Here the honourable Mr Poz does a comparative analysis between the two:
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This video was from 5 years ago. The biggest change is the reduction in price of PV panels.
On the other hand he’s in a frost area so the flat panel will have a heat exchanger. Here in sunny SA you might get away with a direct system and getting better efficiency. (A point that is missed by evacuated tube enthusiasts!)
I think it goes back to what was said previously in that panels are cheap in comparison and will most likely last a lot longer, so just get extra panels if you have the roof space. He also pretty much says that in the start with the “Best in cost” award.
He’s then showing an element directly wired to the PV whereas, I think, most use cases in SA will most likely be inverter & battery backup system where you can then slap some extra panels on to take care of your hot water.
I’m sure this is done to simplify the comparison.
What he has successfully highlighted are the advantages of two systems.
As far as heating your geyser element from an inverter I have reservations. The major one is how excess power is determined to do this control and the other is then having to drive your inverter hard to do this.
The benefit of like a grid-tied inverter, with or without batteries, is that the HOUSE gets powered which includes the geyser heating, offsetting the entire Eskom bill in its lifetime.
That is why grid-tied inverters add so much more value, as the solar panels are being used all the time during daylight hours, with whatever sunlight is available.
So over say 10-20 years of the systems life, you save a hell-of-a-lot more than panels dedicated to just geyser heating.
Mathematically speaking, modern PV modules are around 20% efficient in converting insolation into electricity, and the electrical element is close to 100% efficient in making heat, so overall 20%. A flat plate solar water heater is around 60% efficient. So, simple math, if you have the roof space to fit three times the area of panels, then the PV solution will break even.
Cost wise. A good solar water heater costs no less than 20k, and has a 2.4 square meter collector.
7.2 Square meters of PV is about 1.5kW peak, which is going to cost you around 7.5k, and then you have to add a tank. So the PV solution is definitely the cheaper one now, even here.
I do see some low pressure solar geysers that may compete a bit better though.
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Granted. Grid tie inverters are the best!
What I haven’t got my head around is how to power your geyser with the inverter with solar power only. The geyser is a non essential load so it can wait.
If the geyser element switches on it will draw power from the grid if the load is greater than what the solar panels can provide…
A sonoff and some node red/ha automations… Very easy.
As per Mark above.
What I did:
From 11am to 6pm (can be anything, setting still as per summer - bleh)
If Eskom draw is < 500w
AND Solar Production is more than 3500w
Then switch on geyser.
Check every 5 minutes if the above conditions are true.
Doing it with a Shelly relay controlled by NodeRED.
Notes:
- From 11ish the batts are being recharged so the panels are at max. Batteries can wait.
- IF another load comes on, more Eskom is drawn, what if it is kettle/MW which tends to be over before 5 min, hence the 5 minute delay. Or quick clouds.
- If it is lunchtime cooking, geyser goes off, checked every 5 minutes.
- And IF it is a bad solar day, the geyser temp drops below a set value, switch on and heat the water by whatever means necessary. “Safety net”.
KISS
The bottom line - inevitable fact:
Eskom will be used when conditions are not perfect.
Because woman + cold shower = not worth saving a couple of rands annually.
I lose in winter … win in summer.
I seldom have to do anything, because of the “Safety net”.
Are you using a Carlo Gavazzi to monitor the grid current etc?
Ja. I use the Single Phase ET112.
Yes, ET112.