Yes I agree @derek-m . They're using an Automatic Transfer Switch on grid failure.

I think that's unnecessary, and the switch-over transient might take out IT-equipment which must then re-boot.

I think the transfer between grid inverter and the battery should be handled by the same mosFETs which get used for charging. In effect the AC-connected device(s) are always connected, and it's up to the inverter to 'decide' whether the required 240v 50Hz is derived from the battery or supplied from the inverter's own mains-input.

If the inverter is in Charge Mode, then the load devices share its mains incomer. If the inverter isn't charging the battery then it creates the 240v AC for the load. It matters not whether the grid supply is up or down at that stage.

The external control system isn't used for switching between grid and battery.

But a control system is required in order to synchronise the charge-times to the ToU tariff slots. In the UK, those half-hour slots are only 'known' to your Smart Meter. Your Energy Supplier can't tell where the timing boundaries are.

SMA isn't a British company, although they sell a lot of product here. Without taking their control system apart, I can't tell how they know when you are entering/leaving a cheap-rate half-hour slot.

@transparent  I’m not looking for back up in the event of grid failure, if I was I’d have a battery at least double the size.

Im simply looking to make as near zero use of peak as possible while running the house and EV off solar and off peak using a TOU tariff.

Thanks for all the useful comments.

@transparent 

You cannot just connect and disconnect such a system to and from the grid, without correct synchronisation. That is one of the reasons why a solar PV system cannot continue to work if still connected to the grid, when the grid supply is lost.

If my understanding is correct, the present TOU tariffs that are available are actually higher than the standard tariffs. To convince homeowners to spend thousands of pounds installing the type of system that you propose, would require long term guarantees that a cheap rate tariff will be available long into the future.

I fully agree that every effort should be made to encourage a reduction in energy use at all times, and particularly during the peak periods, which under normal circumstances are fairly well defined. I also suspect that problems will arise in the future as more and more homeowners are encouraged to adopt Heat Pumps and EV's.

Every hybrid inverter already has a mechanism for connecting and disconnecting to the grid, and handling the required synchronisation.

The reasons that a grid-connected PV string inverter cannot continue to operate if the grid fails are both technical and legislative.

• technically it can't operate without the grid because it contains no electronics to create a 50Hz sine-wave. It acquires that ability due to its grid sensing system. The definition I'm using for a hybrid inverter is that it does have the ability to create its own 50-Hz output. (Not every company uses the same criteria).
• legally it can't operate without the grid because an inverter with grid-export capabilities must incorporate anti-islanding in order to obtain the G98 certification. That means it must 'forget' the 50Hz synchonisation when the grid fails, and furthermore may not re-connect until a predefined & random time interval has elepsed.

The system I'm developing has no grid-export capability. With the SEG (export guarantee) at just 4p per kWh, that's not a high priority anyway!

TOU tariff. I believe that the only nationwide Energy Supplier to have ever offered a ToU tariff is Octopus. That has indeed been withdrawn from new applicants last autumn due to the current high price per kWh.

But that situation isn't going to exist indefinitely. Indeed it's just as likely that Ofgem will need to issue a requirement for all domestic suppliers to offer at least one ToU tariff in order to move the UK towards the 2050 Net Zero target.

Have you heard the news today that Octopus are to offer a free "Power Hour" to its customers once per month? They haven't abandoned their ToU strategy. This marketing event is intended to alert customers to the way ToU tariffs will operate in the long-term future.

Standard tariffs may well be cheaper at the moment, but let's not forget that is due in no small measure to the Energy Cap. That will change in April... and probably again later in the year.

Posted by: @derek-m

To convince homeowners to spend thousands of pounds installing the type of system that you propose, would require long term guarantees that a cheap rate tariff will be available long into the future.

There are benefits to having sustainable energy storage whether or not there is such a guarantee.

There will increasingly be more by-the-hour fluctuations in the price of electricity as more renewables are incorporated into the mix.

The simplest ToU tariff is Economy-7. The concept of a cheaper price period of the day won't disappear just because lots of EV-owners now want to charge cars overnight. But when that cheap rate occurs may shift. That's why newly-developed technologies must be ready to embrace moving time-slots... a genuine ToU tariff, such as Octopus' Agile.

Returning now to the project being undertaken by @yachtsman - you have clearly stated that you intend using a ToU tariff. Good.

So has your system designer demonstrated how the proposed equipment is to achieve this?

What mechanism is to be used to let your Solar-Edge inverter know

• when to charge the battery from the grid, at what rate and for how long?
• when to not charge the battery because there is sufficient solar generation?
• how to apportion the amount of energy to store and how much to use within the house?

@transparent 

Hi,

I think that may have adequately clarified your message now, and hopefully got it out to a wider audience. 😎 

@transparent There are a large range of profiles based on time, tariff, seasons, special days etc that can be overlapped, here is a selection of the default options.

Thanks @yachtsman for that document.

So you will be setting up a few charging/usage profiles using a web interface to a server operated by Solar Edge. These will operate seasonally throughout the year.

You can also add single-day profiles which allow you to reflect specific requirements for a day, which they call "Add Special Day". It might be a long weekend or possibly to make use of an 'offer' of cheap electricity slots which you are informed about by your Energy Supplier.

There is no shortcut to automatically populate the online profile slots. If your Energy Supplier sends you a text to give you electricity at 5p/kWh between 10:00 to 14:30 tomorrow and you wish to store this in the battery, then you must manually enter that time-frame into a Special Day profile.

This raises a number of issues in my mind which may or may not concern you...

1. If the internet goes down for a few days (a storm severs the telephone line) can your Inverter/Battery continue to operate the profiles which you've set up on Solar Edge's server? Or does their server connection send the commands each time when a charging change is required? What level of 'commands' are stored in your Inverter in advance in order to cope with such an issue?
2. Solar Edge's online software has profile forms that divide the day into 48 half-hour periods. These are defined according to UTC plus timezone offsets. They do not coincide with the half-hour time-slots used by your Smart Meter. Depending on what 'random offset' your particular meter uses, this will produce billing discrepancies. Do you want me to demonstrate this with a diagram?
3. There is no provision for IFTTT style of 'programing' a profile. Eg you can't say "If EV charger is used in the afternoon, take power from the grid instead of the battery because I want to keep that stored charge for the early evening".
4. There is no variable-consequence option within a profile. You can't say "If the solar input is sufficiently high that the battery will be full before 16:00, then do...."

My breadth of knowledge on Solar Edge's profile system is necessarily pretty narrow. I've only briefly read the PDF you linked to. So not all of my illustrative descriptions above might be 100% valid. But I hope you get the drift of what I'm pointing out.

Equally, the way you run your house may mean that you don't care about any of these scenarios. That's down to personal choice.