In short: a tankless water heater suits a guesthouse where each bathroom gets its own unit and the electricity supply can carry it. A storage water heater wins when several showers run in the same hour, because the tank banks heat overnight.
At 7:40 on a full Saturday, six guests turn on six showers within twenty minutes of each other. Whether the last of them gets hot water depends on one decision made months earlier, during the plumbing: a tankless water heater that makes hot water as it flows, or a storage tank that made it overnight and is now spending it.
Both work in a guesthouse. They fail in different ways, and the failure you can live with decides the choice. An instant water heater never runs out but can run weak when its power is spread thin. A tank delivers a strong flow until it's empty and then needs time to recover.
Tankless vs storage water heater at a glance
| Question | Tankless (instant) | Storage tank |
|---|---|---|
| How it heats | Heats water in the moment it flows through | Heats a tank in advance and holds it hot |
| Runs out? | No, but flow and temperature are capped by power | Yes, when the tank empties; then it recovers |
| Power draw | High, only while a tap is open | Low and long; can charge overnight |
| Space | A small box on the bathroom wall | A tank plus a cupboard or plant room |
| Heat losses | Almost none on standby | Standing losses through the tank walls |
| Upkeep | Descaling in hard-water areas | Anode, relief valve, temperature checks |
| Best fit | Two to four rooms, one unit per en-suite | Six rooms and up, or a shared morning peak |
Why does the morning peak decide the heater?
A guesthouse doesn't use hot water evenly. Most of the day's showers land in a ninety-minute window before breakfast, and the heater has to be sized for that window, not for the daily total.
Start with the shower itself. A normal shower head passes somewhere around 7 to 10 liters a minute, and an eight-minute shower at 7.5 liters a minute uses about 60 liters of water at 40 °C. Multiply that by the guests who will shower in the same hour and you have the load you're really buying for.
A tankless unit meets that load one bathroom at a time. Each unit only has to keep up with one shower, so six en-suites with six units never compete with each other for a tank. A central storage water heater meets it from a bank of heat it built overnight, so the question becomes whether the bank is big enough to last until the last guest is dressed.
How much power does an electric tankless water heater need?
More than most people expect, because it has to heat water in the few seconds the water spends inside it. The physics is fixed: raising one liter per minute by 25 °C takes about 1.75 kW.
That gives a quick way to read any spec sheet. A unit rated at 8.5 kW, like the one below, lifts roughly 4.9 liters a minute from 15 °C mains water to a 40 °C shower. In winter, when the mains arrives at 8 or 10 °C, the same unit manages closer to 4 liters a minute at that temperature. That's a usable shower with an eco head, and a thin one with a rain head.
The catalog lists it as wall-mounted and IPX4-rated, with a touch panel, an LED temperature display, overheating protection and a 6 mm² power cable. That last line matters more than the others: a heater of this size wants its own circuit from the distribution board, and an electrician should confirm it before you order one per bathroom.
How big should a storage water heater be?
Tanks are sized by how many showers they can supply before they need to recover. Stored water sits at 60 °C or above, and mixed down to 40 °C at the shower, one liter of tank water stretches to about 1.8 liters of shower water. So a 60-liter shower draws roughly 35 liters from the tank.
Recovery is the other half. Reheating 80 liters from 15 °C to 60 °C takes about 4.2 kWh of energy, which a 2.2 kW element delivers in close to two hours. A tank that empties at 7:30 will not be full again before the late risers get up, which is why tanks are sized for the whole peak rather than topped up during it.
Keep the stored temperature at 60 °C or higher. UK guidance on Legionella control (the HSE's L8 code) asks for exactly that, and most other markets say something similar. Where guests could touch water that hot, a thermostatic mixing valve before the tap is the usual fix.
Water heater size by number of rooms
The table below works from the numbers above: about 60 liters per shower, 1.5 showers per room in the morning window, a tank held at 60 °C. Treat it as a starting point for your plumber, not as a specification.
| Rooms | Morning showers | Tankless route | Storage route |
|---|---|---|---|
| 2–4 | 3–6 | One unit per en-suite; most practical here | One tank around 150–200 L |
| 5–8 | 8–12 | Works only if the supply carries several units at once | A tank around 300–400 L, or two smaller tanks |
| 9–12 | 14–18 | Rarely practical on one building supply | Around 500–650 L across one or two tanks |
| Owner's flat | 1–3 | A single unit keeps it off the guest system | A small separate tank |
The split in the last row is worth copying. When the owner lives on site, a separate heater for the private flat means a family bath at 8:00 never competes with a guest's shower.
When a tankless water heater is the right call
- Two to four rooms, each with its own bathroom, in a building where every bathroom can get a dedicated circuit.
- No room for a tank. A converted house with no loft space or cupboard to spare often decides this on its own.
- Seasonal or irregular occupancy. A tank that keeps 200 liters hot for an empty house in February pays for heat nobody uses; an instant unit uses nothing while its tap is closed.
- An annex or garden room far from the main plumbing, where a long hot-water pipe would waste water and heat on every use.
When a storage tank is the right call
Choose storage when the peak is shared. Six rooms or more, a building on a supply that can't take several 8 kW loads together, or showers with high-flow heads all point the same way. A tank also draws its power slowly and can charge on a cheaper overnight tariff where one exists.
Storage makes sense in cold climates too, where a winter inlet temperature cuts what any instant water heater can deliver. The tank doesn't care what temperature the mains arrives at; it only takes a little longer to recover.
Supply, space and upkeep
Electricity supply. Add up the heaters that could run at the same moment. Three 8.5 kW units running together draw more than 25 kW, which a converted house on a single-phase supply may not be able to give. Your electrician or network operator can tell you what the building takes; the answer sometimes rules out tankless before anything else does.
Space. An instant unit fits on a bathroom wall. A storage tank needs a cupboard, a floor that can take its weight when full, a relief-valve discharge pipe, and access for servicing, and the bigger commercial tanks stand on the floor rather than hanging on a wall.
Maintenance. Hard water is the enemy of both. Scale on an instant unit's element cuts its output; on a tank it settles as sludge and eats the anode faster. If the local water leaves white crust on your kettle, a filter or softener on the incoming line protects every heater downstream, and the water purification range covers that side. Tanks also want a yearly look at the anode and the temperature-and-pressure relief valve.
Mixing is allowed. Plenty of guesthouses run a central tank for the main building and a tankless unit in one remote room or in the owner's flat. The full water heaters collection lists what's available, and our guide to hotel supplies beyond the bed covers the rest of what a room needs behind the walls.
Not sure which way your building points? Send us the room count, the number of bathrooms and what your electrician says the supply can carry, and we'll suggest a heater setup to match.
