How much hot water do you actually use?

A little maths, so you do not have to guess

When you are about to choose a water heater, the question comes up quickly: how much hot water do I actually use? The answer determines how much power output you need, and it is easier to calculate than people think. It all comes together in a single figure:

Heating one litre of water from 8 to 65 degrees costs approximately 0.07 kWh.Coax, own calculation

That is the anchor figure we use throughout these articles. With it we can work out both consumption and cost, without guessing.

How much water goes into what?

Consumption has less to do with the number of people and more to do with what you actually do. Here are typical volumes for common activities:

The figures vary with the tap, the shower head and your habits, but the relative scale is the point: a shower uses a lot, washing your hands almost nothing, and a bath is in a class of its own.

What does it cost?

Let us do the sums for a perfectly ordinary shower. A water-saving shower head gives around 7 litres a minute. Four minutes comes to 28 litres, which has to be heated from cold water up to shower temperature.

The anchor figure above is for heating to 65 degrees, the temperature a conventional water heater has to hold. A flow-through heater heats the water directly to the temperature you use it at. For a shower at 38 degrees the rise is 30 degrees instead of 57, so each litre costs roughly half:

Heating one litre of water from 8 to 38 degrees costs approximately 0.035 kWh.Coax, own calculation

For the shower that works out at roughly:

28 litres × 0.035 kWh is approximately 1 kWh, that is around 1.50 kroner for the shower (at an electricity price of 1.50 kr/kWh including grid rent and duties).

The point is the order of magnitude: a shower costs a krone or two in pure heating. That is not where the big losses are, they are in storage and pipework, which we have written about in What the conventional water heater costs you when you are not using it.

From consumption to power output

Now we come to what actually decides which model you pick. A flow-through heater does not store water, so it has to deliver the heat in real time. That makes it a question not of total litres, but of litres per minute at the same time.

The relationship is simple: the more litres a minute you want, and the colder the incoming water is, the more power output (kW) is needed. A rule of thumb for Norwegian conditions:

• Washing your hands and a simple kitchen: low power output is enough, often on an ordinary 230 V circuit
• A full shower: more is needed here, typically a more powerful model on its own circuit
• Several outlets at once: the power output has to cover everything in use at the same time

Precisely because the power output depends on water pressure, inlet temperature and how many taps you use at the same time, this is easier to calculate than to guess at.

How to find your figure

1. Think in activities, not people. How many showers in a row? Are the kitchen and bathroom used at the same time?
2. Calculate what happens simultaneously, not the total over a day. It is simultaneous use that determines the power output you need.
3. Let the tools do the maths. The consumption calculator takes your own consumption and works out the cost, and the choose a model guide translates your needs into the right model.

If you want to understand why a conventional water heater has to sit so much hotter than the temperature you shower at, read You heat to 75 to shower at 38.