Energy Information Administration, EIA). Say the water comes in at 15 °C (59 °F) and you want it at 50 °C (122 °F):

1. 160 litres of water weigh 352.7 pounds.
2. From 59 to 122 °F is 63 °F.
3. 352.7 × 63 = 22,220 BTU, and 22,220 ÷ 3,412 = 6.5 kWh a day, at the very least (not counting what’s lost in the pipes and the tank).

A 1,500 W electric water heater would take more than 4 hours a day to supply that. The sun doesn’t provide it all year: in Ireland, according to SEAI, a home system usually covers 50 to 60% of the year’s hot water. In sunnier places, more; but that figure has to be worked out by the installer for your home. To see what your electric water heater uses now, check the appliance energy use chart.

Where to put it

According to SEAI, the best is facing south (in the northern hemisphere; north in the southern) at a 30 to 45° tilt. Turning it southeast or southwest only costs about 5%, and even east or west can work. Shade from trees or buildings does cost a lot: it has to be taken into account in the design.

Common mistakes

• Buying by number of tubes. SEAI says to look at the aperture area in m², not the number of tubes.
• A tank that’s too small. If it can’t hold at least a day’s water, the sun’s heat goes to waste.
• Leaving out the mixing valve. Water from the tank can come out very hot. SEAI says a thermostatic mixing valve must be fitted at the outlet, blending in cold water so it doesn’t scald at the shower and taps.
• Expecting it to cover everything. On cloudy or winter days you’ll need your usual water heater.

When to call an installer

Always, for the installation. A solar water heater has pipework at high pressure and temperature, an expansion vessel and safety valves, according to SEAI. Ask the installer for the collector’s aperture area, the tank size and what share of the year’s hot water they expect it to provide.