@old_scientist wherever a lithium power cell is located is irrelevant to wether your house will burn down.

recall the houses in Kidderminster next to the golf course grass that caught fire, once your garage is alight, not much will put out the blaze; lithium is exothermic in contact with water!

Posted by: @old_scientist

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Of course everyone's usage is different, and I completely get your argument - it makes little sense to spend £7k on a powerwall to ensure you are 100% running from cheap rate, when a £2k plugin battery may get you 95% of the way there and the additional £5k in costs yields marginal returns.

Absolutely 

We heat DHW on cheap rate so that load doesn't figure.  Ditto washing machine and dishwasher.  That leaves only kettle and oven as the high intermittent loads ocurting during peak rate.  Kettle is irrelevant because it's on for 3minutes, leaving oven.  Perhaps 45 mins every couple of days (our hob is still gas).  Absolutely pointless spending a large amount extra to cover peak loads in our case.

@old_scientist There’s such enormous flux in energy markets and hydrocarbon markets that making an informed and rational investment decision at any point in time is meaningless as in less than 6 months the base case could have flipped on its head. For example I generated £2k less value this year compared to 2025, just because one business decided not to renew a particular tariff. Or my old German diesel tanks sitting on the drive cost me nearly twice as much to drive as they did only 3 months ago.

The current US administration is revolutionary in character, it’s not an impossibility that the future uk government will go the same way too and everything we once thought certain could change on a dime. It’s not impossible that for ideological reasons, heat pump and EV owners owners might be intentionally financially targeted in the future as the spark gap increases. 
Very unsettling times have already started. 

Posted by: @agentgeorge 

The restriction on how much you can export is set by the DNO and you need the G99 to export above 3.6kW per phase, which has to be applied for “before” installing a system.

That is correct, however, 3.6kW and below, you only need G98 notification which works in a different way. The installer simply installs a 3.6kW inverter and notifies the DNO within 28 days that they have done so. In other words everyone can have up to 3.6kW output. There should be no DNO restriction to 2.5kW. You need to dig deeper into what is going on, and should be able to get 3.6kW without issue if your inverter can output that much.

G98 is connect, commission, notify within 28 days and is always free of charge from the DNO.

G99 is apply, wait for DNO approval and only then install. Approval can take 45+ working days, however, some DNOs run a fast track scheme for smaller G99s (max 7.2kW) that can be a lot faster.

I have a 5kW inverter and applied for an additional solar PV system with another 5kW inverter. This was rejected and I was told the maximum I could have was 5.7kW, so stayed with the existing system that I have. Since then I have had look back at the maximum grid voltages occurring on particularly sunny days at quiet times for consumption e.g. 2pm and it sometimes tops 252V, so I think in my case, about 400m from the nearest substation, they are right to limit outputs. 

@rob-of-york i know about G98 cert. My PV installer initially proposed 10 panels so I didnt go over that limit and they could install next day. I wanted more panels, basically a “roof full”, so had to pay for G99, wasn’t too long either.

I have 2 Inverters in my system, one for PV, the other for the battery pack. Again it was advice from PV installer to not use a hybrid one that does PV and Batteries.

The one connected to the PV is smaller and is capable of over 5kW. The one connected to the battery pack is twice the size and has a limit of 2.5kW.

As I didnt commission the system, I don't know its limits, my guess is the limit could be there to protect the battery pack from a discharge rate it can’t cope with; have to ask installer for an explanation 

@papahuhu I was just checking the Cosy Fixed rates again in my area, and note they have increased 1.75% since I switched one and a half weeks ago, so now stand at ~16% higher than the variable rates. Maybe indicative of where Octopus view prices going this winter, or at least indicative that Octopus view the outlook has not improved/has deteriorated in the last couple weeks

Still, glad I have the heat pump, as I think the diesel and kero market will be even more stretched that gas markets this winter. Heating oil prices are still climbing, up to £1.18/L (11.4p/kWh) at my local supplier. Once you factor in the efficiency of an oil boiler, that's price parity with a heat pump on Cosy which is 4 times more efficient so a quarter of the running costs.

Posted by: @agentgeorge 

I have 2 Inverters in my system, one for PV, the other for the battery pack. Again it was advice from PV installer to not use a hybrid one that does PV and Batteries.

The one connected to the PV is smaller and is capable of over 5kW. The one connected to the battery pack is twice the size and has a limit of 2.5kW.

The DNO is only concerned with the maximum power that your equipment can export to the grid. So if you have a 5kW inverter and another inverter limited to 2.5kW that are both connected to the grid, then from the DNO's perspective you have 7.5kW of potential export and would therefore have required G99 approval to cover that.

If you have G99 for 7.5kW, and want to be able to get 5kW out of either inverter, but never export more than 7.5kW onto the grid, then there maybe a way of doing this via a dynamic arrangement rather than the static settings that you have now. This involves having a CT clamp reading the current on the grid input/export to the house. This value is then used to control the overall output from the inverters ensuring that the total export never exceeds 7.5kW - an additional controller may be needed to do the interfacing. You could ask your installer if this arrangement can be set up using your existing inverters. 

@rob-of-york

The 2 inverters are not in parallel, but series. The PV inverter routes its power to the battery inverter, which charges and discharges the battery.

The export to the Grid is controlled by the battery inverter, so Grid export is limited by that inverter and ultimately the MCB in the CU.

It decides what power to send down to the CU based on the clamp reading.

its all smoke and mirrors

Just furkled Into the parameter menu. Found the master inverter has a max output of 3kW.

This is the max it can ask the Battery Pack for.

The PV can produce just over 5kWh on a sunny day at 1pm, so potentially the house could draw 7-8kW and not import an electron on a sunny summers day

Hi everyone ☺️

Just checking that I haven’t missed out on a better export rate.

I’m currently with E.ON Next on a fixed tariff for import. Unfortunately, they no longer allow you to have their 17.5p/kWh export tariff if you’re on a time-of-use tariff. Because of this, I had to leave them for export, as the rate dropped to just 6p/kWh.(I have no battery)

I’m now with Fuse Energy, who allow me to join their 13p/kWh export tariff even though I’m not with them for import.

My E.ON Next import tariff is a great deal, so I don’t think anyone can beat it, and I’d like to stay with them for import.

Are there any other providers that would take on someone like me—where I can continue to have my import with a different provider, but get a better export rate than 13p/kWh?

Thanks in advance! 😊

It all depends upon whether, in October and beyond, you a a nett importer or a nett exporter of power?

if you want to win the TOU tariff game and have a heat pump and PV, most people flip tariffs in Oct. When PV production is rapidly decreasing whilst ASHP demand is started.

At 16kW that’s a very thirsty heat pump you are running, I run the same size but older version. How much power do you use over the heating season, for me it’s about 7MWh. So getting that unit rate down as low as possible is critical. Have you got any third party controls that allow you to gently bump the flow temp up and down according to the prevailing TOU tariffs? 
(With a large het pump, having batteries make more sense as your space heating demand increases.)

@papahuhu Very true! For now, I probably won’t export much and will import loads as winter kicks in, especially as the tariff I’m on is only 5.71p/kWh. Thankfully, the 6 hours is enough to cover around 90% of my heating and hot water.

I’ll just wait and see what changes happen. Thanks for the advice! 😊