The Case for PV

[Visti]

It has taken me a long while to be convinced that PV was economically worthwhile, but now that we have the energy model near complete for our house it's become a no brainer. So I thought I'd share my findings.

 

This assumes a FIT installation with Generated and Exported rates of 3.93 and 5.03 p/kWh for a 9kW system. I've not yet tried the cheaper installation of a non-FIT version which is often quoted as a better alternative. Perhaps I'll have a go at modelling and comparing that too in the near future.

 

I've made my Energy Model available for anyone interested; get it here.

 

The following charts show the quickest return to payoff of the PV system and lowest annual outgoings. Had to use Mains Gas for Heating and DHW, through these can be toggled to electricity in the model. 

 

 

 

[Jeremy Harris]

It is a no brainer, but then I paid in another FIT cheque yesterday for the winter quarter and that was £150, more than it cost to heat the house through all this cold weather by a significant amount.

 

Being able to run the heat pump in reverse in summer is a bonus, as although it only draws around 800W maximum when cooling the house, that is always far less than our surplus PV generation in hot weather, so the cooling is completely free.

[ProDave]

It is still a tough one.

 

I maintain with today's low FIT you need to WANT PV and be happy to spend the capital as just another part of the build.  Otherwise if you look at it purely as a money exercise, the big question is "what if we sell before 11.7 years?"  It will by that measure have been a "bad" investment.

[Visti]

18 minutes ago, ProDave said:

I maintain with today's low FIT you need to WANT PV and be happy to spend the capital as just another part of the build.  Otherwise if you look at it purely as a money exercise, the big question is "what if we sell before 11.7 years?"  It will by that measure have been a "bad" investment.

 

Fair point! We're building to stay, so the assumption wasn't when we'd leave, but how long it'd take. But you're right, who knows how long we'll be and whether we're investing in the next home owner!

[Jeremy Harris]

I think you probably need to think of something like PV with the same mind set as when choosing a kitchen or bathroom.  Do you invest in a more expensive kitchen because you know it will add value to the house and give you added pleasure when living there, or do you go for a budget kitchen, accepting that by the time you come to sell it may well look a bit tired and need replacing, or a bit knocked off the sale price to compensate?

 

In my view a fair bit of the value of PV comes from self-consumption, a little bit comes from the FIT and generation payment, and an unknown bit comes from the "feel-good" factor of doing something to reduce your carbon emissions.  There's also the rapid drop in the price of battery storage - that's now down to around 8p/kWh lifetime cost, and dropping.  When it gets down to around 5 or 6p/kWh then it makes sense to fit it to increase self consumption from PV, I think.  I reckon that with about 6 kWh of battery storage at 6p/ kWh life cycle cost we would knock our electricity consumption from the grid, even in winter, down to virtually zero, nd would easily use no grid electricity at all for around 8 months of the year.

[SteamyTea]

7 hours ago, JSHarris said:

There's also the rapid drop in the price of battery storage - that's now down to around 8p/kWh lifetime cost, and dropping

Is that just the depreciation cost though.

E7 night rate is now about 9p/kWh and the day rate is about 24p/kWh.

For the last two days I have used less than 0.5 kWh during the day rate period (since I have got my new DHW cyclinder).

So I would be better off getting a couple of kWh of storage as the unit cost would be 17p/kWh.

Having said that I cam currently using about 25 kWh/day, so I have a bit of tinkering to do on the DHW temperature and heat losses, and the one night storage heater I am using to get my usage back down to my target of no more than 10 kWh/day.  I have been a bit lazy about this, this year.  The secondary cylinder lagging is going back in place today.

 

[dpmiller]

I've got a similar conundrum. I'm happy enough to go for the PV, but can't quite decide whether mains supply needs to be on E7 or not. PowerNI's rates aren't bad-

15.56 day  7.92 night and a standing charge of 7.7 (that's before any discounts that I'd likely be under the threshold for)

vs a best 24hr rate of 14.59

 

I'm thinking an early morning recharge of the TS- either by immersion or ASHP- would be pretty good value on the E7...

[SteamyTea]

6 minutes ago, dpmiller said:

I've got a similar conundrum.

I started off monitoring my usage in detail, several years ago, to answer just that question.

I am still not sure.

Much of it depends on your total usage.

 

If I was forced to have only standard rate, I would probably change the DHW to a large instantaneous water heater to reduce storage losses (these are about 2.5 kwh/day on a normal 200lt cylinder).

 

I think the best way to model your energy usage is to pick an ideal scenario and see if the different rates can fit in with it, rather than look at the times the rates change and see if you can fit your kit to work within those bounds.

[dpmiller]

In truth I'd already picked the kit and never given any thought to E7. But in trawling PowerNI's site recently looking for new connection info realised the standing charge was low enough to make it worth thinking about.

[Jeremy Harris]

The standing charge for E7 is the killer for us, one issue being that both the tariff and standing charge varies by post code, it seems, as well as varying with supplier, so what may work out well for one person may not for another. 

 

Last time I looked a standard tariff with the maximum discounts for monthly payment, paperless billing etc, was best value for us and E7 was way down the list, with the standing charge being the biggest single factor between suppliers.  Sadly our gross import (not our net import) is slightly higher than would make sense for a zero standing charge supplier, but equally that could change with the next round of tariff changes, as the whole thing is very fluid.

 

I've found it a real PITA to compare real prices for our pattern of use between different suppliers, the only golden rule is to never use the comparison websites as they are always way out, you're much better of trawling around and getting the actual unit price and daily standing charge direct from as many suppliers as you have the patience to check out.

[SteamyTea]

My standing charge is 18.9p/day, so £70/year once rounded up.

That does not seem to change with different payment methods.

The unit price does.  The cheapest is 22.54p/kWh day, 8.46p/kWh night on the cheapest and 24.12p/kWh day and 9.07p/kWh night on the most expensive.

So not a dreadful difference really.

But I have always found EDF clear and reasonable.

Yes I know I could shop about and save a penny or two somewhere, but that is all really, hard to find a very large discounts.

[ProDave]

Currently paying just under 12p per KWh (single rate) and just over 20p per day standing charge.

 

Comparing that to @SteamyTea E7 tariff, you would not need to use many daytime units to offset the huge premium he is paying for day rate vs only 3p per unit cheaper at night.

[joe90]

I am in the same boat regarding using E7, I won’t be able to tell till I have some usage figures to work on with what’s available in my neck of the woods. Regarding solar, I think it would ruin my cottage look, did consider putting it on the roof of my garage but at my age I may not get to the payback period ( be different if I was younger) . So in a years time time I can look at some actual data and make a decision.

[SteamyTea]

4 hours ago, joe90 said:

So in a years time time I can look at some actual data and make a decision.

This is the important thing, but even with figures it is hard to know what to do.

One day, when I break a hip again, I shall sit down and work it out.

I think on my usage, and lack of opportunity to add a decent amount of PV, I am better off minimising my day usage and hunting around for a better overall E7 deal.

Of just swap my baths for a shower.  Minimising usage is really the easy way.

[joe90]

17 hours ago, SteamyTea said:

This is the important thing, but even with figures it is hard to know what to do.

One day, when I break a hip again, I shall sit down and work it out.

I think on my usage, and lack of opportunity to add a decent amount of PV, I am better off minimising my day usage and hunting around for a better overall E7 deal.

Of just swap my baths for a shower.  Minimising usage is really the easy way.

 

Yes ST, it was you I was going to get to work it out for me ( pretty please?)

[SteamyTea]

I better get you a monitor knocked up soon then.