Size a multi-bathroom property to the peak hour, not the daily total: allow roughly 50 L of stored hot water per simultaneous shower and 80–100 L per bathtub. A three-to-four-bathroom house typically needs 300–500 L, guesthouses and small hotels 500–1,000 L — or smaller storage with a high recovery rate. Manufacturers including Solimpeks build storage tanks to just over 1,000 L for this duty.
Why does peak draw, not daily use, size a multi-bathroom cylinder?
Because simultaneity, not volume, is what empties a tank. A family house and a six-bedroom guesthouse may use similar hot water per person per day, but the guesthouse fires all its showers in the same 60–90 minutes. Each shower mixes 6–9 litres per minute at 40 °C; from 60 °C storage that draws roughly 5 litres of stored hot water per minute, so a 10-minute shower consumes about 50 stored litres and three simultaneous showers drain 150 L in ten minutes. A bathtub absorbs 80–100 stored litres in one event. The design question is therefore: what is the worst hour, and can storage plus reheat survive it? The per-person daily method in what size hot water tank do I need still sets the floor, but the peak sets the answer.
How much storage do typical properties need?
| Property | Typical peak scenario | Suggested storage at 60 °C |
|---|---|---|
| 2-bathroom family home | 2 showers + basin draw | 200–300 L |
| 3–4 bathroom house / HMO | 3 showers back-to-back, occasional bath | 300–500 L |
| Guesthouse, 6–10 rooms | 4–6 showers in the morning hour | 500–800 L |
| Small hotel, 10–20 rooms | 8+ showers, kitchen and laundry load | 800–1,000+ L or storage + plate heat exchanger |
Commonly used planning figures for hotels run on the order of 70–130 litres of hot water per occupied room per day, with 30–50% of it drawn in the morning peak — numbers of the kind tabulated in CIBSE Guide G and EN 12831-3. Round up between bands, and remember that a thermostatic mixing valve on 60 °C storage stretches every stored litre to roughly 1.5 litres at shower temperature.
Does recovery rate matter more than volume?
At this scale, yes — storage and reheat power are interchangeable currencies. A 500 L tank with a powerful coil can outperform an 800 L tank with a weak one, because it rebuilds its hot layer between waves of demand; the trade-off is defined in what cylinder recovery rate means. The arithmetic is simple: reheating 500 L by 40 K stores about 23 kWh, so a 24 kW coil restores a fully drawn tank in roughly an hour, while an 8 kW heat source needs three. Hotels with a morning and an evening peak need full recovery inside the quiet midday window. Generous coil surface area is what turns installed boiler or heat pump kilowatts into actual recovery speed.
What about hygiene at large stored volumes?
Storing hundreds of litres of potable water raises the stakes on legionella management: large tanks need maintained temperatures, scheduled pasteurisation cycles and turnover discipline, and commercial operators typically carry a written water-safety regime. The engineering alternative is to store heating water instead of drinking water: a hygienic combination tank or an external plate heat exchanger heats drinking water instantaneously on demand, so no potable water sits in storage at all. Solimpeks builds both configurations — large enamel storage tanks and hygienic fresh-water combination stores to just over 1,000 L — and the hygienic route is increasingly the default for hotels and multi-let properties.
Is a big hot water load a solar opportunity?
Usually the best one in the building. A property that reliably consumes hundreds of litres of hot water every day of the year is the ideal solar thermal customer: the load is constant, summer demand is real (guests shower in July), and the storage already exists. That is why hotels dominate commercial solar thermal economics — the payback case, with collector areas and storage ratios, is worked through in commercial solar thermal payback for hotels and factories. Sizing the store at 50–100 L per m² of collector lets the same cylinder do double duty as solar buffer and DHW supply, and a multi-port tank fitted now keeps that option open even if collectors come later.
Frequently asked questions
How many litres of hot water does a hotel room need per day?
Planning figures commonly run 70–130 litres of hot water per occupied room per day, with roughly a third to a half drawn in the morning peak hour. En-suite bathtubs and spa showers push demand to the top of the band.
Can I use two smaller cylinders instead of one large tank?
Yes — twin tanks in series preserve stratification and add redundancy, and doorways sometimes force the choice. A single large tank loses less heat per stored litre, so prefer one tank where access and headroom allow it.
Is 300 litres enough for a 3-bathroom house?
For showers only, usually yes on a boiler at 60 °C storage. Add regular bathtub use or a heat pump storing at 50–55 °C and 400–500 L becomes the safer specification — or 300 L with a high recovery rate.
Do small hotels need storage at all, or can they heat water instantaneously?
Pure instantaneous heating needs very large peak power, so most small hotels combine a storage buffer with a fast heat exchanger. A hygienic combination tank gives the peak-riding benefit of storage without keeping potable water stored.