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Sizing for Household Use

4 min read

Size a solar water heater by working from the heat you need to the collector area that supplies it: energy needed = litres × 4.18 kJ/kg·K × temperature rise, then area = that energy ÷ (local daily sun × collector efficiency). For four people using 40 L each at a 30 K rise, that is about 5.6 kWh a day, so 2.0 to 4.6 square metres of collector depending on whether the local sun is 7 or 3 kWh/m² a day.

Part of Batch Water Heaters

Sizing a solar heater means matching a household’s daily hot-water heat to the sun on a given collector area, with the units kept straight at every step.

Work Backwards From The Need

The units matter here: litres of water, kilojoules of heat, kilowatt-hours of sun. The chain is volume → mass → heat → sun → area.

Inputs, assumed for the example: 4 people; 40 L of hot water each per day; cold supply at 15°C and delivery at 45°C, so ΔT = 30 K; collector efficiency 40% over a day for a plain glazed collector; 1 kWh = 3,600 kJ.

  1. Volume per household = 4 × 40 = 160 L = 160 kg of water.
  2. Heat needed = 160 × 4.18 × 30 = 20,064 kJ = 20,064 ÷ 3,600 = 5.57 kWh per day, about 5.6 kWh.
  3. Area = heat ÷ (sun per m² per day × efficiency):
    • 7 kWh/m² per day: 5.57 ÷ (7 × 0.40) = 1.99 m².
    • 5 kWh/m² per day: 5.57 ÷ (5 × 0.40) = 2.79 m².
    • 3 kWh/m² per day: 5.57 ÷ (3 × 0.40) = 4.64 m².

Rounded, the household needs 2.0 to 4.6 m² in all. Per person that is 0.50, 0.70 and 1.16 m². The parent page’s “1-2 m² per person” sits above these figures, a generous allowance; its other line, 1-2 m² with a 100-200 L tank giving 40-80 L, matches the same chain: 1.5 m² × 5 × 0.40 = 3.0 kWh = 10,800 kJ, and 10,800 ÷ (4.18 × 30) = 86 L at a 30 K rise.

Size Storage And Mind Summer

Storage should hold about a day’s use: 160-200 L for this household, in line with the parent page’s 100-200 L tank. Less storage than a day’s use wastes afternoon sun; far more storage dilutes the heat and gives lukewarm water (Batch Water Heaters shows the litres-per-square-metre rule).

Size for the weakest month you rely on, but remember the strongest: a collector sized for winter overproduces in summer, and the surplus makes the system sit near boiling. A bigger array means more stagnation days, so vent it properly (Plumbing Connections), and consider covering part of it in summer.

What You Need

  • A count of people and a measured daily hot-water use, by buckets — 40 L is an assumption, not a norm
  • A local daily sun figure from measurement (Insolation & Angle)
  • A thermometer for the cold and delivered temperatures
  • Collector area and tank from the chain above

Method: Size It

Step 1: Measure a normal day’s hot water by the bucket for a week.

Step 2: Pick the temperature rise you need; subtract the cold supply temperature from your delivery temperature.

Step 3: Compute daily heat = litres × 4.18 × ΔT, in kJ, then divide by 3,600 for kWh.

Step 4: Divide by (sun × efficiency) for the weakest month you will rely on. Round up.

Step 5: Choose storage near one day’s use and check the tank-placement weight (Tank Placement).

Common Mistakes

  • Mixing kW and kWh. A collector’s power at noon is not its daily energy.
  • Quoting one efficiency for all temperatures. Efficiency falls as the water gets hotter and as the day gets cooler.
  • Using the summer sun for the winter month you need most.
  • Counting water at tank temperature rather than delivered temperature. Hot tank water is mixed with cold, so a smaller volume of hotter water suffices.

Safety

An oversized array overheats, and the water is hotter than anyone expects

A collector big enough for winter can stagnate above boiling in summer; give it a vent or relief path and keep the discharge away from people. Tank water at 60°C or more can scald in a few seconds, and children and the elderly are hurt at lower temperatures and shorter exposures than adults; temper the delivered water (Tank Placement).

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