7.3 kW Bi Facial Rooftop Panels: ~7 year ROI: Fact OR fiction?

silverwhistle wrote: »

If you include the standing charge some of us do, or have done. I've been with Ebico paying 21p-ish on their zero SC tariff as at my low consumption when you divided standing charge costs by consumption and added it to cheaper unit rates theirs was the cheapest. Coming to the end of a deal that was no longer the case and I've signed up to a time of day tariff which can go up to 30p a unit at peak! If I control when I consume and get the average down to 16.26p a unit it will prove to be a cheaper tariff. Late evening, night and early morning it is well under that (8-10p) and even during the day e.g 9.5p between 11.30 and 12.00 today.



But yes, I agree with your optimism point and suppose am illustrating how all our circumstances can vary and need to be examined on a case by case basis.

This is slightly more complicated than mere 'unit rate' comparisons. I suppose the point I should have made is that nobody NEEDS to be paying such high unit rates. To work out the savings on the install then the calculation needs to be based on the best available (average unit) rate which would be available without the PV. I agree that the standing charge needs to be taken into account, although I don't think that was what the OP was being quoted as this then needs to be compared to the best available rate for the lower consumption with PV, again taking into account the standing charge.

I of course fully respect your reasons for going with higher unit rates which work out as better value for money given your low consumption.

I hope this is all clear. I still stand by my original point (even if I said it wrong) calculating savings with a 19p unit rate is ridiculous and for me undermines the credibility of the installer.

It's going to be limited by the lowest factor in the system. If the panels can hit 5.2kw then if the Inverter is limited to 3.68 KW, or whatever the non DNO permission limit is, then you'll never get more than 3.68kw out of it. Now if you've gotten a quote, and permission, for a higher powered inverter then that's a different question.

You could have 2MW of panels installed but if your inverter is 3.68kw then that's all you'll get.

1961Nick wrote: »

I could be wrong ... but I seem to remember that it's the rated output of your array that's required to be under 16A (unless you have DNO approval for a higher export).

What is the theoretical maximum output of this system - 4kWh, 5kWh ... or is it limited by the inverter?

As ABrass explains it's the export potential of the system that the DNO cap at 3.68kW for post install registering. That's why I explained earlier, a 5kWp system [edit - in UK weather conditions] should be an OK compromise through a 3.68kW capped inverter.

Martyn's logic is, al always, sound.

If you took that approach then a way to future proof would be to make sure the system is battery-ready, so that once batteries become economically viable you can essentially just plug and play into your installed system without having to swap inverter etc.

Not fool proof of course as technology changes and may not be plug and playable when the time comes.

Generally solar panels are only "economic" if you use nearly all their output yourself. This may work for an energy intensive business but as most of the output will be in the summer and the middle of the day it is less useful for a domestic house. Most households use their least electricity on sunny summer days...

The only other time it is economic is if your cost of electricity is very high. Which it isn't now but it might be in the future, but equally might not be.

You use 3200kw a year so around £500? Skewed probably to after dark and in the winter. So let's say you actually offset half with the solar (unlikely as mentioned) that is £250 a year, then let's say the export tariff works as expected and you get £150 perhaps. That is £400 a year or so. In the first twenty years you will probably need a maintenance fix and a new inverter. But perhaps inflation will help cancel out that and in twenty years you will get your £8000 back....

You have a gas combi boiler so do you actually have a water cylinder to put an immersion heater in to use with a solar iboost? I'm guessing not. So probably not a practical avenue to explore. If this is right and the installer did not understand this then how well have they understood your needs?

So not a good investment. But nor are most sports cars, if it makes you happy to buy as a vanity project why not.

However I would suggest two alternatives;

1, Look for a more basic £4000 4kw solar install. Less exciting but I suspect in practice you will still get 3800kw a year and 90% of the benefit. You will still have far too much in the summer and far too little in the winter, but at least you will have paid much less.

2, Invest the money wisely for 5-10 years and hope it will then buy the mythical £100/kw battery invention, 10kw of super-efficient dirt cheap panels with built in micro inverters that talk to your smart home for you.

Oh and always remember draught proofing, the best investment.

But what do I know, back of envelope guessing. Best of luck.