Solar system savings - sense check

Reed_Richards

Thank you for that but most FiT suppliers don't pay a decent rate for metered export and those that do only do so for their supply customers.  Even the beloved Octopus only pays 4.1p per kWh if you're not with them for import.  Try your sums with that number instead of the assumed 15p per kwh

Petriix

Reed_Richards said:

Thank you for that but most FiT suppliers don't pay a decent rate for metered export and those that do only do so for their supply customers.  Even the beloved Octopus only pays 4.1p per kWh if you're not with them for import.  Try your sums with that number instead of the assumed 15p per kwh

You're confused. 

On 'negotiated' exports, you can choose any supplier for the export part, typically you get the best deals by going with the same one you buy your imports from (I'm with Octopus).

You can still keep your FIT generation payments with whoever they were already with (I'm with Ecotricity, not the best in terms of speed but I couldn't be bothered to switch away). 

On 'standard tariff' exports these are paid by your FIT supplier at the rate specified by Ofgem which increases at RPI. You can freely switch between these options once per year.

No one has ever suggested taking a derisory rate like 4.1p; that would be very stupid and isn't really worth discussion. It really feels like you're arguing for the sake of it while continuing to ignore the clear point I've made. 

Qyburn

Petriix said:

Qyburn said:

It's not that complicated, on a deemed export tariff any electricity you use instead of exporting is free.

Lol. Here's a thought exercise:

You generate 4000kWh per year. 
You self consume 1,500kWh for normal usage.

Scenario 1:

You divert 2,000kWh to your hot water via an iboost.   
You export 500kWh.
You earn (2000 x 6.79p) £135.80 for deemed export payments.

Scenario 2:

You import 2,000kWh (either gas or off-peak electricity) at 7p per kWh to heat water costing £140.
You export 2,500kWh.
You earn (2500 x 15p) £375 for a net gain of £135.

"Deemed" export means it's not metered. You're "deemed" to have exported a certain amount, or a certain percentage, irrespective of what you have actually exported.

That's the whole point.

So if you are on Deemed Export, you would receive the same payment from a generation of 4,000kWh whether you used it or not.

QrizB

Qyburn said:

So if you are on Deemed Export, you would receive the same payment from a generation of 4,000kWh whether you used it or not.

And that's *exactly* what Petriix calculated in Scenario 1. Deemed export of 50% of 4000kWh, despite 2000kWh being diverted and actual export being only 500kWh.

Scenario 2 is for metered export (without diversion) of 2500kWh.

Reed_Richards

@Petriix, I could never think of a reason for separating the generation payment from the export payment but I suppose speed of the generation payment is a perfectly good one. 

You say:

Petriix said:

On 'negotiated' exports, you can choose any supplier for the export part, typically you get the best deals by going with the same one you buy your imports from (I'm with Octopus).

I say that it's not "typically" but inevitably, always.  If you know of counterexamples please cite them.

I'm not arguing with your sums, I never have; I just didn't think the benefits of metered export were pertinent to @Qyburn's comment .  But I am only aware of two suppliers that pay a decent rate for export, the second of these only quite recently, and I believe that changing from deemed export to metered export with either of them is by no means trivially easy.  Either that or I have been particularly unlucky.    

Grandad2b

SuzeQStan said:

Grandad2b said:

We still have to get a handle on battery round trip efficiency - how much can you expect to get out for every kWh you put in?

 

We estimate a ‘round trip’ lost of about 20% for the battery charge in / out. So the 7p per kwh we charge the battery during cheaper TOU works out more like 10p per kWh by the time we hav stored & used it

That's higher than some other people I've asked - I was told typically 10 - 15% but more when the battery is close to fully charged. 

SuzeQStan

Grandad2b said:

We still have to get a handle on battery round trip efficiency - how much can you expect to get out for every kWh you put in?

 

We estimate a ‘round trip’ lost of about 20% for the battery charge in / out. So the 7p per kwh we charge the battery during cheaper TOU works out more like 10p per kWh by the time we hav stored & used it

That's higher than some other people I've asked - I was told typically 10 - 15% but more when the battery is close to fully charged. 

winter and cold temps affecting the battery to take into account. Ours is in our integral garage - but if we had it to do again would have tried to site it in a heated room. Still represents a good savings on SVT

Grandad2b

SuzeQStan said:

Grandad2b said:

We still have to get a handle on battery round trip efficiency - how much can you expect to get out for every kWh you put in?

 

We estimate a ‘round trip’ lost of about 20% for the battery charge in / out. So the 7p per kwh we charge the battery during cheaper TOU works out more like 10p per kWh by the time we hav stored & used it

That's higher than some other people I've asked - I was told typically 10 - 15% but more when the battery is close to fully charged. 

winter and cold temps affecting the battery to take into account. Ours is in our integral garage - but if we had it to do again would have tried to site it in a heated room. Still represents a good savings on SVT

I've done the sums based on last year's usage and it's exceedingly borderline - if we didn't shift any daytime usage to the cheap period (and I don't like the idea of the dog sleeping next to the washing machine or dishwasher while they're operating) we'd save about £320 over the year on imported leccy. We'd also lose about £50 on export giving a net saving of about £270 pa. I suppose it could go in the airing cupboard and then the lost energy would be doing something useful.
I think we'd also need to replace the existing inverter.

Obviously everyone's situation is different and what doesn't make sense for one household may be exactly right for another.

SuzeQStan

Absolutely - we game the heck outta our battery - even so it probably still won’t pay us back cos we bought it when batteries were more expensive.