Solar thermal, step by step

1. How much hot water you need

Before the collector, work out how much hot water your house really uses.

1. The heater on your bill — For a family of four the water heater uses 5–9 kWh a day. That's a job the sun can take over from April to October.
2. Shower and bath — A 5-minute shower: 40–50 L of hot water. One bath of 100–150 L is like three showers.
3. 50 litres per person — For the house count on 50 L of hot water at 45 °C (113 °F) per person a day (about 13 US gallons), with showers, washing up and cleaning.
4. How much energy that is — Water from a well or the mains is 10–12 °C (50–54 °F); up to 45 °C that's a 35 °C difference: 50 L × 35 °C × 1.16 Wh = 2 kWh a day, roughly 750 kWh a year per person.

Why: In many houses the immersion heater or electric water heater is the biggest user of electricity, roughly a quarter to a third of the bill. And hot water is the easiest job for the sun.

• If you keep livestock and process milk, add another 20–50 L of hot water a day.
• A cylinder also loses 0.5–1.5 kWh a day, more if it's poorly insulated or in a cold room.

2. Collector and cylinder

A collector is basically a black box under glass. What matters is what kind it is and where it sits.

1. Flat-plate collector — A 2 m² panel under toughened glass, with a black absorber and copper pipes. In the UK and Ireland it gives roughly 300–450 kWh per m² a year, and snow slides off the warm glass.
2. Evacuated tubes — A row of glass tubes with a vacuum: 400–550 kWh per m² and better in the cold. In summer they go over 200 °C, and single tubes break from hail.
3. South, no shade — Face the collector south, with no shade. For hot water all year, a tilt of 45–60° gives a more even output and less overheating in summer.
4. How much per person — In lowland UK and Ireland, 1.2–1.6 m² of flat-plate collector and 50–80 L of cylinder per person. For four: about 5.6 m² and a 300 L cylinder, and the sun covers about 55 % a year (in most of the US, 60–80 %).

Why: In the UK and Ireland, and in most of the US, a flat-plate collector usually gives the best value for hot water from April to October. Evacuated tubes give more in spring, autumn and winter, but overheat the system faster in summer.

• More collectors isn't always better: too many collectors for a small cylinder means the system boils every day in summer.
• For summer use only, a tilt of 30–40° is enough.

3. Thermosiphon or pump · Locked

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4. Frost and overheating · Locked

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5. Legionella and one cylinder · Locked

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6. Summer shower and maintenance · Locked

Buy the Solar thermal and hot water module and you get the PDF to download and print, plus lifetime access to this interactive lesson: all 6 chapters, 24 steps.