Expected In House Usage of Solar PV Output

[John Carroll]

 

A few facts/observations first, I have a modest self installed flat(2) plate solar thermal array, and take the daily, Cork,  Roches Point irradiation numbers for the past 14 years, the yearly/period numbers change very little from year to year, for example, periods 1/1 to 17/7 for years 2022,2023,2024,2025, are 665,643,663,703 kWh/m2. I also notice, see "10 Panel Solar PV" below, that the panel efficiency appears to change very little between low and high solar Irradiation, the (10 Panel) attachment shows a 2.6kWp system with a area of 16.4m2, the modelled yearly output is 2.454 Kwh which equates to a efficiency of 14.96%, all the monthly efficiencies are pretty close to this, does this sound plausible? my solar thermal efficiencies certainly vary quite significantly, from say 10% to 28%.

The real reason I'm posting here is that a neighbour is thinking of installing a 4.0kWp system and possibly a 6.0kWp one if his south facing roof can accomodate it. His yearly demand is ~ 3500kwh, 9.6kWh/day, he has a 9kW electric shower only used for 15 minutes/day, plus, electric cooking, electric kettle and the usual other low power users. The spreadsheet is just my estimation of the output, the solar irradiation are my measured ones.

The FIT here is  ~ 18P/kWh

Would someone venture their openion(s) based on the above, as to what the per centage  of self usage might be, based on a 4.0kWp system and on the the 6.0kWp system, or on your own personal experiences. Thanks, John.

Solar Array Output Cork 2024 Rev0.xlsx 10 Panel Solar PV.pdf

Edited 23 hours ago by John Carroll

[JohnMo]

Make life simpler use the link to work it all out fir you.

 

Big array unless you get paid to export and or have a battery - lots of energy will go to waste.

 

https://pvgis.com/en

[Dillsue]

Will your neibours demand profile match the generation profile? Usually there's a big seasonal mismatch because you generate the most in the summer but use the most in winter. Total annual Generation can match total annual demand but because they peak at differing times they'll not balance out.

[ProDave]

As above. Don't over think it.  

 

A typical solar PV with a 3.68kW inverter you can self use almost all of it providing you have a diverter to send any excess to an immersion heater in your hot water tank.

 

You need prior permission from your DNO for anything larger and only consider larger if being paid for export or you have batteries.

[Dillsue]

How big an array you install is a personal thing dependant on lots of factors that vary in importance from person to person.

 

We installed an additional 8.5kw of panels a few years ago on top of the 4kw we already had. No battery for us and no export payments at the time. All self installed and kit bought at discount prices. Having plenty of "free" eleccy and happy for the neighbours to use our surplus is a good enough ROI for me. Octopus now buy our surplus so a big dollop of unplanned icing on that cake:)

[John Carroll]

9 hours ago, Dillsue said:

Will your neibours demand profile match the generation profile? Usually there's a big seasonal mismatch because you generate the most in the summer but use the most in winter. Total annual Generation can match total annual demand but because they peak at differing times they'll not balance out.

 

His usage is a very constant 3315 kWh pa, 9.08 kWh/day, and usage is all day time so all the big users are on, most only briefly,  when the Solar PV should be productive.

I have put a few numbers together which might reflect what kind of savings he migh hope to achieve, currency is in Euros.

Solar Array Output Cork 2024 Rev1.xlsx

[JohnMo]

5 minutes ago, John Carroll said:

have put a few numbers together which might reflect what kind of savings he migh hope to achieve, currency is in Euros

Do you get a feed in tariff (FIT) still. Or that the rate for export with an approved system?

 

So savings are one thing, cost to install another. What sort of cost would the install be? 

[Dillsue]

8 minutes ago, John Carroll said:

 

His usage is a very constant 3315 kWh pa, 9.08 kWh/day, and usage is all day time so all the big users are on, most only briefly,  when the Solar PV should be productive.

Are you sure he uses the same every day all year round? If that's the case then you already have the figures in your spread sheet.

 

If that's not the case and his usage varies then you'll likely export more in the summer and import more in the winter. Because of the difference in unit rates between import and export your figures won't be quite so rosey

[John Carroll]

Don't know much about it John but I believe providers here (the EU) must offer to buy any exported PV, but don't know if they are required to offer a minimum rate for it. I have seen quotes of ~ €10k to €14k for installation, depending on size and location.

[John Carroll]

7 minutes ago, Dillsue said:

Are you sure he uses the same every day all year round? If that's the case then you already have the figures in your spread sheet.

 

If that's not the case and his usage varies then you'll likely export more in the summer and import more in the winter. Because of the difference in unit rates between import and export your figures won't be quite so rosey

 

Usage varies between ~ 8.5 & 10.5 kWh/day based on his two monthly bills over the past year or so.

[JohnMo]

38 minutes ago, John Carroll said:

must offer to buy any exported PV,

If anything like the UK, an approved install and the price you quote would be approved. Export prices without a smart meter is circa 4p and with a smart meter tariff closer to 15p.

 

The reason many on here do DIY installation of PV is the cost difference. 10 to 14k to save 1k is 10 to 14 years payback.

 

Looking at your spreadsheet, it seems there are some errors

 

Have you made allowance for rainy and cloudy days?

Allowed for angle of the panel and sun? i.e. used the link I provide to give a more accurate reality.

 

In May you aren't going to get 29.7kWh every day, maybe not any day, no cloud, no shade, and a tracking system maybe, but a fix array very unlikely.

 

[Dillsue]

8 hours ago, John Carroll said:

Don't know much about it John but I believe providers here (the EU) must offer to buy any exported PV, but don't know if they are required to offer a minimum rate for it. I have seen quotes of ~ €10k to €14k for installation, depending on size and location.

On the mainland we have the Smart Export Guarantee which is government backed/regulated and pays a pittance. Octopus SEG rate is around 4p/unit which they are obliged to pay for export from a qualifying system. Octopus will also buy your export under a separate unregulated agreement and they currently pay 15p/unit on their fixed export tarriff which is a Power Purchase Agreement. Theres no obligation to offer a PPA and it could go up, go down or end due to market conditions/insolvency.

 

That's almost a four fold difference In Octopuses rates. If export payment is important in your friends calcs then you probably need to find out what he'll get and any conditions attached to receiving those rates

Edited 4 hours ago by Dillsue

[Dillsue]

8 hours ago, John Carroll said:

 

Usage varies between ~ 8.5 & 10.5 kWh/day based on his two monthly bills over the past year or so.

If that's a daily variation not linked to changing seasons then using a flat average consumption as you done will likely be fine. On the other hand there's 25% difference in the 2 figures and if those are seasonal highs and lows then probably best to put the high figure in for winter months and the low figure in for summer months.

 

As JohnMo suggests you'd probably be better using PVGIS for your generation forecasts as it takes into consideration lots of variables in producing it's predictions. If you are using daily snap shots of irradiance they're unlikely to represent irradiance for the rest of the day or what is actually hitting the PV panels as the sun moves round and up and down. PVGIS has repeatedly been shown to be pretty accurate.

[SteamyTea]

Knowing means values is one thing, but they cannot recreate a detailed, fine grade, model.  For that you need other statistical parameters.

 

As an example, here is some of my usage data.

 

It is my mean usage figures for a 5 year block, then a couple of random days.

 

 

 

 

Edited 4 hours ago by SteamyTea

[John Carroll]

 

I should have been using a factor of ~ 0.81 in my PV output calculations but even though I have a calculation for it in the spreadsheet I had forgotton to set it properly, I went a step further in Rev3 spreadsheet as I used  PVGIS and inputted every single months factor by dividing each months PV output by the radiation value, they went (see spreadsheet) from a low of 0.79 to a high of almost 0.85, ironically, the highest factors are (0.847) in January&February and November&December. I think a factor of 0.8 is generally accepted?.

The solar irradiation I use is from our national weather people and is the total daily irradiation and not just spot checked, I get it the day after.

PVGIS for a 6M array (Cork) shows a yearly irradiation.

PVIGS shows a PV output of 5964kWh with a yearly irradation of 1218kWh/m2, my calcs, using the accumulated daily irradiation numbers, shows irradiation of 1036kWh/m2 to give  a PV output of 5024kWh from a PV input of 6215kWh, (average factor 0.808). 

JohnMo's query re very high PV output in May.... PVIG shows May&June as the best months and their number of a daily May PV output of 24.1kWh is closely matched by my 24.6kWh (after I use the correction factor)

 

It would be a nonsense to suggest that the house load is constant so those inport/export/savings  numbers will be somewhat less but nevertheless are a reasonable indicator especially since the installed PV of 6kWp is almost double the house requirements of 3315kWh/year.

All very interesting indeed, thanks for all your (hopefully, continuing) insights.

Solar Array Output Cork 2024 Rev3.xlsx

Edited 2 hours ago by John Carroll