RobLe

Thanks for the interest, I love that it's all energy related 🙂 Some details here: The back of the garage is mostly wall - it is being EWIed, U approx 0.22W/m2/K. I want a roof overhang detail to finish it. Between house and garage is a filled cavity wall, U=0.7. Internal door is insulated, it's the original cardboard honeycomb inner with wood outers - I squirty foamed the honeycomb to insulate. The side is a party wall to next doors garage, a mirror of ours. The personal doors are uPVC 3G all glass, fit well up and over door has 50mm celotex on the inside so U=0.5. It has rubber flaps around it, but will be leaky. I wish it has a window in it, it's south facing. I've not EWIed the front of the garage - there's not a lot of brick there. It would need PP. Roof is uninsulated, I think it is the original. House and garage are uninsulated concrete floor. I have perimeter insulated the house and back of garage, likely do front of garage too one day. Rest of house is U=0.15 EWI, gas use last year was 4.5MWh, the gas boiler living in a small room at the back of the garage with token IWI. The boiler has last Dec been replaced with a heatpump, but the pipe run is the same. There is actually a rad in that IWI room - not convinced there should be as it's technically not house, but the plumber 15 years ago persuaded me. It's never used. I know without deliberately heating it won't get very warm, but I expect it would be warmer. Right now it is 20C indoors, 5C outside (it was colder overnight), 10C in the garage. Just a bit warmer would make it a nicer space to use for diy in the winter, and turning on a heater in it now would cause mental anguish.

I would like to insulate our garage roof so it’s not so cold in winter, and also as it’s now the weakest link of our newly EWI-ed house. We are link-detached - there is a mirror image house to our right, linked by the garage. It’s a 1963 build, torched felt, no insulation, with a fall of only 1 in 100 with the lowest at the front, so it pools. It’s not leaking though. What I would like is to add shaped insulation making a warm roof (insulation, osb3, epdm), such that it is 140mm taller at the front, and 290mm taller at the back, giving a decent overall U value of around 0.13W/m2/K, and a fall of 1 in 40. Do I need full planning permission to do this? (Gah) Is there anything sensible else I could do? House plans attached, which I needed to do the EWI !

As ST says, that seems a lot less than expected, a total of 774kWh over 41 days, merely 18.9kWh/day. Is that right? It really isn't that much after all - if the figs are correct then your insulation is great, or the place is freezing! Are they definitely right? Even at 33p/kWh and assuming the same use 365 days of the year, a massive overestimate, that's still "only" 18.9*0.33*365 = £2280. Do you have any other kWh figures - eg. yearly kWh consumption? I'm confused!

It won't be wasted. AC power flows both ways through a transformer just fine, somebody else will end up using it - so excess PV is likely to ever so subtly reduce gas used by a turbine somewhere.

£8000 is one heck of a bill. My guesstimating makes that around 26MWh/year of elec, so I estimate 200kWh/day on cold days. If you have actual numbers rather than my guesses, it would help! I know Octopus "Go" and others offer cheap night time rates - I think 7.5p/kWh in a 4 hour window, but will cost more the rest of the time. Interestingly, this cost is similar to gas (I know you can't get), and it's also similar to the new normal 30p rate divided by typical ashp COP. You can get ashp where all the gubbins is indoors, a couple of pipes drilled through an external walls for air flow, might be acceptable? Or simpler, using your present boiler, you could get 14.4kW*4= 56kWh/day cheaply. Or likely a maximum of 100A*230V*4hours = 92kWh/day if you got storage heaters too, just time everything to operate on cheap rate only. You can only get half the daily energy you need that way though - I agree with others, more insulation !

Do you have a generation electricity meter? I would expect so, it is standard install procedure. If it's all a bit DIY and you haven't (maybe it's not needed if not FIT or SEG, I don't know), I recommend getting a sparky to fit one. Check it once a month or more, then you can compare with the installers expected yield graph you presented. Maybe something you have already integrates the power, to energy in kWh? The instantaneous kW depends on system direction, shading, inverter clipping (it will be set to do so at just under 4kW unless you asked for DNO permission), temperature. Pic? The fusing issue may or may not be related. What exactly trips - solar or whole house? Is this an RCD or RCBO in your consumer unit, or something else - pic?

We have a Vent Axia Sentinel Kinetic Plus BH, and like many units it comes with a few 0-10V analogue inputs for external controls, it even has a handy +24V supply output. One of them I've connected up to a CO2->0-10V sensor powered from that 24V, and the vent axia can be told it's a CO2 input where 10V=2000ppm, and told over a certain ppm to proportionally increase fan speed. Here's the sensor I used: https://www.digikey.co.uk/en/products/detail/amphenol-telaire/T8031/5774483 We've had this add-on for ~5years, It boosts when needed, or it all slows right down if nobody is home. If the humidity is high the MVHR increases, or if CO2 is high it increases. I generally set the desired speed quite low, and let it just get on with things, depending on CO2 and humidity. MVHR units should just come with this built in I think, rather than expecting people to adjust up or down. We left our MVHR on it's factory settings initially, and ended up with a very low humidity house full of itchy people with dry skin!