Hot water is the load most households never look at

Why water is expensive to heat

Water has a high specific heat capacity — it takes a great deal of energy to raise its temperature compared with almost any other common substance. That property is exactly why it works so well as a heat transfer medium in central heating systems, and it is also why producing hot water costs what it does.

The arithmetic is easy enough to do approximately. Raising a hundred and fifty litres of water by forty degrees requires roughly seven kilowatt-hours of energy, before accounting for any losses in getting it there. That is a substantial quantity, it recurs daily in most households, and it is entirely invisible because nothing about a hot tap suggests effort.

Across a year, water heating is commonly one of the two largest energy demands in a home after space heating, and in a well-insulated modern house it can be the largest of all. It rarely receives a fraction of the attention that lighting or standby power does.

Standing losses, for water nobody used

A stored hot water cylinder loses heat continuously to the room around it, because it is a warm object in a cooler space and insulation slows that rather than stopping it. Every unit lost that way heated a cupboard.

Cylinder insulation is therefore among the highest-value energy measures in any house that still has an older tank. A modern cylinder arrives with factory-applied foam and loses relatively little. An older copper cylinder with a loose jacket, or none, loses a great deal, and adding proper insulation is cheap, quick and requires no skill.

The pipes leaving the cylinder matter more than their size suggests, particularly the first stretch. Hot water rises into them, cools, and is replaced by more hot water from the tank, so an uninsulated pipe acts as a small continuous radiator. Insulating the first couple of metres of the flow and the return, and any pipework running through unheated space, addresses most of it.

Flow rate is the biggest lever in most houses

The quantity of energy in a shower is the volume of water multiplied by the temperature rise, so anything that reduces volume reduces energy proportionally and immediately. Flow rate is usually the easiest of those to change.

Shower heads vary enormously in the flow they deliver at the same pressure, and a well-designed low-flow head that entrains air produces a shower most people cannot distinguish from a higher-flow one. Tap aerators do the same job at a basin. Neither costs much, neither requires a trade, and both act on every use from the day they are fitted.

The other half is duration, which is behavioural rather than technical and therefore harder to change reliably. A timer in the bathroom works for some households and is ignored in others. What is worth knowing is the scale: shower length and shower flow rate are multiplied together, so halving either has the same effect on energy as halving the other.

Temperature settings, with a genuine caution

Storing water hotter than necessary increases standing losses and increases the energy required to produce it, so a lower storage temperature saves energy. That is straightforward physics and it is where general advice has to stop and become specific.

Stored water at low temperatures can allow bacteria to multiply, which is why guidance in most jurisdictions specifies a minimum storage temperature for cylinders and often a periodic higher-temperature cycle. The exact requirement varies by country and by the type of system, and it does not apply in the same way to instantaneous heaters that store nothing.

The practical position is that the storage temperature is a safety setting rather than an efficiency one and should be set according to local guidance for your system, while the delivery temperature at the tap is a separate matter usually handled by a blending valve. Anyone proposing to lower a cylinder thermostat should check what applies to their installation rather than acting on a general article, including this one.

Where solar fits, and where diverting is a mistake

A solar diverter sends surplus generation to an immersion heater rather than exporting it, converting electricity that would have left the house into stored hot water. It is a simple device, it involves no batteries, and in the right circumstances it is one of the most cost-effective additions to a solar installation.

The right circumstances are specific. It requires a stored cylinder rather than an instantaneous system, it requires genuine surplus generation, and above all it requires the export rate to be low. Where export pays well, diverting a unit into water is worth the value of the gas or electricity it displaced rather than the export payment forgone, and that comparison can easily come out against the diverter.

It is also worth checking what the household actually needs. A diverter that reheats a cylinder every sunny afternoon in a house that showers in the morning is storing heat overnight and losing part of it to standing losses. Like most energy measures, it works when it matches the pattern of the house and disappoints when it does not.