How Many Litres of Hot Water Do I Need Per Person?

How Many Litres of Hot Water Do I Need Per Person? As a rough starting point, you’re probably looking at about 40 to 60 litres of stored hot water per person per day when sizing your domestic hot water cylinder – though to be honest, the actual requirement can vary wildly. A household that’s mostly taking short showers might use less, while big baths, high-flow showers or several bathrooms in the house can crank up the demand to a whole new level. When it comes to an air source heat pump water heater, you’ve got to think about the cylinder’s recovery time, storage temperature and how much usable mixed water it can actually provide – you can’t just size it based on the number of people living there.

For smaller properties, our range begins with the 100 litre wall hung air source heat pump water heater, providing an efficient solution where a larger floor-standing unit may not be required.

For increased domestic hot water storage, see the 200 litre floor-standing heat pump water heater or 300 litre floor-standing heat pump water heater.

For larger properties and commercial applications, explore the 400 litre commercial air source heat pump or 500 litre commercial air source heat pump.

Is 50 litres of hot water per person enough?

For most households, around 50 litres of stored hot water per person is a good starting point – but it’s definitely not a hard and fast rule.

Take a four-person household as an example. You might start out thinking a 200 litre cylinder would be about right. But then you need to check if that’s actually realistic based on how the household uses hot water.

Four people taking short, low-flow showers might not put a huge strain on the system, but a household where people are taking long, high-flow showers all day or regularly filling a large bath is going to need something a lot bigger.

So the important figure here is peak domestic hot water demand, not just the number of people living in the house.

How much water does a shower use?

The amount of water a shower uses depends on a couple of things: the flow rate of the shower, and how long it’s running.

An 8 litre a minute shower for 6 minutes uses about 48 litres of mixed water. Same shower, but for 10 minutes, and that’s around 80 litres. And if you’re lucky enough to have a higher-flow shower delivering 12 litres a minute for 10 minutes, you’re looking at around 120 litres.

Now this doesn’t mean all of that has to come from the hot water cylinder. Normally, shower water is mixed with cold mains water to get the right temperature – usually around 38°C to 40°C.

Stored hot water is not the same as usable hot water

When you’re choosing a hot water cylinder, this is something you need to keep in mind.

Let’s say you’ve got 200 litres of water stored at 60°C, and the cold water coming in is 10°C. If you mix the two to get water at 40°C, then every litre of stored hot water can give you more than one litre of usable mixed water.

For a simple example, 200 litres stored at 60°C could give you around 333 litres of mixed water at 40°C when the cold water is 10°C. But in practice, the actual usable volume will be influenced by all sorts of things like cylinder stratification, heat losses, controls and how cold water enters the cylinder.

So don’t just compare cylinders by their nominal capacity – that can be pretty misleading.

How much hot water does a bath use?

A typical bath can use a lot of hot water – more than a quick shower, definitely.

A bath containing around 90 litres of mixed water represents a pretty big demand. And if you’ve got big baths, that demand is only going to get bigger.

If two baths are likely to be filled in quick succession, or several showers might be taken one after another in the morning, then you need to take that into account when sizing the cylinder.

How much hot water does a family of four need?

A four-person household might initially think about a cylinder in the region of 200 to 250 litres, but there’s no magic number.

Take two households with four occupants. One has a single bathroom and everyone takes short showers. The other has two bathrooms, high-flow showers and a large bath that gets used regularly. They might both have the same number of people, but their peak hot water requirements could be totally different.

Why does this matter with a heat pump water heater?

Correct sizing is especially important with an air source heat pump water heater because heat pumps generally recover stored hot water more slowly than high-output boilers.

The cylinder needs enough capacity to cover the times when demand is high, and allow the heat pump time to replace the energy that’s been used.

If the cylinder is too small, the system might regularly need the electric immersion heater to give it a boost or get the water hot quickly again.

That can increase electricity consumption and reduce the overall benefit of using a heat pump.

Does storing water hotter mean I need a smaller cylinder?

Potentially, yes – because hotter stored water can be mixed with more cold water.

But it’s not always the most efficient solution to try and get more usable hot water by storing the water at a higher temperature.

As the required hot water temperature goes up, the heat pump has to work harder and its Coefficient of Performance (COP) will generally fall. And the standing heat losses from the cylinder can go up too.

It’s usually better to just choose a cylinder capacity that’s right for the job, rather than trying to make do with a smaller one by operating at unnecessarily high temperatures.

So how many litres should I allow per person?

**Around 40 to 60 litres of stored hot water per person is a good rough starting point for domestic cylinder sizing – though it’s always worth checking against the actual hot water use in your property.**check out shower flow rates, shower length, bath size, the number of people you’ve got in the household, storage temperature, incoming cold water temperature & how fast the system can recover.

For a heat pump water heater, the goal isnt just to store a certain amount of water per person – its to have enough hot water to get you through the longest shower or biggest water-using task of the day without starving the heat pump, and still giving it a chance to recover your cylinder afterwards