Size a hot water tank at 30–50 litres per person per day: 80–120 L for two people, 150–200 L for a family of four on a boiler, and 200–300 L when a heat pump or solar thermal system charges the tank at lower temperature. Households with bathtubs or high-flow showers size to the peak draw, not the daily average.
How much hot water does a household actually use?
A European household draws 30–50 litres of hot water per person per day at 45 °C — 40 L is the standard planning figure. Water heating is the second-largest energy use in EU homes after space heating, at roughly 15% of household energy consumption (Eurostat). Daily totals tell only half the story: a shower mixes 6–9 litres per minute, while a single bathtub fill absorbs 120–150 litres, so the tank must survive the worst hour, not just the average day.
Storage temperature multiplies effective volume. A cylinder held at 60 °C and blended down through a thermostatic mixing valve delivers roughly 1.5 litres of 40 °C shower water per stored litre — the reason a 150 L cylinder comfortably serves four people on a gas boiler.
What size tank suits 2, 4 or 6 people?
Match tank volume to household size and to the heat generator, because generators that store cooler water need more litres for the same comfort:
| Household size | Boiler or electric (60 °C storage) | Heat pump (50–55 °C storage) | Solar thermal combi duty |
|---|---|---|---|
| 1–2 people | 80–120 L | 150–200 L | 150–200 L |
| 3–4 people | 120–200 L | 200–300 L | 300 L |
| 5–6 people | 200–300 L | 300–400 L | 400–500 L |
Round up between sizes, and add a margin where a large bathtub, a rain shower or frequent guests raise the peak draw.
Why do heat pumps and solar thermal need bigger tanks?
Heat pumps store cooler water because their efficiency falls as temperature rises — each additional degree of flow temperature costs roughly 2–3% of heat pump efficiency. Storing domestic hot water at 50–55 °C instead of 65 °C means every litre carries roughly a fifth to a quarter less usable energy, so volume grows 25–40%. Heat pump cylinders also need a generously sized coil heat exchanger: large coil area keeps the condensing temperature low and the reheat fast.
Solar thermal adds a time-shift duty: the collector harvests at midday, the family showers in the evening and the following morning. Sizing at 1.5–2 times daily demand — or 50–100 L per m² of collector — lets the tank carry a sunny day's harvest through 24–36 hours and lifts the achievable solar fraction. Thermosiphon packages such as the Solimpeks TSM series pair 120–350 L tanks with matched collectors on exactly this logic.
What happens if the tank is too big or too small?
An oversized tank wastes energy around the clock: an ErP class C 300 L cylinder loses about 1.7–2.4 kWh per day as standing loss, and slow turnover works against hygiene — stored volume should be exchanged regularly and heated through anti-legionella cycles. An undersized tank fails visibly: the fourth shower runs cold, and a heat pump needs an hour or more to recover, tempting the electric boost heater into action at roughly three times the running cost.
A five-step sizing checklist
- Count regular occupants; add one notional person for weekly guests or home offices with extra usage.
- Identify the peak event: bathtub, back-to-back showers, high-flow rain shower.
- Choose the generator — boiler, heat pump, solar thermal, or a combination.
- Read the matching column in the table above and round up between sizes.
- Check reheat time against the household rhythm — a slow generator needs more storage, not more patience.
Frequently asked questions
Is a 300 litre tank too big for a family of four?
Not with a heat pump or solar thermal — 200–300 L is the recommended band because both store cooler water than a boiler. On a gas boiler alone, 150–200 L is usually the better fit and avoids unnecessary standing losses.
How long does a heat pump take to reheat a hot water tank?
Reheating 200 L by 35 K stores about 8 kWh, so a heat pump delivering 5–6 kW of hot water output needs roughly 1.5 hours. Households with fast evening turnover should size the tank one step larger rather than rely on quick recovery.
What size solar water heater does a family of four need?
A 200–300 L thermosiphon or forced-circulation system with 3–5 m² of collector area covers a four-person family across most of Europe, typically delivering 50–70% of annual hot water demand.
Do I need a bigger tank if I have a bathtub?
Yes. One bathtub fill uses 120–150 L of mixed water — roughly 80–100 L drawn straight from storage. If baths are routine, size at least one step above the shower-only recommendation.