It depends on a lot of factors. If you have natural gas supplied to your house by pipeline, I suspect the gas will be cheaper. If you have to buy LPG in a bottle, then it is affected by 1) the cost of LPG in your area, 2) the cost of electricity in your area.
In Cape Town, there is the slight odd possibility that gas might beat electricity. You see LPG costs R225 for a 9kg refill, which works out around R2/kWh (if you convert from the calorific content to equivalent watt-hours), while the top end electricity price is R3.30/kWh. So at face value the gas is almost a third cheaper… but then the gas geyser is so much less efficient.
A gas geyser is between 50% and 70% efficient depending on how much money you spend on it. That means the heating cost with LPG is really between R2.85 and R4 per kWh. If you are bang in the middle – which is roughly what my little cheapo Bosch seems to be capable of – the cost of electricity and the cost of gas is the same: around R3.30 for a kWh of heating.
So, in the short term, the 13% increase should favour gas. But from experience I’ve learned that the oil price (and gas by extension) tends to follow, possibly because of supply- and demand in the market. So such advantages are fleeting.
Again I have to come back to one item: The standing loss. And here I may well change my position tonight, that gas may indeed by SLIGHTLY cheaper in Cape Town right now. If the cost for gas and electricity works out the same (as calculated above), but you save on the 2kWh standing loss of a geyser… then technically you’re saving R6.60 a day (approximately) on gas.
That is however still a FAR cry from what you save with a solar water heater (over the whole year of course), or using a heat pump.
Edit: Also note above that the R3.30 I used is the rate for the higher >600kWh bracket. Below 600kWh the tariff is R2.40/kWh. So again it depends on your overall electricity use. If you fit below 600kWh… then gas is more expensive than electricity. And many of us use our solar plants precisely to get below 600kWh…