Posted by: @majordennisbloodnok

I have, as I have previously detailed, a solar PV array, a battery and an inverter. I also now have an export tariff with Octopus so that excess production can at least earn us some money.

Currently, however, we are still without power in the event of a power cut. My ideal would be for a setup that would allow us to continue exporting excess, but also to continue using solar produced electricity and any battery-stored electricity in the event of a power cut. We've been told this is not practical, but I'm an electricity luddite. Could anyone give me a fairly simple explanation as to why what seems simple is in fact rather more involved?

Much appreciated.

Hi,

I will try to make the explanation as simple as possible.

During a power cut, your home and all the others in your area are still physically connected to the local distribution network, it is this network itself that is disconnected from the larger grid system. So if you were to try to power the equipment within your own home at the normal 240v, 50Hz, either from a solar PV system or a battery system, you would also be trying to power all the other homes on the local distribution network, so your solar PV or battery system would immediately overload and trip out.

It would be possible to disconnect your home electricity system from the local distribution network, and there are actually disconnection switches designed for just that purpose, but then the problem becomes one of control. A normal solar PV system contains an inverter, which takes the DC power produced by the panels and converts it into AC power, at a voltage and frequency that matches that of the grid supply. Without this reference from the grid the inverter is not able to function. Even if it was possible to operate the inverter without the reference supply, the voltage would probably be difficult to control as the output from the solar panels varied and the load increased or reduced. It would not be possible to run a 3kW kettle, if the solar PV system is only generating 2kW.

An adequately sized battery system, with the correct type of inverter that does not required the grid reference supply, could possibly be used to provide power to your home, but only if your home system is totally disconnected from the grid supply, and the home power demand is limited so that the system is not overloaded. With such an arrangement it may also be possible to use the available power from a solar PV system to power your home and also feed the battery system with any excess.

There are already in use many totally off grid systems around the World, so the equipment is readily available, and there may even be systems on the market that would meet your requirements of transferring from grid supply to off grid supply and back in an automated manner. Maybe time to ask Mr. Google. 

That’s an explanation I can certainly understand. Makes perfect sense. Thank you.

@derek-m, that’s an excellent explanation. I’ve been looking into “full grid backup” solutions this week, and it’s ridiculously complicated.

Fir starters, most battery backup systems are limited to 20A and sometimes to the rating of the inverter.

The downside to all grid backup systems is the requirement for capacity to be held back to allow the system to have some juice for the possibility of failure.

AC coupled systems also don’t allow charging on the ESS side so in the scenario of losing power on a sunny day, the PV system shuts down. Hybrids, I believe, don’t have this issue.

I’m hoping to meet some installers in the weeks and figuring this out. 

@editor you need a chint to disconnect you from the grid "islanding". Some battery systems (Powerwall 2) have one built in. Others (like mine) need an external Chint -  https://midsummerwholesale.co.uk/buy/sofar-storage/sofar-eps-chint-nc1-2508

That would allow me to swap to running only on battery, however the power would be limited to my maximum battery and inverter power (3kw). I could of course get 2 inverters and chints and be able to use 6.4kw

I'd need to spend a load of money on an electrician to come and reconfigure me consumer units. I'd need to isolate the "critical loads" e.g. the fridges and disconnect the non-critical/massive power hogs e.g. the heat pump, AC etc

@batalto, thanks for that. That makes sense. Can you remind me what system you have and how much it cost (hardware + installation)?

@editor we have 1 Sofar ME3000SP hybrid inverter (I think they are £700?) And then 14kw of pylontech batteries. They range in price but I think we paid around £2500 for them as three of the 2.5kw batteries were used and cost £1100 for all three.

I'm looking at getting a second battery charger and batteries this year. The prices just make it a no brainer given our fix will expire in October 2022.

@batalto, interesting. I remember now you had the Pylontech batteries. Can your system go off grid in the case of a power outage?

@editor no it can't. I've considered it, but frankly I don't think we've ever had a power cut. If we do a load of electrical work I might consider getting it done, but I wouldn't do it as a stand alone job.

It's not worth the hassle as 3.2kw of power isn't enough to run the heating and we'd need, in essence a backup battery that's always fully charged. If we get more storage then sure. But you need a good whack of batteries and at least some stored to make full use - no issue in the summer, more of a problem in the winter.

If you want to get some batteries let me know. I know a supplier in China who will do big discounts for volume.

@majordennisbloodnok, are you going to try and get your system working offline? 

We're looking at Givenergy's solution with two 8.2kW batteries that would work when the grid's offline. I have a few phone calls scheduled for next week on the subject and then hopefully we'll place an order.

Out of curiosity, with the heat pump switch off, what does everyone's home idle at in terms go electricity consumption (keeping things like fridges, charges, computers, etc.) running?  

At present, I see no advantage in having batteries. E-on is offering an off-peak charge of 32.11p/kWh which still makes it dearer than my current fixed rate of 26.5p/kWh. The market will need to change significantly to accommodate energy storage before I would consider battery storage. Here is a report from the House of Commons Public Accounts Committee with commentary from Richard Murphy which raises concerns on the government's commitment to renewables. https://www.taxresearch.org.uk/Blog/2022/03/04/the-uk-government-is-falling-way-short-in-its-planning-for-climate-change-war-cannot-be-used-as-an-excuse-for-not-doing-better-this-too-is-an-existential-threat/