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

Screwdriva wrote: »

A few additional responses/ comments to the last few posts:

1) Yes, the system is battery ready and can be scaled.

2) Yes, we have an immersion heater tank. It is a MEGAFLO Eco 250L Indirect cylinder. (not the PV ready model that has an iBoost built in). The iBoost will plug into this tank to allow for hot water and radiators into the house.

3) The Solar Edge inverter is supplied with a 25 year extended warranty, as are all major components except the iBoost, which has 10 year coverage

4) Total cost for the system is £7100/- inclusive of scaffolding, paperwork, materials and installation.

5) We spend ~£650 on electric and ~£250 on gas today via Bulb energy, and are assuming (over optimistically it would appear) 2500 kW consumption and 2000 kW of export. If the system does over perform like we expect it to, maximum export will also be 2500 kWh. We also assumed £150 of savings due to the iBoost but realize it may be lower.

Please see revised workings from installer below, thanks in no small part to your collective feedback. If you believe we're still way off, do let me know what you would change.

Income;
Assuming this system will generate 4,500 kWh If you export 44% = 1,980 kWh of the power, the Export tariff is 0.7p you get paid = £138.60p

Total income of £138.60p per year This figure is based on the SEG in January 2020

Saving’s
Assuming you use 56% = 2,520 kWh of the power per year and you are paying .19p per kWh you will save £478.80p per year on your Electric bill. Using the Solar I Boost water Heater to heat your water you will save an extra £150.00 per year on your Gas Bill
.
Total income and saving of ~£750 per year.

I think even using all these assumptions the installer is still double counting the I boost savings.

Given the export volume the own use must include both the electricity replacement and the iboost diversion. Earlier you said the iboost was getting 3kwh per day = 3x365 = 1095kwh. Valuing this at 6p per kWh as it is replacing gas (crazy high but the highest figure anyone has quoted) this his worth £65.70.

Subtract this from the own use 2520 leaves 1425 units replaced at 19p each = £270.75.

Add on the export £138.60

And using the installers figures the saving is £478.

Note this is the using the installers optimistic estimates on output, own use and over the top prices for electricity and gas.

Check the maths but I am pretty certain mine is right and your quote is wrong and not only applies the wrong rate to the iboost use but also double counts it.

Screwdriva wrote: »

If I understand correctly, the chances of exporting more than 4kW to the grid are nonexistent because we expect to consume ~50% of the 5.2kW generated. It is unlikely that we will add enough panels to reach 4kW of power exported to the grid, due to roof space limitation.

Hiya, just to clear up any confusion, your consumption is irrelevant, the DNO needs to be advised on every SSEG (small scale electricity generator) connected to its network.

This can be done in the month after the install for systems whose theoretical export can't exceed 3,680W, but needs to be pre-approved for systems that can export more.

If the panels and inverter are greater than 3.68kW, the prior approval is needed, however if the inverter is capped at 3.68kW then it's fine, regardless of the size of the PV array. The DNO won't take own consumption, or in my case, differing orientations into account, they will look to the size of the inverter.

That said, I'm pretty sure that the SE4000H can be set to 3,680W by the installer, so that's not a problem, or if in worst case it can't, then they can simply downsize to the SE3680H model.

Based on the specs, both can cope with 5kW DC, in fact the SE HD Wave range can cope with massive overspeccing, the 3680 can take 5.7kW and the 4000 can take 6.2kW.

In the UK it's normal to undersize the inverter to maximise efficiency in poor weather conditions, especially since it won't spend much of the year sitting at max. If, for example the install was in Spain, then you'd size for best days instead, and not worry about a few cloudy days, or the early/late generation.

Panels lose performance as they get hot, hot in the direct sun, but also as air temps rise, and especially when there's little wind to move on heated air. On a hot day in the summer, don't expect to generate more than 80% of kWp rating, and possibly 70% or even less if it's a roaster. 5kWp x 80% is 4kW, and 70% is 3.5kW. You will see spike generations much higher, but once those panels are hot, the hours long generation rate will be lower and that works in favour of an undersized inverter.

So, and just pondering/plotting out loud, if it was me, I'd probably aim for around 14 350Wp* and a 3.68kW inverter (or capped inverter).

*I'm just guessing at the point where prices rise steeply, if it's later, then go for more, fill your boots, so long as the inverter is sized adequately.

Not quite that simple. [Edit - I was replying to a much earlier post than I realised - the one about exporting more than 4kW]

On a sunny summer's day at 13:00 my 5.25kWp can generate about 5.4Kw. Unless the washing machine, dishwasher or tumble drier are one the go we will be consuming our base load of about 200W at that time (water will be long-since hot), so exporting perhaps 5.2Kw to the grid. If the drier is going full blast then add about 3kW to own consumption, so exporting 2.2Kw. Make the tea at the same time and you maybe use all you are generating or even import a little.

So, in summer you will export way over 30% on average.

Move into winter and on a good sunny day you might generate about 5kWh over the day but never go much above 2Kw at any point in time. Base load will be the same, give or take, and if you put on one of those gas-guzzlers you will need to import as you will be consuming 3.2kW and generating 2kW. So, in winter, on average perhaps 70% own consumption.

What is important for own consumption is not the kWh generated on any day but the instantaneous power in kW being generated when you need to consume. And this is very much skewed the summer months.

Take a look at this chart from my diverter data where I am massaging the data to show what the position would be without the diverter doing its job. Yellow is generated in each month, green exported and red imported. Orange is total house consumption. The while line is percentage exported.

https://www.dropbox.com/s/83vfyvlnjzb353a/myimmerSUN-no%20immersion%20or%20radsJPG.JPG?dl=0

Screwdriva wrote: »

This now makes all system components warranted for 25 years including labour. Total cost, 6900/- all in.

Who is giving the warranty?