Solar PV on flat and pitched roofs

The first thing I’d like to say is that our house is not ideal for solar installation as the roof is beset with gables. Two on the south facing front and one on the west facing side. There is also a flat roof extension which would be good except that it has quite a lot of shading from our house and the house next door at times. However we had already decided that it would be worth getting battery storage irrespective of solar generation so that we could take full advantage of cheap night time tariff.Arial view of 24 Oakmount solar

We had a small garden installation (4 panels on a pergola roof) already in place when we decided to have a larger installation on the house. This consisted of 8 panels on south(ish) facing flat roof and 8 panels on west(ish) facing pitched roof. It is in a conservation area and we required planning permission for panels on the flat roof and for removal of (redundant) flues on the pitched roof. These were 90s metal flues that were erected when a tornado in 1995 destroyed the original chimney.

There was a possibility of some extra panels but on front and east roofs, but we decided against this right now as the front roof already has thermal panels and if those are removed there is not much space due to two large gables on each side. This would need planning permission. The east roof is not ideal due to it being NE(ish), and two large original chimneys which may or may not get planning permission for demolition. The associated extra installation cost for possibly marginal gain would mean a longer payback time. But we haven’t ruled either out for the future.

The house and garden systems are not connected as the garden panels have individual micro inverters so there is no DC supply to connect to the new inverter. (I will write a separate case study on the garden solar). The house install comprises of 16 x 410kWh panels giving a potential 6.56kWh peak. 5kW Givenergy inverter and a 19kWh Givenergy battery system (2 x 9.5kWh). Givernergy is not the cheapest but seems to be a very good quality and ties in with Octopus our energy provider. As it is a conservation area we have black panels which look better than the blue ones. We also had pigeon spikes to prevent them from nesting under the panels – we were aware that a neighbour who had panels installed had a big and costly problem getting humanenly rid of their very own flock of pigeons.

Why this installer?

  • Recommended
  • Better price than Solar Together
  • Nicely detailed itemised invoice. Previous invoices have not shown exact cost of install itself so you don’t know if there has been a large unexplained markup.
  • Not the cheapest but very good considering the quality and compatibility of the Givenergy inverter/batteries.
  • Very fast and friendly response to contact.
  • On site survey of the house  rather than a quick glance on Google Earth.
  • Very knowledgeable and gave good advice based on our specific needs.
  • Not reliant on 3rd party (freelance) roofers.

Why such a large battery storage?

This may not be a typical install because we decided for extra large battery storage because we run an Air Sourced Heat Pump (ASHP) and an EV. Our typical annual electricity use is therefore a whopping 14,000kWh. No gas is consumed.

Inverter and batteriesThe initial install quote was for just one battery, but the installer mentioned that it would be easy to upgrade the Givenergy system any time. However we decided to go for the two batteries immediately, as buying an extra one outside of the solar install would not be VAT free so we just went for it. We would be able to charge them overnight on our very cheap Octopus Go night time electricity, all of which would be used in the winter months due to the ASHP. In the Summer any stored surplus can be sold back to the grid at peak times when the prices are higher.

Note that currently we can only charge the battery to about 75% on night rate as our Octopus Go only tariff only gives us 4 hours, however once we switch to Go Intelligent or Agile we expect to be charging it to 100%.

Post install observations

Everything was connected incredibly tidily (it put the scruffiness of our side passage to shame). Apart from the quality of workmanship the main thing I can recommend about this is the Givenergy monitoring and dashboard app. Many people may not be too bothered about this kind of thing and basically want to install and forget about it. When I first saw the dashboard I felt the same, being a bit overwhelmed by the apparent geekiness of all the tables and graphs however after a couple of minutes it all became obvious and very user friendly (at least the most important bits such as when is the battery charging, when is energy being generated and how much).

The dashboard.

In this graph you can see the battery level combined with solar generation and electricity use on a reasonably cold  winter day with a little bit of sun (23/11/2023)

  • The blue line shows battery %
  • Teal shows total power required by house
  • Red is import from grid (what we pay Octopus for)
  • Yellow is solar generation

battery and solar graph

  • 00:30 to 4:30 it is charging the battery on the cheap rate Octopus Go tariff (to 75% at 9.5p per unit)
  • 04:30 it discharges slowly as it is powering only minimal devices in the house such as fridge/freezer.
  • 06:30 battery starts discharging at approx 2.5 – 3kWh for heating via ASHP
  • 12:00 battery has dropped to 24%.Solar generation kicks in and battery charges to 38% by 15:00,
  • 16:00 ASHP comes on again and uses remainder of battery until 20:00 switches to grid power.

The bulk of the grid import (ie the electricity supply to house from Octopus) is at the very low night time rate of 9.5p kWh

We can now load up the Octopus tariff (all within the Givenergy app) and see the cost for that day

Octopus Go Tariff

So what we know is for that day we were using either cheap tariff or free solar right up until 20:00

We can view other days with different graphs, e.g. this shows one of the very sunny days (though still in winter so not expecting miracles) For example this shows good generation for Nov 25 (12kWh) and we can see how it is split between battery and home. (Presumably the inverter gives immediate home usage preference over battery charging:

Graph showing solar generation

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