Oh. Math question. I like to procrastinate on these for dayyyyys.
The main thing with SSEG is that you still need to be a net consumer which means that you need to consume more than what you are feeding back (if I remember correctly).
Looking at City of Cape Town as they have it published (using 20/21 tarrifs):
You need to install a new meter which is for your own account. This is roughly 12k - 15k from what was mentioned when we discussed it here.
You then have:
- Service Charge per month of R260.31
- Per kWh consumption of R2.1152 for the first 600kWh (R2.9190 > 600kWh)
- Per kWh refund of R0.8423 for what you feed back
It might not be exact, but let’s say you’re a 20kWh a day user (600kWh a month) and you are running 50% of your loads from solar (300 kWh) and buying 50% from the municipality (300 kWh) as you don’t have batteries and also feed back 300 kWh (although I’d most likely do 250 to play it safe on the net consumer side):
- R260.31 service charge
- R634.56 usage (300 kWh x R2.1152)
- -R252.69 feed in refund (300 kWh x R0.8423)
= R642.18
Compare that to being a home user without SSEG and PV only:
- R171.21 network access and administration charge
- R634.56 usage (300 kWh x R2.1152)
= R805.77
That means that with SSEG you will end up paying R163.59 per month less than without SSEG, but this excludes the above mentioned 12k - 15k meter which means ~ 73-92 months before the ‘saving’ can be pocketed.
The math obviously changes when the meter is provided by the municipality or cheaper, you have more than 50% solar usage (I chose 50% to accommodate bad days) etc. You then also have no load shedding protection as the grid can only be used as a battery when the grid is available, so you will still be in the dark during load shedding.
Now add R34k for 2x US3000C batteries into the home user tariff and reduce your grid usage to 25.5% (153 kWh) by using 70% of your available battery power daily:
- R171.21 network access and administration charge
- R323.6256 usage (153 kWh x R2.1152)
= R494.8356
So with batteries you’ll end up R310.9344 per month less than without (and R147.3444 cheaper than with SSEG). That is ~109 months if you want to look at a ROI on the batteries alone along with the gift of electricity during load shedding without needing to start a generator etc.
Like I said, the math changes based on your usage etc. and it’s open to interpretation with bad weather (which also affects SSEG returns) and battery capacity fade etc.
I now also looked at a friend’s installation where over the last 30 days they’ve consumed 501.05 kWh with 18% from the grid (90.189kWh), 56% from solar (280.588 kWh) and 26% from batteries (130.273 kWh - ~62% daily DoD) which would mean 44% from the grid (220.462 kWh) should he not have had batteries. For interest’s sake I did the same calculation for him based on his real world data:
SSEG:
- R260.31 service charge
- R466.3212 usage (220.462 kWh x R2.1152)
- -R185.6951 feed in refund (220.462 kWh x R0.8423)
= R540.9361
Home + PV only:
- R171.21 network access and administration charge
- R466.3212 usage (220.462 kWh x R2.1152)
= R637.5312
Home + Batteries:
- R171.21 network access and administration charge
- R190.7677 usage (90.189 kWh x R2.1152)
= R361.9777
In his case going SSEG would have been R96.5951 per month cheaper than no SSEG or battery storage and the 12k - 15k meter would take ~124 - 156 months before he starts saving.
With batteries he’s R275.5535 a month cheaper than with PV only and he’ll start ‘saving’ after 124 months along with the added benefit of load shedding cover.
Should the meter have been free that would change things, but one also needs to consider that the SSEG refund tariff might not increase proportionally to the price of buying and there’s no guarantee how long the SSEG will be in place seeing how a lot of municipalities / providers worldwide are starting to no longer refund customers for feeding back into the grid.