Solar PV & Heat Pump

This page contains our notes about our proposed solar PV and air source heat pump installation.

Not everything in here will be correct or make sense. I am no expert on these matters, just technically savvy and curious to learn and understand it all. That’s why we’re approaching installers for their expertise in design and installation.

I am learning as I go along, starting in November 2022.


  1. General
  2. Solar Photovoltaic (PV) System
  3. Air Source Heat Pump (ASHP) System
  4. Control and Monitoring


Our Household Energy Usage History

  • year Elec kWh Gas kWh
    2018 4,500 16,000
    2019 4,500 16,000
    2020 5,500 16,000
    2021 5,500 17,000
    2022 4,500 13,000
    expected ~5,000 ~16,000?
  • Years based on October to October billing. 2018/2019 are a 2-year average (2019 bill unavailable).

  • Why is our 2022 gas usage low? Were we so frugal? Was weather so mild?

Solar PV System

Our own roof measurements

  • 4 potentially useful roof surfaces
  • 2 larger roofs roughly south facing:
    • roof pitch: 45°
    • roof azimuth: 20° east of south (compass bearing 160°)
    • roof height: counted 27~30 tile rows; best estimation is 3.0 m gutter-to-ridge
  • 2 smaller roofs roughly east facing:
    • roof pitch: 45°
    • roof azimuth: 20° north of east (compass bearing 070°)
    • roof height: counted 27~30 tile rows; best estimation is 3.0 m gutter-to-ridge

Our own system planning exploration

Reminder: I’m new to this. This is experimental. These plans are only illustrative, for discussion; not verified, not complete, not final.


  • We have no preference for make and model of the panels, other than obviously preferring higher output, all else being equal.

Planning attempts:

  • Easy PV Project Report – Julian’s version 1
  • This version of our plan shows a 6 kW inverter where 5 kW is probably enough; likewise other details may be inappropriate.
  • No shading is defined for any panels in this version. In reality some shading is likely.
  • This version of our plan shows panels of a more elongated shape than usual, 1855 x 1029 mm (Jinko Tiger 410W N-Type Black Framed Mono). This type is shown just to illustrate one of the extremes of shape that might fit. The same pattern could work using any of the more common panel sizes around 1100×1700 mm, if sufficient top-to-bottom measurement is available on the roofs.

It’s currently looking to me like sixteen panels may be too much of a squeeze. Also, on the two more north-westerly roofs, the lower they reach the more shading they have.

  • This version of our plan uses panels of more common shape and a more conservative fit in the available space.
  • 12 panels shown: 4 and 3 on SSE roofs, 3 and 2 on ENE roofs

We’re also looking to add more (2) panels on the west-facing roof space.



  • Home automation connection, locally: the power monitoring should be independent of anyone’s “cloud” service. Including any “apps”, and any available control functions.
  • Backup capability? Ability to run the house from battery and/or solar power alone, when the grid has a power cut.

Backup capability:

  • See for example (compares DC-Coupled (SolaX) and AC-Coupled (Tesla Powerwall 2))
  • Ability to operate in “island mode” always requires a G98/G99 certificate (and not G99 fast track applications)
  • Manual or automatic switch-over? Automatic is obviously nice, but not too important.
  • Whole house or split? Probably whole house: we can add a smart switch per device if desired, any time later, easily for appliances up to 13A. Perhaps split and don’t back up any too-high-power circuits such as electric shower; alternatively we can just avoid using it.

Tech specs relevant to solar inverters:

  • Communication protocol: (standard) SunSpec Modbus protocol IEEE-1547 (explanation), (optional) manufacturer’s other open protocol such as Fronius API (JSON).
  • Communication physical connection: any of Ethernet, WiFi, Zigbee, perhaps other.


  • Fronius seem to be the only inverter manufacturer that makes known a serious commitment to local API with open protocols: Fronius Solar API info.
  • Home Assistant (preferred open home automation system) has good integration for Fronius inverters.
  • SolarEdge looks like the second best make in this regard: local connection available through hard-wired ethernet (previously also wifi but that no longer enabled in recent versions).
  • Home Assistant has some integration for local connection to SolarEdge and a few others; but neither SolarEdge nor others seem to promote or prioritise this from their end.


  • Fronius Primo GEN24 Plus (page for installers | page for home owners)
  • probably 5.0 kW (available models: 3 to 6 kW)
  • with “full backup” add-on, because “why not?”: small additional cost for grid outage backup.

Some suppliers of Fronius Primo GEN24 Plus (just from a general DuckDuckGo search):

Assigning strings of panels to MPP Tracker inputs:

  • There are 4 potentially useful roof areas, two roughly south facing (-20°, or SSE), two roughly east facing (-110°, or ENE).
  • The two more north-westerly roofs will have some shading from the more south-easterly roofs, at times of low sun elevation. If this is significant, do we need to consider dual inverters to give each group of panels its own MPPT input? That is “option B”.
  • Option A: 1 x Fronius Primo GEN24 Plus 5.0 kW, with 2 MPPT inputs: all south-facing panels (10) to one, all east-facing panels (6) to the other.
  • Option B: 2 x Fronius Primo GEN24 Plus 3.0 kW, total 4 MPPT inputs: each group of panels to a separate MPPT input (5 south, 5 south, 3 east, 3 east).


Aim: maximise capacity, for the sake of future use and economy of scale.


  • No particular requirements on size, make, style, colour.


  • See my plan (shown above) made on Easy-PV
  • Consider whether 16 panels can fit.
  • Consider panels of more elongated shape (~1850 x 1050 mm) instead of the more common shape (~1700 x 1100 mm) if that helps.
  • Considered triangular panels: they seem to be expensive and hard to obtain (few suppliers).
  • Lichen growth, especially on non-south-facing panels. Are some panel types more resistant than others? Concern because ours are quite high and difficult for cleaners to reach.


I estimated our shading factors, with assistance from easy-pv.

  • rear south-facing roof: around 0.6 to 0.75
  • rear east-facing roof: Around 0.8 to 0.9

Panel Level Optimisation (PLO) or Module Level Power Electronics (MLPE):

One line of wisdom says we need PLO/MLPE: either a micro-inverter or a DC conditioning unit on each panel.

Another line seems to show this doesn’t in fact help, when compared with a good PPT string inverter.


  • Battery sizing. Still TBC whether we install PV + ASHP together or PV first and ASHP later. Consider probably sizing for PV + ASHP. (Is that the conditions where a smaller battery is appropriate?)
  • Battery (system) type: what’s compatible with Fronius inverters? See Fronius Compatible Batteries — looks like it requires BYD Battery-Box Premium HVS/HVM

Installation requests

  • Battery and inverter in under-stair cupboard next to existing electric supply consumer unit.

Air Source Heat Pump (ASHP) System


  • Considering east-facing chimney breast above garage, at north-east corner of house. Discussed with surveyor. Ideal siting for connection to existing heating pipes in garage below, and access is good. It’s a potential bedroom, so vibration is a concern, but it’s old solid walls.

Control system (Central Heating, Radiators, TRVs, Time Switch)

Aim: upgrade to better control than simple whole-house thermostat and time switch.

  • controlled from smart TRVs? (related to “zoned” control?)


  • general introduction:
  • standards: OpenTherm?
  • compatibility: ???


  • Replace existing simple time switch with new controller (TBD).

ASHP Monitoring and Control


Other Installation Requests

  • Remove gas boiler flue completely and make good the roof. (Check: do we have spare tiles?) Aim: weatherproof.
  • Remove excess pipe runs from old boiler location in garage: route directly to new ASHP location. Aim: tidiness.
  • Site the hot tank for shortest possible outlet pipe run to domestic hot water supply. That would be south-west corner of garage. Aim: minimise time delay and waste in using domestic hot water.

Control and Monitoring

Open source home automation system:

Solar PV monitoring:

Heat pump monitoring:

Other system changes

  • Additional Insulation
  • Electric water heater boost for kitchen sink

Additional Insulation


  • additional thickness in loft
  • external insulation on north wall of upstairs corridor
  • external or internal on stair well
  • external or internal on small bedroom

Electric water heater boost for kitchen sink

Would like to add a small instant electric water heater below kitchen sink.

  • Aim: top up the hot water temperature after its long cooling flow from the garage.
  • Been considering this already with current gas combi boiler. Presumably same problem and solution applicable with ASHP and hot tank.
  • Whatever power the circuit can handle: maybe 13 to 30 A (3 to 7 kW). This range of power would be insufficient to give full flow full temperature from cold, but sufficient to give a useful temperature rise at moderate flow rates.