Solar water heaters save money, but are expensive to install

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

17 March 2019

Over the last few weeks, we have established that it is far more expensive to collect rain water than to have it supplied by the local government from a large dam. It turned out that to get the same quantity cost about 600 times the monthly bill – it takes 50 years to pay back!

After reading this, Engineer Ngari Mugo wrote to advise that, instead of buying manufactured tanks, it would be cheaper to excavate a big hole in the ground and line it with water-tight plastic material. By his estimation, that would cost just Sh120,000 for the whole job. Thus the “pay-back” period comes down to a more reasonable 8 years.

Still, we should not forget that this is based on the assumption that there is enough roof area to collect the water. Nairobi is not as rainy as we think!

The next question that has been asked by other readers is this: apart from rain water harvesting, can a house in Nairobi harvest enough solar electricity to sustain itself?

Well; I did that calculation in this column 15 years ago (2004) and, in the end, I made very many enemies! It turned out that, domestic solar electricity systems are simply too expensive compared to power supplied from the national grid.

Since then, there has been a drive by the government to force people to install solar water heaters in their homes. In addition, there is also a proposal to ban the use of the popular instant hot showers.

The question then follows: how much saving can one make by installing a solar water heater?

Most of the instant hot showers available in the market have two temperature settings: “warm” and “hot”. The “warm” level consumes about 3kW while “hot” is rated about 4.4kW.

Most of the time, I find that the “warm” setting is sufficient unless the day is particularly cold. During a normal 10-minute shower, I estimate that the water will be running continuously for a total of about 6 minutes.

So, for a household with five people, it is reasonable to say that the instant hot shower will be on for about 30 minutes (6min x 5 people) each day. At a power rating of 3kW, this means that the gadget consumes about 1.5kWh of electricity daily; that is, about 45 units per month.

At the current prices of about Sh23 per unit, the total cost of using the instant hot shower is about Sh1,075 per month.

Assuming that the house will only use the hot water for showering, a small 100-litre solar heater will be enough for a household of five. The market prices for such a unit range at around Sh75,000 [including installation].

In other words, you invest Sh75,000 in order to save Sh1,000 monthly. It will take you about 75 months (six years) to recover your investment through savings. Although financially it would be a good move, I doubt whether there are many people willing to pack away Sh75,000 for six years.