Beelbeebub

We've been getting sun pretty much all week, but my problem not is my damn house is getting in the way! The sun is so low it doesn't rise above the ridge of the house. I get a brief 2h of direct sun, but fortunately generate 4kw over that time so can top up my battery nicely. At some point I'll lose the direct sun but that was a trade off I was willing to make.

I have solar and a battery and gas heating so I currently use old fashioned economy 7. I charge my battery overnight at 13.5p and then any elec the panels generate that isn't used in the house is exported at 15p. When the sun goes down we run off the battery until the next charge period. If it's been a bad day solar wise and we've used a lot of elec (washing etc) then we migbt use a bit of peak rate elec (30p) for a few hours in the night. My average cost per unit is about 15p and if I include my PV it"s about 2p at the moment.

If I'm honest, there doesn't seem to be much wrong with your setup. The 17th was pretty cold where I was and I assume where you were, so £5.46 all in isn't too bad. Remember this is for one of the coldest days you are likely to see. In August your per day costs were much lower but it stands to reason your heating costs will be higher in the winter! One thing that does stand out is you're using the agile tariff whilst also using lots of 4-7pm elec. Because the unit cost changes hour to hour and day to day the cost on any given day can vary wildly. For example here is the cost for the 25th Oct As you can see it was near zero in the early morning and only a few p/kwh for most of thr day with a peak of 30p 4-7 Now compare that with a few days before. At no point was the unit price below 10p but the peak was huge (off the gaph, at 60p). You could could charge an electric car up (say 75kwh) on the morning of the 25th for less than running your dishwasher after tea on the 22nd. Over a year you migbt save vs a Cosy tariff but the day to day costs will occasionally be a real shocker. For your sanity, look at the monthly or weekly costs rather than daily and try to minimise any use in thr peak eg dishwasher, washing machine. You should def make sure your heat pump is not heating hot water in that time slot unless it absolutely has to. There will be settings you can change for that. If your house doesn't lose heat too quickly you could try setting the target temp back a bit in that slot to use a bit less. Otherwise your usage seems about waht would be expected - unfortunately energy is expensive these days. Good. Luck!

If you used 31kwh yesterday (20th) that's pretty good. The 20th was pretty cold (dunno where you are but it was sub zero across the Midlands). If we assume your house uses something like 6kwh a day for non heating stuff like fridges and lights, then you used 24kwh in a day, basically 1kw continuous heating load. If we assume a cop of about 2.5, that's a heat demand of around 2.5kwh. Not unreasonable for a house kept at 22-24C during a subs zero cold snap. If I read your graphs correctly on the 20th you used 31kwh and paid £6.52. That's 21p per kwh. Coukd you post some pairs of graphs for the kwh in a given day, then the £for the same day. I suspect you are losing out by using some energy during the agile peak period. You could save a fair bit by making sure your hot water heating doesn't occur then and also maybe turning your heating off for those hours. Agile is best used with some sort of automatic system that adapts your use to the variable daily price curves. You migbt get a better price using Cosy.

Just for perspective our gas heating has shot up during this cold snap (shock!) We were sub 50kwh a day then the cold snap hit and we are more like 100kwh plus - and that's with a log burner going to take the edge off things.

Could the Op post the kwh usage graph from. Octopus. Thry say they are on agile which has a massive spike between 4-7pm (eg today 21st Nov it's neatly 50p/kwh) Agile is for people with a battery (mostly) For a HP you need a Cosy tariff.

Yes inverters have efficiencies, depending on the operating range it can range from 90-97%. Even if you take the absolute worst cases (90% in, 90%out and 95% battery) you're still over 75% round trip. But at more general levels my system can beat 90% round trip efficencies. I think the drop in price of hybrid systems has made them a "no-brainer" for the majority of installations - not least because even without PV you can save by time shifting your demand. Obviously there will be edge cases where it doesn't make sense - but the majority of those will be the result of tarrifs that may not hang around. If the ratio of import/export prices widens or the price of imported electricity rises batteries will make even more sense.

i am assuming he is talking about round trip efficiency ie how many kwh it takes to charge a battery so that you can discharge 1kwh. For lead acid batteries it can be 60% round trip ie you only get 6kwh out after putting 10kwh in. LFP batteries are very good in these circumstances with efficiencies in the 95% plus range. This is independent* of the nominal and usable batter capacities * nearly - i believe you get worse round trip efficiencies if you use all of the usable capacity vs a smaller subsection (eg 20-80%) but it's not a huge effect and i believe most data sheets assume a higher depth of discharge 10-100%

The grid is already sized and set up for all the houses on it. If you have enough PV and battery that you never export or import you essentially dissappear from the grid in the same way as if you disconnected. Yes your neighbours can't use your solar but there is now more headroom in the infrastructure for increaced demands from them. Your contribution to the local grid will not be as stable, when thry are charging an EV and your panels are cutting in and out because of clouds the grid is having to take up the slack. They may not even be charging at that time and your lower might be effectively powering a factory across town - which requires the grid to carry the current. If I installed a 2Mw solar farm, more than enough to run all the houses in thr area, the grid would still prob be overloaded because it couldn't carry the current from me out. Imagine a block of flats with water pipes, a big water main splitting into smaller pipes for each floor and then smaller ones for each flat before even smaller ones for individual taps. If I had a magic water fountain, that produced free water from nothing, I still couldn't provide water for the entire block just by plugging the fountain into my kitchen tap. On the other hand, if I (and some other tenants) used our magic water fountains to satisfy all our individual water needs, the remaining tenants without magic water fountains would have better flow and pressure.

You're right, 30p saving by using a battery. Given the sun doesn't sine 24h a day at some point we cannot use the panels directly. Our options are use the grid or use a battery. The viability of using the grid as your battery (ie exporting when you have excess and then importing when you have shortfall ) is critically dependant on the price ratio between import and export. Without a battery the best you are likely to get is 15p all day, and the best import you can get is 25p because you can't time shift without a battery. This is equivalent to a storage efficiency of 60% and is unlikely to stay as good. The SEG rate is a better long term metric which is around 5p IIRC (could be more but it's nowhere near 15p). This drops the efficency really low. For a battery, your estimate of 6 to 7kwh in to get 5kwh out implies a round trip efficency of 70-80% which I think is too low it's more likely 80-90% or better. To be fair panels are so cheap now that *if you could get a really high export limit*, like 10kw or something you could have a huge array, big enough to export so much elecreicty in the day and summer months that you could build up enough credit to import "off sun" and still end up ahead. But you would need the space for the panel and the high export limit.

But your import rate is likely to be higher - at least outside of night rate. So exporting at 15p and importing at 25p makes the financial efficency 60%. Say you produce 10kwh during day and over the same 24h use 10kwh During the generation period you use 5kwh and export 5kwh (earning 75p) Then after suunset you use 5kwh, buying it all from the grid at a cost of 125p Net loss of 50p If you stored the 5kwh you would forego 75p but not spend 125p. And lose 20p in depreciation. Overall a difference of 5p - chicken feed really, though it does widen if your power requirement is higher or if the imoort/export gap widens (eg the 15p rate is cut or price of day rate elec Rises) *but* the battery option has zero impact on the grid - there is no import or export

But batteries allow you power your air con in the evening/night with solar from thr day atow cost (4p/kwh assuming 4.5k cost for 14kwh usable over 8k cycles) And crucially at zero extra load on the grid.

Batteries.

Sadly, people will always try to diddle the system, whether it be a."free" air conditioner or a £120 million contract for faulty PPE. Which is why subsidising the consumer's bills for a period would be alot better - yes people could and would try to fiddle things by bypassing meters etc, but thry do that anyway.

The whole grant situation, especially with the closed shop MCS, invites grant harvesting. I still maintain using the subsidy to subsidise the heating costs of the HP system to guarentee a HP is always cheaper than gas would be a better way forward