Understanding Solar Water Heaters

Solar water heaters are a great, useful and money-saving device. But if you’re a household looking for Solar Water Heater you might get a bit lost with all the promises of “save up to 30% on your electricity bill”, “save over 50% on your heating”.

I was recently walking around Builders Warehouse and spotted a flyer for Solar Water Geysers. It all sounded very good until I read one of the figures at the back – 11.4MJ of energy per day. As an engineer and someone interested in saving money on my electricity bill, I converted that number –> 11.4MJ is 3.2 KiloWatt Hours – that’s 3.2 from my electricity meter; so if you pay R1.3 per unit of electricity that’s just over R4/day. So this geyser, working perfectly and if I use all hot water it makes can only save around R4/day…

Lots of people say your geyser accounts for around 50% of your energy use, so if you spend R1,500/month on electricity, then you spend R750 on your geyser – which is about R25 per day. So you’d need more than 6 of these solar panels to cover my usual geyser usage, if they worked perfectly!

How to heat water

Energy is like distance, power is like speed...
Energy is like distance, power is like speed…

Your water doesn’t care how it gets heated – all that matters is how much energy gets put into the water. The big difference between a Solar Water Heater and an electric geyser is the amount of heating power it has. When we say ‘power’ we mean the amount of energy it can deliver each second. Heating up your geyser is like taking a journey in your car when go from A to B, but instead you start off at cold and want to get to warm. The energy you need is like the distance you need to travel (say 200 kilometres, or in energy 200 kilo-joules) ; the power is how fast you get there or your speed (say 100km per hour). At that speed we know the car will take 2 hours to get there. An hour is a long time for a geyser, so rather than dividing by hours, we divide by seconds. So, the ‘speed’ of your geyser is measured in Kilo-joules per second  but just to make things a little confusing, rather than Kilo-joules per second, we shorten that to Kilo-Watts. In summary, think of Energy like distance and Power like speed – the higher the power, the quicker you’ll get from cold to hot.

The energy amount that is on your electricity bill or prepaid voucher is ONE kilowatt-hour — this is 1 kilowatt for 1 hour. A kettle is normally 2 kilowatts, so if you ran it for an hour it would use 2 kilowatt hours.

Solar vs. Electric

An electric geyser gets its power via the grid from Eskom. That little wire going into geyser, may not seem like much, but it can deliver around 4,000 watts of power (or 4 kilo-watts). Your geyser is really just a big, powerful kettle – with that much power it can boil a cup of tea in 10 seconds; or what it actually does –> heat up 600 cups of tea (or 150 litres) in an hour and a half.  If you’re geyser is on for a full hour, it will use up 4 kilowatt hours – that’s 4 energy credits.

A solar water heater tries to do the same thing but using the Sun not Eskom. The sun is hugely powerful, but here on planet Earth we’re pretty far away, meaning that even in bright sunlight at midday it only gives us around 1,000 watts for each square meter of land that it hits. A typical solar panel at around 2 square meters (2m long by 1m wide) is getting 2,000 watts of power, unfortunately it doesn’t capture all of that (the pipes and roof are also getting hot), so probably only 60% or 1,200 watts end up heating the water. So, at midday in summer, in the brightest possible sun, a solar water heater is only able to heat water at about quarter the speed of an electric geyser.

Think about it this way – when you step outside in the sun you think “oooh, thats warm”. If you were to touch a live electric wire you’d think “OUCH, that’s REALLLLLLY very warm” and your smouldering finger would drop off… [don’t try this at home!]

The good thing about Solar is that you get that energy for FREE, directly from the Sun. The bad thing about Solar is that its pretty much out of your control – for half of every 24-hour cycle (i.e. every day) there’s no sun at all (it’s night-time!), and the other half sometimes there’s sun, sometimes it’s cloudy and only at around midday 2-3 hours per day will you get the 1,000 watts mentioned above. Load shedding might be annoying, but it’s never been that bad!

How much sun do I get?

The amount of solar power hitting your roof depends on where you live. Everyone all over the world gets on average 12 hours of daylight per day, but unless you’re on the equator it changes throughout the year – more daylight in summer, less in winter. Also, some places are cloudier and therefore less sunshine hits the ground. All of this is very complicated to work out, but luckily NASA watch the whole planet from satellites and keep track of exactly how much sun energy we all get. You can see some of these figures at the NREL website here, and some really amazing maps from the National Earth Observatory here.

NREL Map of South Africa
Solar is best in the Central Plateau

You can see on the map of South Africa (right) that we get between 5 and 7 Kilowatt hours per day. On the coast – yellow on the map – there are generally more clouds so they only get around 4.5-5.5 Kilowatt hours per day per square meter; whereas in the Central Plateau (red and dark red) it can be as high as 7. Again, the black tubes in a solar panel don’t manage to grab ALL of that energy – so you’re like to only get around 60% or between 3 and 4 kilowatt hours per day for each square meter of panel. These figures are averages over the whole year. I guess we all realise that Solar Panels work better in the Summer than the Winter – but these figures can help you realise how much different it is for you at your home.

Looking at the annual average figure, it seems that Cape Town and Johannesburg seem quite similar (they are both orange on the map) – however when we drill down into the monthly data we see a very different picture. In Cape Town there is much more ‘seasonality’ – in the wet, cloudy Winter they only get  3 or 4 kilowatt hours per day (3.4 in July), but in the brighter, clearer Summer months this rises to almost 8 (7.9 in January). In Jo’burg all the figures are much closer to the average – the lowest month being March at just under 5 kilowatt-hours per day and the highest is July at 6.4.

For me or for the average?

So those maps can tell you how much you’d get in an open field with nothing around you for miles. But generally our houses aren’t like that – in Cape Town we may have Table Mountain blocking the sun for some areas for most of the day, or you may have trees or other buildings blocking the light.

Whether the seasonality is important for your household really depends on when and why you use hot water. If you are flexible about your daily hot-water usage – you’re happy taking shorter showers in the winter or on cloudy days – then a solar heater will be better for you.

It’s important to understand how much hot water you use before deciding on your hot water system – you wouldn’t buy a vehicle without deciding how you’re going to use it – small and quick (sports car), lots of seats (MPV/minibus),  big load with few seats (a bakkie) or something in-between (family sedan) – so you shouldn’t invest in any hot water system without knowing how you use your current geyser. The first step would be to find out how much hot water you already use (see our follow-up article, coming soon).

Check-list before buying a Solar Water Heater

So if you’re asking how much will I save with a solar water heater? You need to know a few things:

1. How much do you spend on hot water already? – You’ll never save more money than you spend, so you really need to know how much you spend on hot water BEFORE you make a solar water investment

2. Where do you live? – have you got enough sunshine and roof space for it to make sense; do you live in an area that gets much less sun in winter than summer? Are there trees, mountains or buildings blocking the sun for some of the day?

3. When do you use hot water? – solar water heaters cool down at night, so if your main hot-water usage is early morning showers when it’s still dark, you’ll need to make sure you store that water for the morning. If not you’ll still be using electricity to heat it up!

Ok, it’s taken a while, but I think I’ve made my point – deciding how much YOU would save is really hard. In depends on exactly on your house (its location, existing geyser etc.) and your family’s hot water usage. It’s impossible for me to work our YOUR savings, but luckily at Homebug we can do that for you. Give us a shout below and we’ll organise for a remote audit of your home using our awesome, South African built technology.


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