Roof and Panel advice needed.

lstar337

I could be moving into a new house, and I would like to put solar on the roof as it is electric only (Air to water heat pump heating).  What I don't know is what parts of the roof are worth putting panels on.

The roof is like this (see below) with a 40° slope at ~latitude 50°15.  Is it only worth putting panels in areas 1 & 2, or will 3,4, & 5 work too?

 

By my (rough) calculations, I can only fit around 8 350W panels on 1 & 2.  I was hoping for a bit more than 2.8kWp.


I know this is something an installer will work out, but I like to know ahead of time so I know if I'm being told codswallop.

Thanks in advance.

Netexporter

I'd put them on 1, 4 and 5. You'll get an evening boost, in summer, during the evening peak when prices are highest.

40 degrees is good for winter production, when you need all you can get. 

Screwdriva

A few thoughts:

1) North aspects typically experience a ~50% PV production reduction when at a negative 20-degree incline, so I don’t typically recommend installing any north facing aspects. The production reduction is greater with roof inclines > 20 degrees. This rules out #5. 

2) Assuming you intend to install panels on #4, you may experience shading from #2 and/ or # 3. If so, optimizers are recommended. 

3) I recommend installing 16 425W panels (by your calculations) on #1-4 for a nice SE/ SW aspect spread to cover full day usage.

4) I don't say this often, but I recommend a 13.5 kW battery for an electric only household, as you will barely generate enough to cover usage in the winter months.  My recommendations are the Tesla Powerwall 2 and the GivEnergy All in One. 

For perspective, the installation cost for this 6.8kW system + SolarEdge optimizers + 6 kW SolarEdge inverter should be ~£7500. Add another £6.5K for the AIO battery. 

P.S. I'm aware you didn't ask about this, but I believe electric only A-A heat pump households would benefit most from a Mixergy iHP heat pump hot water cylinder. No need for the immersion heater or gas or oil to heat hot water ever again

Heedtheadvice

I have similar thoughts to Screwdriva.

Not so sure about the specific hardware recommendations but the overall suggestions.

Having panels on different aspects does tend to smooth out the generation through the day rather than a pure south aspect that tends to peak mid day. It is only in summer that there is  significant relative power produced by NE and SW aspects. So the big area #5 would not really be worth the cost especially with the gradient.

Similarly #3 will not be great being NE.

The main power will come from #1,#2 and #4. 

Shading has been mentioned ( from the building itself) but also consider if there will be shading from the landscape or trees too. As an example if you will have a tree shading #3 early morning then another reason to discount.

Bear in mind the greatest need for heat (the HP)is winter and then the shoulder months and that is the opposite to panel production. Import/export tariffs combined with big battery storage can be considered virtually essential. I assume insulation is maximised.....

Netexporter

So the big area #5 would not really be worth the cost especially with the gradient.

On the contrary. The sun is always at a low angle north of due east and west.

lstar337

Screwdriva said:

A few thoughts:

3) I recommend installing 16 425W panels (by your calculations) on #1-4 for a nice SE/ SW aspect spread to cover full day usage.

Aren't 400W (and greater) panels larger?
I must admit, my diagram above was using 350W panels because they were slightly smaller at 1600 x 1000mm.
When I looked at 400W panels, they were 1900 x 1000mm, meaning I could only fit 6 and have a lower overall output.

lstar337

Heedtheadvice said:

The main power will come from #1,#2 and #4. 

Shading has been mentioned ( from the building itself) but also consider if there will be shading from the landscape or trees too. As an example if you will have a tree shading #3 early morning then another reason to discount.

Luckily there is no external shading.  The only shading will be from the roof itself.

Heedtheadvice said:

Bear in mind the greatest need for heat (the HP)is winter and then the shoulder months and that is the opposite to panel production. Import/export tariffs combined with big battery storage can be considered virtually essential. I assume insulation is maximised.....

Here is the insulation data:

lstar337

Netexporter said:

So the big area #5 would not really be worth the cost especially with the gradient.

On the contrary. The sun is always at a low angle north of due east and west.

Could you elaborate on this comment a little further Netexporter?

Heedtheadvice

Regarding gradient response:

I too would be interested in an elaboration of that comment relative to seeing an estimate of power produced per annum vs installation cost.

Reason being that, although the statement that the sun is at a low angle when NE and SW is quite true, the solar energy on the roof does not generate much pv except at the very start and end of the day and only in summer and radiation never reaches perpendicular to the panels (angle of max power) whereas a shallow roof benefits a bit more during the rest of the day.

Does PVGIS calculations give estimated figures as that could indicate the value?

Screwdriva

lstar337 said:

Aren't 400W (and greater) panels larger?

I must admit, my diagram above was using 350W panels because they were slightly smaller at 1600 x 1000mm.
When I looked at 400W panels, they were 1900 x 1000mm, meaning I could only fit 6 and have a lower overall output.

173 x 114cm is a common size I see across the 420+ Watt panels. Another option is the 410W REC Alpha Pure series which are 183cmX102cm but will benefit from bifacial gains (even more if you paint the area concealed behind the panel with white solar reflective paint, as I have done with my roof). 

As you rightly say, an installer roof survey would determine the maximum generation possible - a responsible installer should work with you to recommend the right panels to maximize generation of your roof. 

lstar337 said:

Luckily there is no external shading.  The only shading will be from the roof itself.

This is as I suspected. Shading from any source will diminish performance of the system, and is one of many reasons I recommend SolarEdge optimizers. 

Nick_Dr1

You can use this predictor to give you a very good idea of what can be generated from a particular aspect
PVGIS (PV-GIS) (photovoltaic-software.com)