How to Estimate Hot Water Use

What you need

Your household's daily routine — how many showers, baths, basin uses, dishwasher cycles and washing machine cycles — and the hot water usage calculator. The calculator uses standard assumptions for each use, which are stated on the page: a shower at 9 litres per minute, a bath at 80 litres, a basin use at 5 litres, a dishwasher cycle at 15 litres and a washing machine cycle at 50 litres.

Step 1 — Count the showers

Multiply the number of showers by the minutes and by the flow rate: showers × minutes × litres per minute. The standard flow is 9 litres per minute, but check your own showerhead — a power shower can use far more, and a low-flow head far less. Two 8-minute showers at 9 litres a minute use 144 litres, which is nearly half of a four-person household's daily hot water.

Step 2 — Add baths and basins

A bath is 80 litres and a basin use is 5 litres, by the calculator's assumptions. Count the baths and the basin uses for the day — hand-washing, teeth-brushing, shaving — and add them. They look small next to a shower, but a household of four uses the basin many times a day, and it adds up.

Step 3 — Add the appliances

A dishwasher cycle uses 15 litres of hot water and a washing machine cycle 50 litres, by the calculator's assumptions. Count the cycles per day — not per week — because the system must supply the water on the day it is used. Cold-fill appliances use less hot water than hot-fill ones; check how yours are connected.

Step 4 — Total it and divide by the household

The hot water usage calculator adds it all up and divides by the household size. For a household of four — two showers, one bath, three basin uses, one dishwasher cycle and one washer cycle — the total is 304 litres a day (80.31 US gallons), which is 76 litres per person. That total is what the tank or heater must supply.

Step 5 — Use the total to size the system

Compare the daily total with the tank size using the water tank calculator: a 200-litre cylinder cannot supply a 304-litre day without reheating, and a 500-litre one heats water that is never used. Size the system to the total, and check the recovery time — how fast the heater replaces what was used — before you choose.

Worked example

Two showers × 8 min × 9 L/min = 144 L. One bath × 80 L = 80 L. Three basin uses × 5 L = 15 L. Appliances: 1 × 15 L + 1 × 50 L = 65 L. The total is 144 + 80 + 15 + 65 = 304 L (80.31 US gal), and for a household of four that is 304 ÷ 4 = 76 L per person. Those are exactly the numbers the hot water usage calculator gives you.

Common mistakes

• Forgetting the appliances. A dishwasher and a washing machine together use 65 litres a day in the example — a fifth of the total. Leave them out and the system runs short.
• Using the wrong shower flow rate. The standard 9 litres a minute is an assumption, not a fact about your shower. Check the actual flow before you size anything.
• Sizing for one person when the household is four. The per-person figure is useful, but the system must supply the whole household's total, not one person's share.

Safety

Hot water scalds: water above about 50°C burns skin quickly, and children and older people are hurt worst. Store hot water at 60°C (140°F) — hot enough to stop legionella bacteria growing — and use a thermostatic mixing valve so the water at the tap is cooler, around 50°C or below. Never turn a cylinder thermostat down to save energy: lukewarm stored water is where legionella grows. Plumbing work, including cylinders and mixing valves, must be done by a qualified plumber.

Try these calculators

• Hot Water Usage Calculator — daily hot water in litres and gallons.
• Water Tank Calculator — capacity in litres, gallons and cubic metres.
• Water Flow Calculator — flow rate from pipe size and velocity.