Hi All, 

To continue from my post in the hello tread. Project overview:

1960's 3 bed, brick, detached house with 2010 two story side extension to create integral garage with master bedroom + ensuite above. 2010 single story rear extension, extra garden facing reception room. Unvented cylinder for DWH, gas boiler, UFH in rear extension + kitchen and rads. Nominally 250 mm loft insulation but this has been moved (prior to removal of a chimney) and relayed badly by myself so far (I should have picked a cooler day!). Plan to relay an increase in depth to ~300 mm.

EPC rating C, 145 m2 excluding garage, estimated heating 14000 kWh. Last year gas usage was 12500 kWh CH & DHW but running house around 18C. Octopus survey appears to estimate at 10 kW heat loss, I get similar value from a quick calculation based of a sample December gas usage last winter.  

Associated detached annexe built around 2000, estimated 20 m2. Kitchenette, bathroom, and bedroom. Rads & DHW fed from gas combi boiler. Used around 2000 kWh last year on CH + DHW heating to between 10-15C, but not occupied other than decorating, occasionally running taps etc. Cavity insulated and some loft insulation (plan to improve)

DNO is National Grid (Leicestershire). We're at an elevation of 170m and certainly have good winds for the washing line!

Exposed radiator pipework both buildings is min diameter 15 mm. Some larger feeds from boiler in main house (how far they run I'm not sure). 

All started of with an exploratory survey with Octopus to find out if, in line with the horror stories, I'd be told I need to replace all my radiators and pipework. Octopus assessed that we'd be ok if increasing loft insulation to 300 mm (design stated flow temp of 50C though). (Did a quick check myself (found a spreadsheet on here) and I think the bathroom radiator (K1) could do with upsizing to a slightly larger K2 or K3, but other than that likely ok at 40C but may have to accept the hallways being slightly on the cool side. Using Octopus stated heat losses, plan to have a go myself)

I've since put a bit more thought into potential locations for the outdoor unit (>1m from boundary, <10m to DHW cylinder). Unfortunately no room down the side of the house where the boiler is so without relocating cylinder would either end up pride of place on patio or middle of the driveway. Therefore looking at relocating to between annex and garage, plenty of room in garage for DWH cylinder (that MCS requirements would want to replace anyway). As garage is integral the run back to the main bulk of the CH pipework is hopefully not too ridiculous.

Hence started on the can I run both buildings off a single ASHP and cylinder. With the BUS and Solar PV + battery can I make the cost balance (i.e. reduced bills) and justify taking money out of the mortgage to fund. 

Key requirements would be getting rid of both existing boilers, gas standing charges, maintenance fees etc. Having a system able to heat the house comfortably and guilt free at temps down to -5C. Aside from costs my motivation is keeping a growing family warm, I don't want to be agonising over switching the boiler on anymore.

Positive side is we have a south facing roof with enough room (~40m2) to fit 16-18 400W Solar PV panels. Some slight shading on one corner of the roof due to the neighbour's TPO spruce tree. Also slight change in roofline between side extension and existing (planning ...). 

I like a bit of certainty so I've been more drawn to the likes of Octopus' Flex tariff (rather than the likes of Agile). I've had a reasonable looking quote from a local installer proposing to use Perlight 425 W panels, Huawei inverter, Optimisers and Huawei Luna batteries. Would be interested to hear if there are any strong positions on microinverters vs optimisers (seem to both have pro's and con's) and any experience with the Huawei kit and associated control app FusionSolar?

Also query what the going view is on maximising installed battery capacity vs predicted ROI. At a going rate of approx £500 per kWh and a 20p cheap to peak differential per kWh the break even point is about 7 years? Arguably (short of being export limited) from a ROI perspective I could likely get away with less than the 15 kWh battery capacity I was aiming for (which I estimate would cover a 3 hour winter peak window if ASHP at full)?

Sorry for the long post, I'm sure it's still missing some useful info!