What size water heater tank does your home actually need

How to judge capacity by number of people

As a rough starting point, allow 30 to 40 litres of hot water per person per day for someone who showers rather than baths. A household of two showering in the morning is usually comfortable with 80 litres, since the tank has all day to recover before evening use. A household of four in the same routine tends to need 100 to 150 litres, or a shorter gap between showers than the tank can refill.

Baths change the sum considerably. A full bath uses roughly 120 to 150 litres of hot water on its own, so a household that baths rather than showers needs a tank sized to one bath's worth at minimum, plus washing-up and any other draw happening around the same time.

Two adults in a flat or small house who shower and rarely bath usually manage well on 80 litres. A family of four to five, especially with teenagers who shower back to back in the evening, is often better served by 150 litres than by hoping a smaller tank recovers fast enough between them.

Why recovery time matters as much as size

A 150-litre tank on a weak electrical supply can take longer to reheat than an 80-litre tank on a stronger one, and that recovery time is what decides whether the second or third person in the household gets a warm shower or a lukewarm one. The heating element's power, not just the tank's volume, sets how quickly hot water comes back.

Most domestic storage heaters use a 2 to 3 kW element. On a 2 kW element, a full reheat of an 80-litre tank from cold takes a few hours; a 150-litre tank on the same element takes noticeably longer. If the household showers in sequence rather than all at once, a smaller tank with a capable element can outperform a large tank that heats slowly.

For a house where hot water is needed at short notice throughout the day, a mid-sized tank with a stronger element is often a better trade-off than the largest tank that will physically fit. This is where an electrician's opinion on what the existing circuit can carry is worth having before you buy, rather than after.

How much space and electrical supply the heater needs

A storage water heater needs a dedicated space with room for the tank itself, clearance for the pressure relief valve and its drain, and access for future servicing, such as replacing an anode rod. As a guide, allow roughly 40 to 60 centimetres more height than the tank itself where a vertical unit is wall-mounted, and check the floor can bear the weight of a large tank full of water if it stands on a shelf or in a cupboard.

Electrically, tanks above roughly 100 litres with a 3 kW element usually need their own circuit and, in older houses, may need the consumer unit checked for spare capacity. Panel flats and older wooden houses in areas such as Kalamaja and Nõmme were not always wired with a dedicated water heater circuit in mind, and adding one is electrical work, not plumbing.

Before buying a tank larger than what is currently installed, it is worth checking whether the existing circuit and breaker can take the new element's load. Our electricians can check this and add a dedicated circuit where needed, from 49 € per hour, and our plumbers can advise on the tank's physical fit and connections, from 45 € per hour.

• Allow clearance above and around the tank for future anode or element replacement
• Check the floor or wall fixing can bear the full weight of the tank filled with water
• Confirm the existing circuit can carry the new element's load before buying a bigger tank
• Fit a drip tray or drain point if the heater sits somewhere a leak would cause damage
• Leave access to the pressure relief valve and its discharge pipe unobstructed

What capacity suits a summer cottage or seasonal use

A summer cottage used mainly at weekends by two or three people rarely needs more than 50 to 80 litres, since the heater is switched off between visits and reheats from cold each time regardless of size. A smaller tank reheats faster on arrival, which matters more than having a large reserve nobody stays long enough to use.

Seasonal properties also need to consider what happens over winter when the water is not drained. A tank left full and unheated in an unheated building is a freezing risk, and draining it down before leaving for the season is worth doing regardless of size.

If the cottage occasionally hosts more people for a weekend, it is usually more sensible to accept slower recovery on those occasions than to install a tank sized for peak use that sits mostly empty of demand the rest of the season.

When the advertised capacity is not what actually matters

Retailers tend to lead with litres because it is the easiest number to compare on a shelf, but the element's power, the tank's insulation quality, and how the household actually uses hot water decide whether a given size feels adequate. Two 100-litre tanks from different manufacturers can perform quite differently in practice.

Energy label and standing heat loss are worth a glance if compared, since a poorly insulated tank loses heat between uses and works its element harder to compensate, regardless of how many litres it holds. This shows up as a slightly higher bill more than as any dramatic difference in comfort.

The soundest approach is to size the tank to the household's actual pattern of use, check the element power against that pattern, and confirm the electrical supply can support it, rather than choosing the largest tank the budget allows and assuming bigger is simply better.