I used the amount of energy needed to boil 2l of water:
0.4381kWh electricity or 27.3g of LPG
At R2.919/kWh for electricity in Cape Town and R27.52/kg for LPG in March, using LPG was 41.3% cheaper to use.
27.52/kg / 13.87 kWh per kg of LPG = about R2/kWh of heating energy.
That relies on you burning 100% of that gas PERFECTLY (i.e. perfect oxidation, getting all that energy into the water (none in the pan, air, stove) and draining each tank to 0% empty). I don't think thats possible meaning you lose at least 20% of that*.
So that leaves you at R2.50/kWh (or possibly MUCH worse!*)
2L of water going from 20 to boiling needs 0.366kWh of heat without losses.
This does seem surprisingly similar to your gas number -- i.e. 27.3g of gas = 0.377 kWh
(So your gas stove is 97% efficient!? That would be the most efficient gas stove ever invented*)
Whereas for the electric example you managed to waste a whole lot of energy.
I dont know about you, but generally I boil water in a kettle, as it quick (so less losses!!). It sounds from your example you used something less efficient like a pan, maybe with no lid?
Looks like 20% losses on the electric example.
A more typical kettle is closer to 90% efficient and theres no other losses from wasting the tank, transmission, delivery fees etc (in fact often old city meters move slower than they should!)
CoCT has a few rates:
Domestic rate under 600kWh thats R2.40 inc VAT or for a 90% efficient kettle thats R2.66 per kWh of heat
Home User is only R2.11 inc VAT so only R2.33 per kWh of heat.
(And unless your home is worth less than R1m you should be on Home with the fixed charge but lower rate for first 600kWh nowadays!).
Yes - there is a higher rate over 600kWh. To my mind if you are consistently up.in that higher block you either:
a. Have bigger fish to fry on efficiency
b. Have enough money to not care
(Originally in a 2 bed flat when I wrote stuff on the blog, but now a 4 bed house with 2 young kids, having baths and lots of laundry and yet STILL not often over 600 kWh, with no solar etc... So I dont get how others are always up so high!?)
Its hard to get to the bottom of this, but not too hard. You just need to consider the efficiency of boiling using the two methods. Gas is great, but not very efficient. Once thats accounted for the numbers are, I think, still in favour of electricity, but I agree MUCH closer than last time I looked!
And I've now set myself the to-do of measuring my kettle boiling!
Might have to add an article on this on the relaunch of the homebug blog, as couldn't find it but sure I worked this out before!)
Ps - seen that LPG is up a bit this month too, along with Petrol price... Whereas electricity only moves once per year (July) albeit always the SAME way.
*. Note: quick google suggests actually only about 40% of gas cooking heat ends up in the food, vs about 74% for electric and 90% for induction. That would dwarf the other comparison with even 100% used gas, no left over in the tank, costing R5/kWh heat in the food!!)
