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We just installed a petrol generator from Adendorff Machinery Mart (to replace the diesel brute that my neighbour complained about). Now we only have 5Kw to play with instead of 25, so I drew up a chart of what can be on at any one time. It proves possible that we can still survive an extended power outage and we don’t switch off our fridge or two freezers. However, the geyser pretty much has to run entirely on its own except for lights and computers.

We had to engage an electrician to make sure that everything that we wanted on was on the same phase, because it is a single phase genny.

At the same time I got the electrician to install a geyser timer, so that even under normal conditions, we would save electricity. So, when should it switch on and off?

Back to school physics. The hotter something is, the quicker it will lose heat. Lesson number 1 – if you just made coffee for your darling wife who is in the shower and won’t be drinking it just yet, how do you make sure that it is hot enough to drink, do you add the cold milk now or just before drinking? Answer – add it now, because “the hotter something is, the quicker it will lose heat”.

Now let’s apply that to the geyser. When is it coolest? After you took your morning shower. So arrange for it to switch off immediately before that. Geyser switched off, you shower, your wife showers, the kids shower. If the geyser is the right size, you’ll all get hot showers and it will have filled up with cold water. (Be careful here, you don’t want your wife getting into a cold shower!). It won’t lose much heat during the day because its temperature is low and you did fit a geyser blanket didn’t you? Now it must come on again in time to heat the water for tomorrow. Now we found that when we are running the genny, the geyser takes 2 hours to heat up. I shower at 05h00, so our geyser is set to come on at 03h00.

Going back to the hot coffeee. You will often hear or read that it is better to leave your geyser switched on if you’re only going away for say, two days, as it takes more power to heat it up from cold than to keep the temperature topped up. Absolutely not true! If the temperature is kept high, the heat loss remains high and that’s energy out which must be replaced by energy in. As it cools down (over the two days), the rate of loss of heat (energy) gradually reduces with the falling temperature, so the energy required to replace the lost heat also falls. So before you all take the last showers on the day of departure, switch the damned thing off and save South Africa!


  1. Kika Van Schalkwyk

    My husband and I argue on whether we should switch off the geyser during load shedding to save energy on the generator.
    We also been told by a plumber that it’s not a good thing to switch the geyser on and off every day


    1. Hi Kika,
      Now you’re appealing to my electrical engineering side. Not often that anyone does that! There’s more than one thing to consider.
      1) The generator will be running anyway, so whether you use the electricity that it is making available or not, will not affect it’s fuel consumption.
      2) If, however, the generator struggles, then the fuel consumption will go up and the generator will age more quickly. You can hear when it is struggling.
      3) The plumber is nuts. The electricity will have already been switched off by Eskom and then on again when your generator kicks in. Likewise at the end of load shedding, there’ll be an off/on unless your generator has an automatic switch over.
      4) By switching the geyser off before load shedding and only on again once Eskom is being kind to you, you will only have one off/on, rather than two, so your geyser will like you for that
      5) Regarding the geyser when you go on holiday, there are those who say that the cost of reheating the water on your return exceeds the cost of keeping it hot while you’re away. They are also nuts. I guess you already read about this in an article that I wrote some years back. If not, search for “geyser” on our website.

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