Split PV arrays

[FB.]

The PV "bug" is catching and a relative of mine is considering solar PV.

However, from a quick measure-up with them over the weekend, it looks as if they have two options. One seems crazy, but the "theoretical" numbers look slightly more attractive than the other more-logical option.
The roof is a bit awkward; it has a gable and the roof size is right on the borderline of not managing to fit an additional panel (not even compact Sanyo) without risk of wind grabbing the edges in a storm (and the property is quite exposed on the edge of open, slightly sloping fenland).

The roof is 30 degree slope.

Option 1:
10 x 250W panels facing South, with one panel (maybe two with low winter sun) suffering from minor shading in the mornings due to a gable.
Annual generation estimate probably 2200kWh if a small (4-5%) loss is allowed for shading.

Option 2:
8 x 250W panels facing South. No shading (because the two in partial shade of the gable are absent). Annual generation estimate 1900kWh.
8 x 250W panels facing North. No shading (the back roof is clear). Annual generation estimate 1100kWh.

They only have electricity (no gas), and have just been hammered by a massive electricity bill which has them thinking about other options - of which solar would be one.

So given that 2.5kWp South (with slight shade) = 2200kWh per year

- or -

2kWp South + 2kWp North = 3000kWh per year

- and -

given that panels only account for around one-third of the total cost, so it's more cost-effective to put in a bigger system

- and -

they would like to not get hit by such huge electricity bills again, so would like as big a system as possible as long as it is efficient


.....................


So I wonder whether they should consider the unshaded 2kWp South + 2kWp North array, rather than 2.5kWp South with minor shading.

.

What do you think?
North seems crazy, but with a flat-ish roof and only a minor increase in cost, it actually seems the better option (more generation and no worse in cost-effectiveness than buying a smaller South-only system with minor shading).

Thanks,
F
:wink5:

 

[Glen Powell]

Get to the route of the massive electricity bill first, this is family, do the full works and assess the house to how he / she can improve their consumption it may work out better in the long run with a smaller array on the south, energy efficient cooker 300-500 average scaffold to north the same.

 

[Glen Powell]

Other options are available BTW, wood burning stoves / cookers, insulation, airsource heatpumps, etc, etc.

 

[FB.]

Get to the route of the massive electricity bill first.

I think that it's mostly because they have no gas to the property, and electric heating is not anywhere near as cost effective.
They have Economy 7 heating, but it's not really adequate, although they plan to add one or two more E7 heating units.

They believe their property just about scraped a band D for efficiency as a result of having additional insulation in walls, maybe loft (they only moved in during 2011), so they think that they may be eligible for solar PV without further changes (and costs) to the property.

 

[FB.]

wood burning stoves

Yes, they're also considering that. However, how "cheap" are they to run, given that they need a supply of wood to burn?

 

[Gavin A]

if they've got storage heaters, then presumably the vast majority of their consumption will be in winter, at which point a north facing array will frankly be doing sod all as it will get no direct sunlight at all - though I guess on cloudy days it won't do much worse than the south facing array.

 

[screwdriver]

no chance of a groundmount anywhere? wood burning works out pretty cheap compared even to gas but given the chance I would go with 2 wood stoves than one full on wood boiler to do radiators. and heat the water with leccy

 

[Glen Powell]

Depends on a whole heap of stuff, my wood is free from those nice solar panel companies that supply crap pallets, a cubic meter of logs is between £40 and £90 but free if you speak to the right people.

wood gasification boilers will be eligible for RHI (if it comes out) which gives an income, electric heating can be more cost effective than you think, but what I have to say on this goes against some of the forum sponsors so I will leave that one there.

Insulation is key, walls, floors, roof. Get rid of as much thermal mass as possible, we took it on as a project to create 18 passive homes (retrofit) last year with success, no running costs at all but they had room to fit PV, Thermal, Wood burner, LED lighting, etc.

It is not impossible, the thing that can kill the deal is that they keep loads of snakes, lizards, etc or running 6 xboxes and 3 computers. I am sure you get the idea.

 

[FB.]

if they've got storage heaters, then presumably the vast majority of their consumption will be in winter, at which point a north facing array will frankly be doing sod all as it will get no direct sunlight at all - though I guess on cloudy days it won't do much worse than the south facing array.

Yes, but being on E7 means that any power they use in the daytime is more expensive.

From memory of what they said, it's 7p night rate (1am-6am) and 15p day rate. Not being on E7 would be a flat rate of 13p.


Solar might not make a big contribution to their winter usage, but they can "get their money back" during the summer when the solar panels will be working hard. They also won't have as much higher-priced daytime power, such as for cooking, mowing the lawn, watching TV, and many other things which can't be done on E7.

At 21p FiT (and 3.1p export), 3000kWh is £677 per year of payments, plus some reduction in their daytime-usage bills. The "investment" in PV would probably give them an annual return of RPI+3%, plus some bill savings, which isn't bad.

My "usual" electricity payment is a direct debit for just under £40 per month, but that looks like it'll soon be reduced to £20 per month based on my usage so far. So, by being reasonably energy-conscious, I actually appear to be able to use half of what I have generated since mine was installed a few months ago.

 

[FB.]

Insulation is key, walls, floors, roof. Get rid of as much thermal mass as possible, we took it on as a project to create 18 passive homes (retrofit) last year with success, no running costs at all but they had room to fit PV, Thermal, Wood burner, LED lighting, etc.

I think that the previous house owner added extra insulation, which explains how a non-gas-supplied property made band D efficiency.

It is not impossible, the thing that can kill the deal is that they keep loads of snakes, lizards, etc or running 6 xboxes and 3 computers. I am sure you get the idea

They are early-30's and have a young family.


.

 

[Glen Powell]

My big problem with storage heaters is they have not moved on in decades, in most properties they are actually being ripped out rather than installed, we all know electricity is the future, this is why we are in this forum. Other forms of energy are equally viable, wood burners or other similar technologies are sneared on by the uneducated for causing polution, for being inefficient, la, la, la.

Burning wood or coal for electricity is not efficient if the power source is 50 miles away with outdated turbines, however heating the home is very efficient, if you get loads of junk mail more efficient still (with a bit of time and effort), smoke / polution means you are burning rubbish and not the recommended seasoned fuel with little moisture content or you have not maintained your flu as well as you should.

Airsource requires oversized radiators but buy a good airsource designed for the UK then you will be a happy bunny, providing you have insulated to the best of your ability first.

 

[Gavin A]

Yes, but being on E7 means that any power they use in the daytime is more expensive.

From memory of what they said, it's 7p night rate (1am-6am) and 15p day rate. Not being on E7 would be a flat rate of 13p.


Solar might not make a big contribution to their winter usage, but they can "get their money back" during the summer when the solar panels will be working hard. They also won't have as much higher-priced daytime power, such as for cooking, mowing the lawn, watching TV, and many other things which can't be done on E7.

At 21p FiT (and 3.1p export), 3000kWh is £677 per year of payments, plus some reduction in their daytime-usage bills. The "investment" in PV would probably give them an annual return of RPI+3%, plus some bill savings, which isn't bad.

My "usual" electricity payment is a direct debit for just under £40 per month, but that looks like it'll soon be reduced to £20 per month based on my usage so far. So, by being reasonably energy-conscious, I actually appear to be able to use half of what I have generated since mine was installed a few months ago.

makes a difference, but also bear in mind that a significant proportion of the north sides generation will be between around 3am-6am in summer, which may well negate the impact of that extra 2p daytime rate.

I'd not totally discount this idea, but would say you need someone to work up some pretty detailed output and performance projections for various times of day at various times of year to prove the concept, as well as the costing side of things.

We have done north east / north west facing roofs to compliment eg south east / south west roof installations on dual MPPT inverters, and the decrease in the cost per kWp, combined with increased inverter efficiency from using eg Aurora or SMA TL dual MPPT inverter vs a single smaller inverter (1-2% advantage), can in some circumstances outweigh the reduced output for the North East / North West facing array in percentage terms, but each situation is different and it's usually fairly marginal.

 

[FB.]

If I cost it at "best guess" retail prices with low-middle range panels........

2kWp North + 2kWp South.
Fixed costs: £5k (inverter, scaffold, wages etc)
Panel costs: £4k
Total cost: £9k.
Total annual generation: 3000kWh
Cost per annual kWh: £3.00

2.5kWp South
Fixed costs: £4k
Panel cost: £2.5k
Total cost: £6.5k
Total annual generation: 2200kWh
Cost per annual kWh: £2.95

So the split array seems to be practially as cost effective as the single array. The bonus being that the split array has a longer period of reasonable generation each day, will cope better with cloudy days, plus a higher total annual generation; therefore more to offset against bills at 15p per kWh and higher FiT payments.

I guess the main reason for this topic is that it's such a close call from what I can see.

.

 

[sedgy34]

I would go with south and using 8 suntech 280w modules 2240kw system or find a 300 module.

 

[Earthstore]

If panel choice is the problem, I would use sunpower, as you can produce far more from a smaller area,
But then I would as I can supply these, they are not on the open wholesale market.

 

[jupestar]

Hi FB.

It seems you've done the analysis (assuming correct performance figures) and now you've come to a decision which is a client decision not really an installer decision. Now it comes down to other considerations.

Capital outlay.
Aesthetics.
Possible alternative use of roof space.
Weight on roof.

The only thing i would say is that you are considering first years returns only, i know this is correct from a sales view, but it is worth considering your return 5/10 years down the line if you think power prices are going to increase quicker than RPI then a system producing more electricity will start to give extra returns as a higher proportion of the total return will be savings (if they use the energy).

 

[sedgy34]

The Green Deal Mythbuster - energy saving improvements for no extra cost - Money Saving Expert

More on the green deal