Cost/benefit analysis of different size of array

[Omnibuswoman]

I'm trying to compare the relative cost/benefit/pay-back period of different sizes of PV array in order to work out what size of array will be best, and wonder if anyone is interested in casting an eye over my maths to spot the errors...?

 

The house will be passive house standard, around 200m2, occupied by a household of two adults with, we hope, regular visitors. The roof is a simple two-way (E/W) 32 degree pitched roof, around 75m2 per side, with a dormer window on the E side (photo attached). No shading from trees. The panels will be flush with the slates. We are in Cornwall.

 

We plan to have a battery installed, but I haven't included in the calculations the cost of that. This is likely something we will add later on. 

Look forward to your thoughts...

 

Solar Panel cost benefit analysis spreadsheet.xlsx

[ProDave]

You are missing out batteries.

 

I always maintain up to about 4kWp batteries are not worth it as you can quite easilly self use all of it.

 

But above 4kWp which both your examples are, then battery storage will ensure near 100% self usage, which is way more effective than basing your sums on the pittance of export payments and allowing most of what you generate to be exported.

 

Before you consider that much PV, check if / or for how much fee, your DNO will allow it.

[Omnibuswoman]

Thanks @ProDave.

Yes, I will apply to Western Power and see what they say. Hope it's not as big a palaver as getting the supply in the first place. We have a three-phase supply here, but the local infrastructure is absolutely creaking and I can imagine they might be anxious about it taking any output from us. 

And yes, we do plan on getting a battery, I've just not looked into the cost of those as yet. That is the subject of another weekend's research!

[SteamyTea]

Do you know your current usage profile, and will it change much in the new place?

[PhilT]

14 minutes ago, Omnibuswoman said:

We have a three-phase supply here

Interesting application. Do your calcs include the cost of a 3 phase inverter?

[Dillsue]

If you are having for about to have the house wired, is that utilising the 3 phase? If not, I'd speak to your sparks and tell them you may be going for 3 phase and could they consider that in any work they are doing now. If you go for a 15kw system, WP could well ask you to spread it over the 3 phases so you would need the house wiring over 3 phases to use your PV. That said there was a post on here a while ago saying if you have a smart meter with net metering any unused exported PV on one or two phases will cancel out an equivalent amount of imported energy on the third phase.

 

You look to be calculating the forecast monthly PV generation with a panel "efficiency" figure. You'll likely get more accurate figures if you Google PVGIS, input your data and use their forecast figures.

 

If you are focussed on ROI, you might want to run your calculations using a variety of grid kwh unit figures. I cant see the current unit rates lasting too long!

[Omnibuswoman]

19 hours ago, PhilT said:

Interesting application. Do your calcs include the cost of a 3 phase inverter?

No, I’ve not heard of this. I don’t really understand the three phase thing at all. We had it installed as 3 phase as that is what the DNO specified, but have no idea how this benefits us or what impact it will have for our electrics. Our first fix stuff will be done in the spring/ summer, so I will have to gen up before then!

[Omnibuswoman]

19 hours ago, SteamyTea said:

Do you know your current usage profile, and will it change much in the new place?

no, I’m not wholly sure. Currently our heating and hot water are from condensing gas boiler.  Our electric use is plug-in stuff only plus oven, so the new house being all electric will increase our use as our hot water will be from an immersion heater and our heating will be from low temp rads from same tank. 
 

 

[SteamyTea]

10 minutes ago, Omnibuswoman said:

the new house being all electric will increase our use as our hot water will be from an immersion heater and our heating will be from low temp rads from same tank. 

Your SAP calculations, and double checking with Jeremy's spreadsheet should give you a good idea what the new place will use. DHW usage generally don't change much, not as if we move house and decide not to wash.

Cooking will probably not change much either, I boiled potatoes for the same amount of time for the last 50 years, they taste the same.

Why are you heating the DHW with an immersion heater and not a heap pump?

[Omnibuswoman]

@SteamyTea

We've decided not to go for a heat pump just now on the grounds of cost v benefit. Whilst it would save us some money, the low level of heating needed, the use of PV for hot water for 2/3 of the year, and the use of WWHR means that it wouldn’t be likely to pay us back the outlay in its lifetime at current prices. 

We will leave the infrastructure in place so that when/if they drop in cost, and/or electricity prices rise further, we will revisit the idea. 
 

[S2D2]

Be careful assuming self use will be so high, if the hot water and car charging are set up as a divert that will greatly help self use but for heating (unless also divert with some logic if you're not installing a heat pump?) and plug in electric (assuming this is house load?) your generation and consumption are unlikely to line up. I found self use for house load was as low as 30% using PVGIS and smart meter data. With an E/W split you definitely need to be using PVGIS data as generation will be a lot lower, I'm unsure if this is already factored in?

 

If that dormer is on the East roof am I right in thinking it's towards the South end of the roof? That would shade panels significantly on the larger portion of the roof so would need careful consideration.

[Omnibuswoman]

Thanks @S2D2 Those are good points. I need to do some more learning about how PV and household demands work in real world scenarios - so far this is very much an academic exercise based on a set of figures for average household use. We may well be below that, although we do plan in the future to get a fully electric car and will use PV to charge that at home. 
I’m not sure what PVGIS is so I will look that up. Are you able to point me to an idiots guide to domestic solar PV? 

[Omnibuswoman]

6 minutes ago, S2D2 said:

If that dormer is on the East roof am I right in thinking it's towards the South end of the roof? That would shade panels significantly on the larger portion of the roof so would need careful consideration

Yes spot on. I tried modelling the shading effect but gave up as it was too tricky. Instead decided we should site the panels on the northern end of the East facing roof, away from the dormer. 

[Alan Ambrose]

I can't see anything wrong with your calcs.

 

I did something similar to this calc a few months ago for an existing property i.e. I have some reasonable energy usage stats. I came to these tentative conclusions (some or all of which might be nonsense) :

 

+ until Ofgem make the feed-in a proper % of the standard rate, selling the surplus is not worth the bother.

+ a HW diverter is a fairly simple way of using a lot of the extra generation.

+ you can always add the battery later if the cost/benefit is good.

+ even if you have good projected usage and generation stats, the sizing calcs are a bit iffy - there are just too many assumptions. The key assumption, of course, is electricity prices going forward - hopefully things will normalise a bit once the Ukraine war is concluded. But that implies the payback will be longer

+ ideally you could add single panels at a time to your model - until the incremental payback of the extra panel was longer than you could be bothered with. In my case (12 panels/6.5kWp is the max that will fit on this roof) I came up with these calcs:

 

 

+ but overall I decided that the panels themselves are fairly cheap and you might just as well fill the roof while you're about it (even if you don't use/install an inverter/battery to cope with max. panel output). That is, the fixed cost of installation is high enough that you only want to do it (that is, the bit on the roof) once (except that you might start with a smaller inverter / battery etc. if funds are tight). That might mean asking for something a bit more custom than the usual packages that get sold. You can probably change the electrics inside without much bother/cost later if needed. At least I can - since there's an easy / substantial / flexible attic space to use and easy to access.

 

As I said - these conclusions might be nonsense , and they might be too specific to my particular situation for anyone else to draw any conclusions from.

 

 

 

 

Edited December 28, 2022 by Alan Ambrose

[S2D2]

4 hours ago, Omnibuswoman said:

I’m not sure what PVGIS is so I will look that up.

Here it is, just input your postcode then the other parameters, it lets you play with different configurations etc. and most people find it lines up with real world performance reasonably well:

 

https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP

 

I don't have a basic guide reference, I think you understand the fundamentals from the spreadsheet but the forum will be happy to answer any specific questions I'm sure. If you have a smart meter at your current property, you can get into some proper analysis of what you might self use in the new house with respect to plug in electric, see my other thread:

 

 

I have a system installed now and have started logging data so can share more on real usage soon, but others have posted various data in this forum topic before. The key takeaway is it's massively usage dependent, every household is different and will self-use different amounts. You can change usage patterns to address this, but solar is very unpredictable. A battery helps smooth this out but they are very expensive at the moment. We installed a small one (3kWh) as we don't consume enough to go down the Octopus Go/E7 route.

 

For the shading, it will affect you differently throughout the year. Whilst the average for a 2m dormer might shade 1.7m, in winter with the sun at 15 degrees it might shade more like 7.5m. I believe PVGIS can model this to show the overall effect, but I've never used it for this. This may push you towards all the panels on the west side, or it may not be an issue. Also check vertical mounted panels on the South wall, depending on the space you have there and view on aesthetics.

 

As you haven't tiled yet, in built panels will be "cheaper" for you as you will save on tiles, so probably stick as many as you can up there, just consider the shading.

[Susie]

You mentioned regular visitors in you original post. Is that paid for guests if so are you claiming cost of electricity as a business expense.   The pay back on a battery for us was calculated at 13 years so really not worth it, but we can use a lot of what we generate as we can send some to another business building, on the third phase.   You might want to consider a 3 phase inverter that would allow the battery addition at a later date (add battery on solar side before inverter so as to limit losses) when they hopefully get cheaper.   
it also depends on your build time as the longer you take to finish the more you might be able to judge your usage and claim back the VAT of the battery just before sign off. 

 

[Nickfromwales]

22 hours ago, Omnibuswoman said:

@SteamyTea

We've decided not to go for a heat pump just now on the grounds of cost v benefit. Whilst it would save us some money, the low level of heating needed, the use of PV for hot water for 2/3 of the year, and the use of WWHR means that it wouldn’t be likely to pay us back the outlay in its lifetime at current prices. 

We will leave the infrastructure in place so that when/if they drop in cost, and/or electricity prices rise further, we will revisit the idea. 
 

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pumps/5-kw-panasonic-aquarea-high-performance-mono‑bloc-j-generation-1-phase-r32-wh-mdc05j3e5/
 

Change of £2500 with a controller, weather-comp built in etc. Super quiet, and an all inclusive mono block. Cracking bit of kit for the money, I’m very impressed with the ones I’ve installed to date.
 

I’d install this plus a HP UVC now, as you’ll defo regret it in 5-10 years eg after it’s too late to recover the spent ( ever rising in price ) electricity.

I used to be a fan of an all electric ( non-HP ) PH, but after seeing how ( exceptionally ) well these units perform and the unarguable CoP values…..not any more. No ma’am. 
 

Add slab cooling as a residual ( free ) bonus, and it’s a zero brainer.

Introduce an EV and attempt to charge that during daytime pockets of ‘excess’, and then you’ll be saying a very long “goodbye” to every ounce of excess previously earmarked for “other diversion”. That means either the majority of your DHW comes from the grid, or, you opt to charge the EV to near full overnight, and only use it to capture any excess during the day by tethering routinely and filling the remaining ‘gap’ you’ve purposefully left. Unless retired that’s a little impractical in honesty. 

 

PV into a HP as a multiplier is the logical path, giving up to 3x the revenue per kWp of roof space. Turns a 4kWp array into a 12kWp equivalent. 
 

Couple this with; both heating an oversized UVC and charging the slab from cheap rate EV tariff, and you’ll be sub 5p/kW for a huge chuck of the winter heating and DHW, where PV output will diminish to less than 25% of the net micro-generation.
 

Measure twice, cut once 😉