S2D2

Hybrid inverters have an extra CT clamp close to the meter so they know how much is being exported and can adjust the battery charge accordingly. As you say, anything DC side of the inverter can soak up excess PV up to its max charge rate even if the inverter is going full pelt pushing to AC. AC side is more complicated, my Solax inverter is set to G98 and will only generate up to 3.68kW AC, regardless of how much the house steals before it hits the meter. Someone in a similar thread stated their inverter could go over 3.68kW if the house was using the difference, so I can only assume that's a g99 setting with export limitation.

It depends entirely on your usage, see here for an outline of the method to optimise the size: Installers will almost always push you to get the biggest battery they can sell you, but the diminishing return drop off is very aggressive for the consumer. Flux changes the calcs considerably, but could disappear at any time so best not to plan on it but use it if available.

Interesting, the Solax is also LifePO4 but far more conservative with the current, which I just sort of assumed was normal. What capacity and temp is your battery for this charge?

Here's one that does, a small Solax battery rated for 2.5kW charge, the reality is far more complicated:

Okay, keep the current ratios between rad output and scale at a house level to the heat demand that pops out the automated calc. This would adjust for improvements, and may come out as no change required.

Again seems simple, assume the current rads are appropriately sized and adjust for the new flow temp using the manufacturers heat output function. Usually about 3x. Blame the homeowner if it ends up undersized. Open that replacement job up to any local plumber and I reckon you'd see £2.5k drop off the quote as a total guess.

Yes apologies this is only for replacing gas heating. DHW and cooking can be estimated from summer use, or assume an average to reduce the monitoring period to just December.

Sizing is a very simple process if a smart meter is installed, there are companies that provide this service for far less than the £300-£400 I've seen quoted for ASHP surveys: https://www.buildtestsolutions.com/building-performance/smart-htc-heat-loss-calculation The company behind Bright are experimenting with this I believe, and there's no reason it couldn't be done with £25 worth of (reusable) sensors rather than their fairly ridiculous £230 fee. Scale to government level and this gets you a sizing for every property that is likely to need a boiler replacement in the next year for peanuts. It's a simple regression fit of data and highly accurate when I did it DIY on my property, came within a couple of kWh on the design OAT. Came out as a 4kW ashp for me, whereas MCS calc came out at 6kW. Just the other £14.5k to worry about then.

I think this is the only question not answered above, I can only speak for Solax but I assume they're all the same (someone will be along to correct if not!). For G98 it's 3.68kW to the AC side, regardless of what your house steals before it gets to the meter. I was a bit disappointed by this as it doesn't matter to your DNO until it hits the meter but I assume they're not confident enough in the CT reading to pump more than 3.68kW into the AC and guarantee it's used down to 3.68kW before hitting the meter, which is understandable. Also cost cutting etc. You can squeeze more out of the panels by leaving spare battery capacity, I've had to write a bit of control software to do this as it's not a standard feature. Then your max panel generation is 3.68 + max battery charge (2.5kW in my case). Note that max charge rate is both temperature and charge level dependent, so this gets complicated very quickly. I believe there is now a fast track for up to 7.36kW export which didn't exist when I put my system in, I definitely would have gone this route if it did as managing the battery charge is just unnecessary complexity, even if automated.

Might as well be accurate! I've mentioned it before but if you have a smart meter you can get 13 months worth of historic data via Bright.

Interestingly it does need factoring in when modelling the Flux tariff as the cheap/peak periods do follow the clock changes. A good chunk of my export has moved into the peak period since they went forwards.

Yes not in the spreadsheet sorry, it's here: https://joint-research-centre.ec.europa.eu/pvgis-online-tool/pvgis-tools/hourly-radiation_en

From memory it states UTC in the downloaded spreadsheet somewhere, that's what I used it as when predicting system performance.

PD needs expanding to two units imo, enough for a full A2A multisplit install or a phased move from Gas -> A2A offsetting gas -> A2A + A2W. All the current fees do is incentivise scrapping a perfectly good A2A unit to get the A2W under PD and spending that money on the A2W install.

Thanks @RichardL, very helpful. £330 to planning that could have been spent on renewables, that's half the cost of my A2A install...

How much did it cost for planning if you don't mind me asking? If I install an A2W it will also be a second outdoor unit. As others have said, for PD I considered following the MCS requirements that were relevant to planning (noise, visibility from street) to be all that's required. System performance is not within their enforcement remit. Someone could test that with a certificate of PD if they wanted a watertight guarantee. I remember some concern about the wording including MCS Contractor, here is the definition from their standard: I.e. distinctly different from a certified MCS Contractor?

Use either the current immersion power or the forecasted solar generation for the rest of the day as your blending coefficient. If you want to get really fancy model the current energy in the tank and the current loss rate.

It's possible both energy companies got overruled, the process is very convoluted and they are bound by it: https://octopus.energy/blog/secret-life-opening-meter-reading/ If your usage has dropped before switching that may have been the reason it's so high. Either way both companies use the same reading so you wouldn't pay twice. The process should be much simpler with smart meters and it shouldn't be described as an "actual read" when it's not, only one of those is in the hands of the energy company though.

Yes, but you have to be on Flux import too, so the cheapest you can buy it in is 19.67p. You can still profit of course, just a smaller gain.

£17.04 in export payments for the previous three days, £3.06 in gas hot water usage. Important to remember it's an unusual few days and not get tempted into investing in a divert system based on the "worst" case, it's the argument solar installers use to sell more unnecessary batteries too.

Can't speak for other manufacturers but the Solax portal is only updated every 5 minutes, so useless for export logic. There is a local REST API that can be called to get live data though, I call mine every 10 seconds to log the values so much better for control systems. Your Arduino can then call this rather than querying a separate CT clamp.

Check if your inverter has a local API you can call via HTTP, then you can get your export power reading for free.

This is optimal PV divert benefit

There's no exit fee so unless someone has been refused I don't think they'd have an issue with switching every six months. That may get more complicated soon as fixes start coming back in, I wasn't with octopus when they did them for the smart tariffs but I believe it was a one year fix? Flux is probably the first test of this heavy seasonal skew and it still may not be worth switching depending on usage, so I don't think they'll be plagued with an annoying amount of customers switching often.

There may just be a good reason for this but you might find a quick call/email will hurry things along, Octopus are quite good for that. Think they estimated five weeks when I set up a new export and it was done in five days. 15p export skews the sums significantly, I couldn't really justify a few hundred for the divert so you're probably just looking at one of the good cheap suggestions mentioned in the thread rather than anything more significant.