BartW Posted January 24, 2021 Share Posted January 24, 2021 Hi, I am after a bit of advice. I am currently sizing a system for our future home. Current facts: We live in a 2 bed maisonette at the moment. The two of us and a dog Factual 12 month energy use is 3000kWh (Jan '20 to Jan '21, so enough to stipulate 50% daytime occupancy). That is a combination of usual appliances, induction hob, UFH, and plenty of LED mood strip lighting. The current property is 70sq metres. The future: The two of us, and two kids, and (maybe) two dogs Property size 180sq m. Fully detached, 0.15uVal all around Timber Frame home, with all as the above, but also 7kW ASHP, MVHR, very open downstairs, and a total of 4 bedrooms (+gym on the top floor). Also some smart home gear (if money were to allow). Assumed 12month energy use 6000kWh. The following Roof: Assumed 10no 445W panels along the pitched roof (South - tiny-bit-East roof), and 6no 445W panels on the flat roof. All connected up using individual microinverters. No shading. Total 7120W @ 20% panel efficiency x 70% realistic efficiency due to weather, etc gives me 996W per h. If I average 7 hours a day in beautiful Oxfordshire, then there is a potential for 7kWh of energy. Paired with "alleged" 16kWh daily consumption, this is technically not enough to warrant a battery storage, BUT we all know that energy recuperation times are never equal the energy required, due to the lifestyles, and other factors. Take for example washing machine, and tumble dryer. These would usually work evening, or weekends. Would the battery storage help with the "energy management"? If so, would a 7kWh choice be optimal? I can only imagine that anything bigger would never get fully charged, unless we are on holiday. So with the above in mind, I can probably build a 7kWh PV with 7kWh battery for not a lot more than £7k + VAT. Am I being logical? Or a bit naive? Many thanks on this freezing mid Winter Sunday Bart Link to comment Share on other sites More sharing options...
ProDave Posted January 24, 2021 Share Posted January 24, 2021 Battery storage may be worth it. Anything over 4kW PV and you will struggle to self use all of it in the summer even dumping surplus into water heating. And remember there is no FIT so at best if you jump through all the hoops you might get 5p perkWh that you export. So in my book anything above 4kW needs costing to see if the extra you can self use is worth it or if some battery storage will be worth it. We self use virtually all we generate but that is largely by using the big appliances in the daytime and about 1/3 of the generation ends up in the immersion heater. Even of you are out is is worth putting them on timers to run close to mid day. And don't forget for more than 4kW you need permission form the DNO so get that permission before you place an order. Link to comment Share on other sites More sharing options...
TonyT Posted January 24, 2021 Share Posted January 24, 2021 You can fit 7kW and limit to the single phase maximum. EV also a good way to use the surplus.o Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 (edited) Go to PV GIS and see what the calculator works it out as. https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP Edited January 24, 2021 by SteamyTea Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 53 minutes ago, ProDave said: Battery storage may be worth it. Anything over 4kW PV and you will struggle to self use all of it in the summer even dumping surplus into water heating. And remember there is no FIT so at best if you jump through all the hoops you might get 5p perkWh that you export. So in my book anything above 4kW needs costing to see if the extra you can self use is worth it or if some battery storage will be worth it. We self use virtually all we generate but that is largely by using the big appliances in the daytime and about 1/3 of the generation ends up in the immersion heater. Even of you are out is is worth putting them on timers to run close to mid day. And don't forget for more than 4kW you need permission form the DNO so get that permission before you place an order. The difference between 7kw and 4kW works out about £1500 + VAT. How big is your system Dave? 37 minutes ago, TonyT said: You can fit 7kW and limit to the single phase maximum. EV also a good way to use the surplus.o What do you mean by "limit to"? Sadly, only a 6.2V8 on the driveway for now. Where do I put the plug? ? Link to comment Share on other sites More sharing options...
TonyT Posted January 24, 2021 Share Posted January 24, 2021 The inverter limits the output to the max permitted Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 5 minutes ago, SteamyTea said: PV GIS That suggests I would produce 1500kWh per year. Which is a 1000kWh less than my brief calculation. Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 6 minutes ago, TonyT said: The inverter limits the output to the max permitted Why would I then want to spend money on the 7kW system in that case? Or is it to limit what I feed back to the grid? Sorry, slightly confused here... Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 Just now, BartW said: That suggests I would produce 1500kWh per year. Which is a 1000kWh less than my brief calculation. Well assuming 70% of 20% is optimistic. As you are putting some of it on a flat roof, you get a lower yield than on a pitched roof. Link to comment Share on other sites More sharing options...
ProDave Posted January 24, 2021 Share Posted January 24, 2021 2 minutes ago, BartW said: That suggests I would produce 1500kWh per year. Which is a 1000kWh less than my brief calculation. My 4kW with a LOT of shading generates way more than that, so you must have done something wrong for 7kW Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 Just now, ProDave said: My 4kW with a LOT of shading generates way more than that, so you must have done something wrong for 7kW Here is what I get: Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 Just now, BartW said: Why would I then want to spend money on the 7kW system in that case If you have more modules than seems necessary, you get more power at the lower end, even allowing for the inverter's lower efficiency at part loads. It is all a balancing game and using averages. Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 1 minute ago, SteamyTea said: Well assuming 70% of 20% is optimistic. As you are putting some of it on a flat roof, you get a lower yield than on a pitched roof. That could negate the worth of the extra expenditure indeed. Link to comment Share on other sites More sharing options...
ProDave Posted January 24, 2021 Share Posted January 24, 2021 1 minute ago, BartW said: Why would I then want to spend money on the 7kW system in that case? Or is it to limit what I feed back to the grid? Sorry, slightly confused here... At the dark end of the year, having more panels will let you eek out more from the limited daylight. In summer it will get to full output earlier and maintain full output later. The inverter will slice the top off the output graph. the only reason for doing that is if the DNO refuse a higher power connection or say there will be a big upgrade charge to allow it. Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 1 minute ago, ProDave said: My 4kW with a LOT of shading generates way more than that You get more hours of daylight in the peak generation period than down south, that will account for a bit of it. You are 7° North of me. Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 I see. In view of all of the above, would I be better off investing into a big battery, say 10kWh? Or stick to a 7kWh, and upgrade if ever necessary (chances are not worth it anytime down the line). Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 Or just buy in power as cheaply as possible. battery storage prices are still way too high, They need to drop to about £60/kWh to make financial sense. You would save more just buying a Leaf or a Zoe, or not getting dogs. https://www.amazon.co.uk/Time-Eat-Dog-Sustainable-Living/dp/0500287902 Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 4 minutes ago, SteamyTea said: Or just buy in power as cheaply as possible. battery storage prices are still way too high, They need to drop to about £60/kWh to make financial sense. You would save more just buying a Leaf or a Zoe, or not getting dogs. https://www.amazon.co.uk/Time-Eat-Dog-Sustainable-Living/dp/0500287902 I would hate to eat a dog, or drive a leaf or a Zoe Link to comment Share on other sites More sharing options...
TonyT Posted January 24, 2021 Share Posted January 24, 2021 I would try and have the incoming power in a garage so that inverters, meters, consumer unit, future batteries can all be added, configured and commissioned with little or no disruption to the house. I would oversize the PV as it’s cheap in the grand scheme of things, it will allow more useable generation in winter months. Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 38 minutes ago, TonyT said: I would try and have the incoming power in a garage so that inverters, meters, consumer unit, future batteries can all be added, configured and commissioned with little or no disruption to the house. I would oversize the PV as it’s cheap in the grand scheme of things, it will allow more useable generation in winter months. I was hoping to stick the battery in the attic space, right next to MVHR. I am assuming it would be best to site the inverter close to the battery, so in the attic, and run the AC side back to the meter in the downstairs utility? Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 Check the regs about putting inverters and batteries in lofts. Think they may have stopped it because of the fire risks. Link to comment Share on other sites More sharing options...
Russdl Posted January 24, 2021 Share Posted January 24, 2021 7 minutes ago, SteamyTea said: Check the regs about putting inverters and batteries in lofts. I don’t suppose you have a link that specific reg do you? Link to comment Share on other sites More sharing options...
SteamyTea Posted January 24, 2021 Share Posted January 24, 2021 5 minutes ago, Russdl said: I don’t suppose you have a link that specific reg do you No, just something I heard from some electricians a decade ago. It would be in the latest electrical regs, whatever edition we are in now. 1 Link to comment Share on other sites More sharing options...
TonyT Posted January 24, 2021 Share Posted January 24, 2021 (edited) But if you put the battery in the loft, allowing for the weight and higher operating temps you then need to know exactly cabling requirement back to consumer unit. where as if you say fit a 100x100mm trunking along the wall and fit everything below the trunking cabling through the trunking, it makes everything easier. can’t imagine getting a battery in a loft The latest Tesla battery is 114kg! Edited January 24, 2021 by TonyT Link to comment Share on other sites More sharing options...
BartW Posted January 24, 2021 Author Share Posted January 24, 2021 55 minutes ago, TonyT said: But if you put the battery in the loft, allowing for the weight and higher operating temps you then need to know exactly cabling requirement back to consumer unit. where as if you say fit a 100x100mm trunking along the wall and fit everything below the trunking cabling through the trunking, it makes everything easier. can’t imagine getting a battery in a loft The latest Tesla battery is 114kg! Tesla battery is £6k, too equivalent capacity something else works out half that it weighs less too (about 75kg). Nevertheless, I can see the reasons behind not fitting in the loft. Quite frankly it could probably become a concern to me at some point. What about fitting one on the outside wall beside the ASHP? Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now