SBMS Posted March 9, 2022 Share Posted March 9, 2022 (edited) Hi I am looking into battery storage options for a new build and as I understand it need an AC inverter or charger and then some batteries. At the moment it is looking like PV is a no go because of roof design so I am purely looking at charging from the grid. I plan on charging a 28kwh battery array on Octopus Go for the 4 hour period and then effectively running all the house off that stored energy, procured at 7.5p. On single phase I’m assuming that the maximum draw can be 32A to produce 7.4KW. My question is when looking at inverters/ chargers they have a KW rating and I wanted to understand - is this the maximum KW that the inverter will pull from the grid? I was looking at, for example, the GivEnergy hybrid which has a 3.6 and 5 KW version (https://www.givenergy.co.uk/pdf/Version 2.0/Hybrid Inverter.pdf). Would this mean that the 5 KW would, over a 4 hour period, only be able to charge 20kWh of my battery array? Or have I got it completely wrong? thanks! Edited March 9, 2022 by SBMS Link to comment Share on other sites More sharing options...
Marvin Posted March 9, 2022 Share Posted March 9, 2022 Hi @SBMS I think your thoughts above open up a whole load of questions. The first would be about the use of the energy: What do you hope to run from the batteries? Our fuse box (Consumer unit) has a 100amp fuse and some times we require 60amps all at the same time for cooking, washing, heating, etc. So when I looked at running the house on battery storage I realised that I would need a system to be able to handle the peak loads and gave up. I assume you have taken into account the following in your calculations: The maximum power you can draw from the batteries: Ours are about 40% Each time you convert the power you loose some. So AC to DC and then DC to AC again. Yes a 5kW system charging for 4 hours would give 20kW, less the energy lost converting, but the link info doesn't seem to be saying that is what it does. Are you saying you require 28kWh of power over the 20 hour period? If so you would need about 80 kWh storage depending on the loss converting DC to AC. Other bright sparks will correct my thinking I'm sure.... M Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 Hi @Marvin This is interesting. I think the inverter referenced outputs AC at up to around 20A so you’re right that would be a limiting factor. I guess multiple inverters could fix this? When you say max power you can draw from batteries is 40% - not sure what you mean here? I’m just trying to understand where the 80kwh storage to serve 28kwh of required energy comes from? Are you saying the AC to DC to AC conversion process is only 35% efficient? Link to comment Share on other sites More sharing options...
SteamyTea Posted March 10, 2022 Share Posted March 10, 2022 (edited) 6 hours ago, Marvin said: Our fuse box (Consumer unit) has a 100amp fuse and some times we require 60amps all at the same time for cooking, washing, heating, etc. So when I looked at running the house on battery storage I realised that I would need a system to be able to handle the peak loads and gave up. Not quite the case, unless you are totally off grid. The battery storage can supplement the grid supply, it does not have to replace it all. Whether it is financially worth it at 14p/kWh at night and 23p/kWh during the day is the tricky bit. With my daily mean usage of 2 kWh (50p worth), I doubt it is. Edited March 10, 2022 by SteamyTea Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 (edited) 20 minutes ago, SteamyTea said: Not quite the case, unless you are totally off grid. The battery storage can supplement the grid supply, it does not have to replace it all. Whether it is financially worth it at 14p/kWh at night and 23p/kWh during the day is the tricky bit. With my daily mean usage of 2 kWh (50p worth), I doubt it is. My idea was to store as much as possible (up to 28kWh of power) on the Octopus Go overnight tariff which is 7.5p. This would then be used throughout the next 20 hours. We have high electricity usage (predicted peaking around 28kWh per day in winter months with the heat pump). I was trying to understand whether a single inverter/charger could charge 28kWh in the 4 hour time window. I think Marvin was indicating that to get 28kWh of AC out, you would have to put 80 in, which seems strange - my electric car doesn’t charge at this level of efficiency using its inverter? Edited March 10, 2022 by SBMS Link to comment Share on other sites More sharing options...
SteamyTea Posted March 10, 2022 Share Posted March 10, 2022 1 hour ago, SBMS said: I think Marvin was indicating that to get 28kWh of AC out, you would have to put 80 in, which seems strange No, he was explaining that the most efficient 'window' is to only charge and discharge at 40% of rated capacity I think. There are also inverter limits. If you want a very high capacity inverter, that starts to cost. Though you can use two inverters. 1 hour ago, SBMS said: the Octopus Go overnight tariff which is 7.5p This is an unsustainable tariff and not something I would base an investment decision on. The power companies have to make the exact same calculations, except their 'storage' is basically buying in extra capacity when they are caught out, usually from gas powered sources. Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 (edited) 9 hours ago, SBMS said: Hi @Marvin This is interesting. I think the inverter referenced outputs AC at up to around 20A so you’re right that would be a limiting factor. I guess multiple inverters could fix this? When you say max power you can draw from batteries is 40% - not sure what you mean here? I’m just trying to understand where the 80kwh storage to serve 28kwh of required energy comes from? Are you saying the AC to DC to AC conversion process is only 35% efficient? Hi @SBMS What is your peak demand in amps? Generally you should only discharge a battery down by 40% Converting power costs power. It think losses of about 15% can happen. You would need to check the equipment specification. So you want to use 28kWh over 20 hours you will need to discharge say 20% more to convert from DC to AC so you need to draw from the battery array about 31kWh which is only about 40% of the battery capacity so batteries need to have about a 78kWh capacity. To charge about 31kWh you will need about 20% more so about 34kWh which is about 9kW output for 4 hours. The lost power is basically converted into heat. Many inverters indicate the max power conversion rate which is actually only for a limited time. I think one of the deciding factors for us was coping with our peak demand. If you are mixing the battery supply with the mains supply you will find you do not use the power evenly. My guesses: Inverter anywhere between £1.5k and £3k for a 10kW! Batteries anywhere between £10k and £50k Cabling, switches fuses housing and installation costs (not sure about the main grid permissions/costs if any) Basic set up cost say £6k ( I'm sure someone will correct me if I'm wrong... ) 34kWh at 7.5p is £2.55 a day. Normal power rate say 25p per kWh so 28kWh at 25p is £7. Therefore saving per day £4.45 if battery system fully used every day. So final money guess: Basic system installed total cost £25k System used fully 60% of the time so saving about 60% of £4.45 each day is about £2.67 £25,000 divided by 2.67 is break even in about 26 years. As @SteamyTea says will the 7.5p price lasts? In conclusion, in my humble opinion, the whole problem with PV is not the production of power but the storing and subsequent use. We use batteries on "isolated from the mains low power continual demand systems" charged through our PV and hope to include the MVHR this year. Our 5.12kW system supplies ALL the power we use in the year, just not exactly when and at what rate we need it. Good luck. M Edited March 10, 2022 by Marvin Completely missed priced the batteries Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 38 minutes ago, SteamyTea said: No, he was explaining that the most efficient 'window' is to only charge and discharge at 40% of rated capacity I think. Is there some data on this? I know EV batteries, for example, don't have this efficiency window (although the BMS does slow charging rates as capacity is reached - is this what is being referred to?) 39 minutes ago, SteamyTea said: There are also inverter limits. If you want a very high capacity inverter, that starts to cost. Though you can use two inverters So does a "5KW' inverter refer both to the maximum power it will charge an array (AC to DC), as well as the maximum power it will discharge an array back to the house (DC to AC)? And can that power capacity simply be 'doubled' by adding an additional inverter? 41 minutes ago, SteamyTea said: This is an unsustainable tariff and not something I would base an investment decision on. I can see your thinking - although does green energy not make a difference? The wind still blows midnight to 4am, power is still being generated, but the demand isn't there, so I do think that there's a likelihood these tariffs will continue - in some form. At what level, and for how long is difficult to predict, and it's the dilemma I face. My ROI analysis sees a system being paid back within 3.5 years. 2 minutes ago, Marvin said: Hi @SBMS What is your peak demand in amps? Generally you should only discharge a battery down by 40% Converting power costs power. It think losses of about 15% can happen. You would need to check the equipment specification. So you want to use 28kWh over 20 hours you will need to discharge say 20% more to convert from DC to AC so you need to draw from the battery array about 31kWh which is only about 40% of the battery capacity so batteries need to have about a 78kWh capacity. To charge about 31kWh you will need about 20% more so about 34kWh which is about 9kW output for 4 hours. The lost power is basically converted into heat. Many inverters indicate the max power conversion rate which is actually only for a limited time. I think one of the deciding factors for us was coping with our peak demand. If you are mixing the battery supply with the mains supply you will find you do not use the power evenly. My guesses: Inverter anywhere between £1.5k and £3k for a 10kW! Batteries anywhere between £1k and £12k Cabling, switches fuses housing and installation costs (not sure about the main grid permissions/costs if any) Basic set up cost say £6k ( I'm sure someone will correct me if I'm wrong... ) 34kWh at 7.5p is £2.55 a day. Normal power rate say 25p per kWh so 28kWh at 25p is £7. Therefore saving per day £4.45 if battery system fully used every day. So final money guess: Basic system installed total cost £6k System used fully 60% of the time so saving about 60% of £4.45 each day is about £2.67 £6,000 divided by 2.67 is break even in about 7 years. As @SteamyTea says will the 7.5p price lasts? In conclusion, in my humble opinion, the whole problem with PV is not the production of power but the storing and subsequent use. We use batteries on "isolated from the mains low power continual demand systems" charged through our PV and hope to include the MVHR this year. Our 5.12kW system supplies ALL the power we use in the year, just not exactly when and at what rate we need it. Good luck. M Hi Marvin Thanks for this - really interesting. Do you mind explaining your current setup - as you've clearly done your homework and have something that works? Our limitation is our roof design wouldn't really get more than 10 panels on which limits our system size. Whats your monthly kWh usage out of interest? We seem to use around 4500kWh in rental here - so we will likely use that, plus I'm estimating the same again to run the heat pump and DHW. Thanks Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 Hi @SBMS I had to revise the battery costs in my last post Link to comment Share on other sites More sharing options...
Nickfromwales Posted March 10, 2022 Share Posted March 10, 2022 12 hours ago, SBMS said: At the moment it is looking like PV is a no go because of roof design Can you upload your plans? 9 hours ago, SBMS said: I guess multiple inverters could fix this? Not quite. Its the number of connected cells that dictates the discharge rate, which is then further 'choked' by the limit of the throughput of the BMS. It's the batteries that are the thing that slows down rates of charge and discharge. 1 minute ago, SBMS said: And can that power capacity simply be 'doubled' by adding an additional inverter? As above, no. 8 minutes ago, Marvin said: As @SteamyTea says will the 7.5p price lasts? Not very long! This will go up again after your initial 12 month contract with Octopus runs out. They will likely ask you to adopt a fixed rate tariff first, and then migrate you to the ToU tariff. The fixed will be >40p/kWh from the get-go. 12 minutes ago, Marvin said: Many inverters indicate the max power conversion rate which is actually only for a limited time. Correct. You would also be hammering the guts out the system every single day, so its lifespan would be reflected in that usage pattern. You would need to choose equipment that had a warranty based on unlimited throughput. 14 minutes ago, Marvin said: Basic set up cost say £6k ( I'm sure someone will correct me if I'm wrong... ) Not even in the ball-park for a system such as the OP's requested capacity. Triple that, unless going for the cheapest of Chinese. @SBMS If AC coupled, have you yet done a DNO application? You would likely need a 3-ph solution, further increasing costs / complexity. Link to comment Share on other sites More sharing options...
Nickfromwales Posted March 10, 2022 Share Posted March 10, 2022 7 minutes ago, Marvin said: Hi @SBMS I had to revise the battery costs in my last post How did you get a price of £6k for 28kWh sized system? Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 7 minutes ago, Nickfromwales said: How did you get a price of £6k for 28kWh sized system? Yes I messed up. I have revised figures. Searched for 80kWh but given price for 10kWh storage 1 Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 I also think the peak demand is a BIG price concern. Out hob usually uses about 21kW when 3 rings on. So I think there are alot of unknowns from @SBMS causing caution in my costing. 30kW sine wave inverter price? Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 5 minutes ago, Nickfromwales said: Can you upload your plans? The roof with the 4 veluxes are the south facing roof. It's tight getting between the dormer window bits and the veluxes.I might be able to shift the veluxes up a bit. Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 Think I'm going to change my name to Slayerofdreams. 1 Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 7 minutes ago, SBMS said: The roof with the 4 veluxes are the south facing roof. It's tight getting between the dormer window bits and the veluxes.I might be able to shift the veluxes up a bit. What pitch is the roof? Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 1 minute ago, Marvin said: What pitch is the roof? 40 degrees.No shading: Link to comment Share on other sites More sharing options...
SteamyTea Posted March 10, 2022 Share Posted March 10, 2022 18 minutes ago, SBMS said: Is there some data on this? It is all do do with Coulombs, as said by Nick, generally charge and discharge rates are sized to the storage capacity, though that is not the be all and end all of it. Lithium Ion batteries can have a very high discharge rate, why they are used in BEVs, but to increase longevity, they are charged slowly (there used to be a limit on how often they could be changed at maximum rate). 23 minutes ago, SBMS said: The wind still blows midnight to 4am, power is still being generated, but the demand isn't there, so I do think that there's a likelihood these tariffs will continue - in some form We are long was off that point yet, and it is easy and cheap to turn off a few turbines than pay to turn off a few large freezers. There is a balancing mechanism that sorts all this out, supermarkets get paid to turn of loads. The system is set up to minimize price and emissions, but it limited i.e. you can't turn off a large nuclear or hydroelectric plant. Link to comment Share on other sites More sharing options...
SteamyTea Posted March 10, 2022 Share Posted March 10, 2022 9 minutes ago, Marvin said: Out hob usually uses about 21kW when 3 rings on. What are you frying up, whole sides of pig? 3 Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 12 minutes ago, SBMS said: 40 degrees.No shading: I don't know much about PV panels and the different costs or outputs of roof integrated ones, but I would use the ones that replace the tiles on a lovely new build. Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 38 minutes ago, Marvin said: Our 5.12kW system supplies ALL the power we use in the year, just not exactly when and at what rate we need it. @MarvinWhats your 5.12kW system? On-roof PV? Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 7 minutes ago, SteamyTea said: What are you frying up, whole sides of pig? Oh f...! Yes your right I meant AMPS not WATTS. Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 8 minutes ago, SBMS said: @MarvinWhats your 5.12kW system? On-roof PV? Yes Link to comment Share on other sites More sharing options...
SBMS Posted March 10, 2022 Author Share Posted March 10, 2022 Just now, Marvin said: Yes Any battery storage? Link to comment Share on other sites More sharing options...
Marvin Posted March 10, 2022 Share Posted March 10, 2022 The good thing for us is the PV power (when available) direct to the ASHP. And in the summer ASHP cooling when we get LOADS of power. Link to comment Share on other sites More sharing options...
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