Adding more panels to the Geyserwise will definitely be much cheaper. Just check what is the max number the MPPT can take. A new Paloma + installation is over R20k for the smaller one.
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Yes I am below 600 atm. Based in Cape Town. My assumption was that even with the gas geyser inefficiencies it makes that up by heating on demand. Instead of warming a 100-150lt with electricity we using what we need.
The gas install is off the table based on my first quote of R18k. That upfront capital is a no no. Might contact the Geyserwise okes and hear what they have to say. Thanks @plonkster you explanations are always detailed and insightful 
This is exactly what I had in my head. Where is this source from? Very helpful @DaveSA . Thanks. The gas installer I had out here could not visualise what I had asked.
I do feel this is probably the cost effective option. Going to google them Geyserwise okes. Will let you know what their quote is. Thanks.
It is a Dewhot product.
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It could be, if you use little enough hot water. And you are about to see me discover the answer was more extreme than I thought!
The electric geyser has a standing loss of around 2kWh a day (of course it could be slightly more or slightly less, but it is a good enough estimate for a good well-insulated installation). The gas geyser doesn’t have that, which means that for all practical purposes, you get 2kWh of hot water for “free”. At R2.40/kWh, the gas geyser has a R4.80 head start.
If it takes 1.16Wh to heat one liter of water by 1°C, and we assume we have ambient at 15°C and we need to take it to 55°C (delta=40°C), then 1000Wh of energy makes 1000/(1.16*40) ~= 21.5 liters of hot water.
This costs R3.30 with a gas geyser, and R2.40 with an electrical element.
The cost per liter for the gas geyser is therefore 3.30/21.5 ~= 15.4 cents. That used to be at least three Wilson Toffees back in my day.
So the point where the electrical geyser starts to cost less than 15.4 cents per liter, despite the R4.80 handicap, that’s the point where electricity becomes cheaper.
Throwing these numbers into a spreadsheet suggests that the tip-over point is somewhere between 110 and 130 liters.
So… if you use less than 120 liters of hot water per day, gas will be slightly cheaper.
But, even for an amount as little as 50 liters a day, the difference per liter is around 4 cents, and rapidly closing towards 100 liters. Practically speaking I would say that if you use less than 100 liters of hot water a day, gas might be for you.
I doubt any of the salespeople have analysed it to this level.
Of course I may have made a mistake or two as well.
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And with Gas you have to drive or have it delivered… sometimes a real pain! 
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OK, I managed to produce a picture.
Liters on the X -axis. Cost in ZAR on the Y-axis. Gas is cheaper (than an electrical geyser) for low consumption.
I haven’t taken an electrical geyser as “the standard” for years though. I’ve had at least some form of cheaper hot water since at 2011.
Edit: Spread sheet for others to play with. I included a calculation at the bottom to work out the equivalent kWh price of LPG based on geyser efficiency.
The balance is also quite sensitive. Imagine for example an LPG increase to R240 for a 9kg bottle, and suddenly your break-even point becomes much lower.
gas-vs-electricity.xlsx (9.6 KB)
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In the OP’s use case though the standing losses are potentially not as relevant since he switches on the electric geyser only prior to use in winter?
I think there is a problem here when looking at gas vs electric heating cost when comparing a point tof use gas geyser to using a 150L electric geyser like it is a point of use/on demand heater.
A better comparison will be a gas point of use heater and an electric point of use heater. I suspect in that case electric will be more efficient than gas but in total the choice will boil down to whether you give the heating money to the electricity supplier or the gas supplier (and practically I highly doubt the potential saving in either direction, gas or electric, for domestic use will pay for putting your kids through university).
For a theoretical comparison in efficiency and cost implications it will be very interesting to compare a gas water heater to something like this, which claims to incorporate things like flow rate and inlet water temperature monitoring in the feedback loop controling heating. Looks like a unit like this is about R6000 but I suspect install cost will be higher than for gas… and it is not going to run of the average 5kVA inverter if that is needed.
TLDR: for occasional winter heating in the OP’s use case I think gas vs electric is 6 tomaytoes vs half dozen tomaatoes. I am not convinced you will make back your install cost any time soon vs just using eskom to top-up what you already have when needed.
I’m about 5 mins away. Lucky me 
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Nah, your example makes perfect sense and cents. Saves me time of setting up yet another spreadsheet! (Hate those things.)
If my 2 panel solar geyser is around 100-150 lt, then we are using about that much warm water daily.
Thanks for the spreadsheet. Time saver! 
If I punch in what I paid recently for gas, 22.6c/kg, then the crossover point is at ±150 litres. Bought a 19kg + 9kg backup at R631.
A 0.65 efficiency at 22.6c/kg would bring that crossover point to ±270 litres.
Looked at this cheapie Kexin but not sure how to find the efficiency. Kexin 16L Gas Water Heater - Outdoor – Sustainable Energy Solutions
Love it man, had me ROFL! 
Have the same feeling as well, but this exercise is valuable to me. So many factors, main one being cost for me. Not a vendetta against Eishkom, just yet.
In that case, people like myself who generally use over 600kWh from the grid 9 out of the 12 months, will see no benefit in switching to gas. The price is pretty much the same at the moment.
I also agree that since this is for very occasional use, the impact is probably negligible.
Also, as @mmaritz said, you have to deal with the supply side. There may be additional cost in having it delivered, or you need to fetch it yourself. And occasionally we do have shortages. I can deal with my household heater not working, but I cannot deal with a cold shower, and I definitely cannot deal with a woman that’s just had a cold shower! Even if the electricity is that little bit more expensive, I would probably prefer it. Simply put, the slightly lower cost of LPG would not be enough to persuade me.
But as I also already said, I don’t want cents on the liter. I want 50% off, 67% off, free if possible. That’s the point of this long-term renewable endeavour 
And hence I will always tell people to stop wasting money and just install the solar water heater or the heat pump.
There is an old saying, that the bitterness of poor quality (or the wrong product) remains long after the sweetness of the low price is forgotten. I’m actually installing a bathroom in my house right now, and man does that statement hurt. Every time I have to make a call… R900 shower head/mixer combo, or the R2500 one? I know I’m going to appreciate the latter so much more later on… long after I have stopped smarting over all these expenses!
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With the tariff increases, could I update the Cost(e) to 3.18? https://resource.capetown.gov.za/documentcentre/Documents/Procedures,%20guidelines%20and%20regulations/Understanding-residential-electricity-tariffs.pdf That will be the new tariff in Cape Town.
But I would assume gas would then increase accordingly as well 
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I must remember that one
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It’s usually attributed to Benjamin Franklin, but it sounds more modern to my ears. Who knows…
Of course, that’s the point of uploading the sheet, so people can toy with it themselves, and maybe even make a better one.