Could anyone sanity-check my fag-packet solar ROI calculation?

itm2

The house is pretty packed with technology - IP cameras (inside and out), smart speakers, video doorbell, etc. Turning everything off would be a major PITA, but yes it would be good to find out which are the main culprits.

Martyn1981

Brill, OK here are some guestimates, but please take them with a large pinch of salt, and of course they are specific to you, with high consumption and a relatively small PV system.

So in the best 3 to 4 months a 2.4kWp system will probably hold around 2kW generation at peak, since panel performance drops off as temp rises. For a SW system, let's say it peaks from 12pm to 4pm. During that time you'd be using about 50% of generation. Earlier and later in the day, gen will be lower and you'll be using a higher percentage. Summer gen is very reliable, but let's say one day per week gen is poorer due to weather, and again you'll consume a higher percentage.

I'm really guesstimating now, but perhaps 75% consumption during those 4 months where you'll generate about half your annual generation.

For the rest of the year you'll probably consume most of it - I appreciate that's an extremely oversimplified judgement but hopefully good enough - so lets say 90%.

(75% x 50%) + (90% x 50%) = 82.5%

Again (and again, and again) that's just a really assumption based guess off the top off my head. It depends largely on what you've said being correct, but will be impacted by your demand profile, and could be boosted a bit by time shifting, such as washing machine / dishwasher use in that peak period in the summer.

Regarding a battery, you'd probably do quite well with a small batt (4kWh?) in those peak periods, but it would be largely unused the rest of the year, so I'd also question the economics for now.

2nd_time_buyer

itm2 said:

The house is pretty packed with technology - IP cameras (inside and out), smart speakers, video doorbell, etc. Turning everything off would be a major PITA, but yes it would be good to find out which are the main culprits.

FWIW we have 5 computers running today with all of us forced back home with the wind. Our consumption is averaging around 300w. All the tech you mention above should be negligible consumption.  In practice, heat = energy, so look for anything getting hot.

QrizB

itm2 said:

We have conventional gas central heating, with a combi boiler. I do have 3 desktop computers which run 24/7 (long story), plus a couple of laptops which are used for a few hours each day.

  Wow. When you said you used 10000kWh/yr of electricity I assumed you had an electrically-heated property.

Hexane

2nd_time_buyer said:

itm2 said:

The house is pretty packed with technology - IP cameras (inside and out), smart speakers, video doorbell, etc. Turning everything off would be a major PITA, but yes it would be good to find out which are the main culprits.

All the tech you mention above should be negligible consumption.

... unless the TV is a large plasma model

Martyn1981

Ok, this'll probably come under the heading of 'too much info', but I took the dog for a walk, so thought I'd play some numbers in my head and two things struck me (other than the sideways rain and broken tree branches):

1. Based on 12m2 that sounds like you are assuming 7 panels, (which is fair, but will depend on the roof dimensions to see how they'll fit) at 340Wp each (7 x 340Wp = 2.38kWp), but if that's the case, I'd now suggest using 370Wp as a starting point, as panels are getting more efficient over time, so hopefully a nice bump up to 2.59kWp.

2. Magic man maths / weird economics of PV - OK, this might sound like a trick, but it does work. For a small roof like yours, it's worth considering higher efficiency panels, which do carry a cost premium per Wp, v's 'normal' panels.

For instance, let's say you get panels that have 10% more power, so cost 10% more for the extra Wp, but perhaps also an extra 10% as they are more expensive (closer to the cutting edge), so 10% more generation, but 20% more expensive.

But wait, the extra cost only applies to the PV panels, with perhaps a £50 extra for a larger inverter (though this is largely shoehorned in to make the numbers easier (see later)).

So let's say the panels would have cost £143 each, £1,000 total. But now they cost 20% more, so £1,200. Plus that £50 for the inverter.

This raises the total cost from £3,500, to £3,750, an increase in cost of 7%, but results in a generation increase of 10%, despite the PV panel cost increase of 20%.

But to be clear, I've fudged the numbers just to make them rounder and simpler to aid in comprehension. Don't take them too seriously (for instance I've just found 370Wp panels for retail inc VAT at £122), but I wanted to make the point to encourage you to do some extra research and ask for multiple quotes, as if I was in your situation, I would try to squeeze every last Wp onto the roof. Also need to be careful to check panel sizes, as higher power panels, might simply be larger, say 2m tall rather than 1.7m(ish).

Perhaps for now, put this post out of your mind, and come back to it later when you've started to get a good grasp of all things PV.

itm2

Thanks - that's a really good point. I can see the sense in maximising the power output, given the relatively low impact on the overall cost. I've contacted a few companies for quotes.

In the meantime I've been bamboozled by the range of panels (and prices) that are available:
https://www.zerohomebills.com/product-category/solar-photovoltaic-pv/solar-photovoltaic-panels/solar-monocrystalline-panels/

Hopefully the companies that I've contacted will be able to explain the difference between the £116 440W panel and the £224 380W panel !?!

QrizB

I've bought panels from these guys before, they have a useful comparison table:
https://www.bimblesolar.com/panelcompare

Martyn1981

itm2 said:

Thanks - that's a really good point. I can see the sense in maximising the power output, given the relatively low impact on the overall cost. I've contacted a few companies for quotes.

In the meantime I've been bamboozled by the range of panels (and prices) that are available:
https://www.zerohomebills.com/product-category/solar-photovoltaic-pv/solar-photovoltaic-panels/solar-monocrystalline-panels/

Hopefully the companies that I've contacted will be able to explain the difference between the £116 440W panel and the £224 380W panel !?!

Don't go down that rabbit hole, you might not make it back out! 

To answer that specific question, the 380W panel is actually more efficient at 20.6% v's 20.2% for the 440W panel, but the 440 is an ickle bit wider, and a lot longer, in fact very long at 2.115m.

Some great prices there though. Getting ever more tempted to change my 5 low roof panels which at 235Wp (going back to 2011 here), would give my system a nice upgrade. But those panels are at a shallow 20d pitch, so work best in the summer, when I have enough generation already. Ho hum.

itm2

QrizB said:

I've bought panels from these guys before, they have a useful comparison table:
https://www.bimblesolar.com/panelcompare

My attention was drawn to the windscreen internal solar panels at this link. I remember buying one of these 10-15 years ago, leaving it in the sunlight in an open car park for about 5 hours to charge my phone, and finding that the phone charge had increased by about 1% when I returned. 
Are these panels of any genuine practical use for charging phones these days?