Heating
What size water heater do you need (liters)?
Storage water heater size in liters from household size, hot-water habits and heater type (electric or heat pump), with standard tank sizes.
Quick answer
A storage water heater is sized in liters from how many people use it and their hot-water habits, not by adding up every shower. A useful rule of thumb: about 20 liters of base plus 24 liters per person for normal daily showers — so a family of four lands around 115 liters and a 120-liter tank fits. The reason it isn’t "50 liters × 4 people = 200" is that the tank reheats between uses: daily heating quietly refills it, so a well-sized 100-120 liter tank covers a typical family.
Recommended tank size
100 L
- Estimated need
- ~92 L
- Back-to-back showers it covers
- ~3
Related: planning the whole system? Compare heating fuel costs
These are honest hobby ranges, not a plumber’s spec. A storage tank reheats between uses, so you need far less than "liters per shower × people" — daily reheating does the rest. Heat pump water heaters are sized larger because they reheat slowly, but use about 3× less energy. For an instantaneous (tankless) heater the number that matters is flow (kW / liters per minute), not tank liters. Oversize by one step if you have a bathtub or teenagers; undersize and you get cold showers at peak hour.
How it works
Two choices change the number. Type: an electric storage heater is cheap to buy and sized as above, while a heat pump water heater is taken one size larger (it reheats slowly) but uses about three times less energy — worth it if you have space and use a lot of hot water. And habits: a bathtub or two teenagers taking long showers pushes you up a size. Undersize and you run out of hot water at the morning peak; oversizing by one step is cheap insurance, oversizing by three is just paying to heat water you never use.
One setting matters more than the litres, and lowering it to save money is the wrong move. Below 50 °C a storage tank becomes a comfortable place for legionella to multiply; above 60 °C limescale forms quickly and the tap water can scald. The working compromise is 55-60 °C, and mixing cold at the tap rather than turning the tank down. On a storage heater the saving from a lower setting is small anyway — the standing loss depends on the gap between tank and room, and a few degrees change it very little.
Most people arrive here with the question the other way round: not how many litres do I need, but I already have a 80-litre tank — how many showers is that? The answer is not the tank volume, because you never shower at 60 °C: the hot water is mixed with cold at the tap, so one litre in the tank becomes roughly two litres of water at 38 °C. Against that, only about three quarters of a tank comes out hot before the temperature starts falling. The two effects together give a multiplier of about 1.5.
| Tank | Water at 38 °C | Comfortable showers (6 min) | Quick showers (4 min) |
|---|---|---|---|
| 50 L | ~75 L | 1 | 2 |
| 80 L | ~115 L | 2 | 3 |
| 100 L | ~145 L | 2–3 | 4 |
| 120 L | ~175 L | 3 | 5 |
| 150 L | ~220 L | 4 | 6–7 |
Those figures assume a tank at 60 °C, mains water at 15 °C, and a shower head using about 9 litres a minute. Change any of the three and the table moves: an old head at 12 L/min removes roughly a third of the showers, and turning the thermostat down from 60 °C to 55 °C costs about a tenth of the hot water — worth knowing, because that is the same setting that keeps limescale and legionella in check. It is also why two households with identical tanks give completely different answers to the same question.
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Frequently asked questions
What size water heater for a family of 4?+
For four people with normal daily showers, a 100-120 liter electric storage tank is the sweet spot. Drop to 80 liters if showers are short and water-saving; go to 150 liters if you have a bathtub in regular use or teenagers taking long showers. A heat pump version would be taken one size up (120-150 liters) because it reheats more slowly.
Electric storage or heat pump water heater?+
A plain electric tank is cheap to buy but the most expensive to run — it turns 1 kWh of electricity into 1 kWh of heat. A heat pump water heater costs more up front and needs a ventilated space, but moves about 3 kWh of heat per kWh of electricity, cutting running costs roughly two-thirds. Rule of thumb: high hot-water use with space available → heat pump pays back; occasional use or a tight cupboard → simple electric tank.
How many showers does an 80-liter tank give?+
Roughly two back-to-back showers before it needs to reheat. A typical shower draws about 40 liters of hot water once mixed with cold, so 80 liters covers two people in a row; then the heater takes 1-2 hours to bring a full tank back to temperature. This is why sizing is about your peak hour, not the whole day — the tank refills itself the rest of the time.
Storage tank or instantaneous (tankless) heater?+
A storage tank keeps a reserve of hot water ready and is sized in liters; an instantaneous (tankless) heater makes hot water on demand and is sized in power (kW) and flow (liters per minute). Tankless never runs out and wastes nothing standing by, but needs a big electrical or gas supply to heat water fast, and can struggle to feed two taps at once. For a single bathroom with modest use it’s neat; for a family with simultaneous demand, a properly sized storage tank is usually simpler and cheaper.
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