Does solar heater waste more water than electricity saved?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

22 June 2025

Levis Maina is wondering about the cost benefit of solar water heaters. He says that his mother recently installed a solar water heating system and, while it saves on electricity costs, he is concerned about the wastage as she waits hot water to run from the tank to the tap.

He writes: “Based on the prevailing rates of power (from Kenya Power) and the consumption of an instant shower, versus the cost of water held in a pipe (based on the average estimated length of pipe between the heater tank and the shower head, and the cost of mains water from most Kenyan water service providers) which option is more economical?”

One of my concerns about solar energy adoption – be it electricity or water heating – is that architects are yet to designing “solar-ready” houses. Consequently, home owners end up fixing the ugly panels on their beautiful (and expensive) roofs where they stick out like a sore thumb. Further, inside the house, there is no thought given to the storage of batteries and hot water tanks.

In the case of solar water heaters, there is no specification for insulating the hot water pipes to retain the heat. This can greatly save the wastage that Levis is concerned about. Still, if we assume that the plumbing is done with standard 18mm pipes and the average distance from the hot water tank to the tap is about 30m (remember; the pipe is not in a straight line!), then the volume of water is about 7.6 litres.

Now since the pressure in the pipe is not too high, running 7.6L may appear to take a long time before the hot water arrives. But, let’s even say it is 10 litres; how much does that cost?

Municipal water is sold in units of 1,000L and in Nairobi, for example, the median price is Sh73 per 1,000L. Therefore, the cost of 10L is 73cents. This is the money wasted in every shower.

The typical instant hot shower is rated at 3kW of electricity. We may assume that it is kept on for about half the time of a 10-minute shower; that is, for 5 minutes. In that time, it consumes 0.25kWh (3kW x 5min / 60min). At the current cost of about Sh28/kWh, this comes to Sh7.

Therefore, on every shower, Levis’ mum is wasting 73 cents in water but saving Sh7 in electricity. The solar water heater was certainly a good idea!