The Emergency Power System (EPS) is the method of using power from your Solar Batteries to provide electricity to either a socket, a group of circuits or your whole house in the event of a power cut.
How you choose to set up your EPS along with the appropriate settings will depend mainly on:
1) The circuits you need/want to be powered in the event of a power cut.
2) The amount of a battery “buffer” you need/want to keep in reserve to provide backup power when needed.
I will cover the following topics in this article.
1) The “levels” or “methods” of EPS setups along with key differences and considerations.
2) The battery settings around minimum state of charge levels and what you should set them to.
3) The EPS settings on the inverter and how they should be set based on your EPS setup.
4) Any FAQs that are asked and added to this article over time.
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1) What are the various levels of EPS and what considerations are there for each one?
Two main points to make before going into the levels.
1) EPS or no EPS, if there is a power cut, the inverter will shut down completely. This is to protect any power line workers that may be working on them. As you can imagine, without this, the power lines could become energised by solar producing homes and that could be an unwelcome shock to someone who will assume the power is off!
Setting up the EPS will ensure that this doesn’t happen and enabling the opportunity to continue powering your home in a safe manner for everyone.
2) The need for an earthing rod.
If your earthing is supplied through the incoming supply, this will be disconnected in the event of a power cut and therefore, you will need to have an earth rod at your property to maintain the earthing in your home. Whether you are connecting just one socket or hooking up the entire house to the EPS, you will need one. A qualified electrician will know this but if you choose the DIY route, just make sure this is in place for your safety.
Here is a handy guide on the types of earthing in a home.
https://electricalapprentice.co.uk/an- ... d-bonding/
You may already have an earth rod (referred to as a TT earthing arrangement), in which case, you shouldn't have any issues in this area (a good electrician will test your earth to ensure it is adequate).
If your earthing is supplied by the grid, your electrician will need to make additional modifications on top of adding an earth rod to ensure your home electrical system is safe in EPS mode. This requires quite a technical explanation (see later in the article) but as this should be left to a competent person to carry out, it shouldn't be too much of an issue!
I have attached some links to helpful resources to explain the various EPS levels, but I have summarised the main area below.
https://kb.givenergy.cloud/assets/EPS%2 ... n%20v2.pdf
https://tanjent-energy.com/blog/power-c ... y-storage/f
Level 1 – Automatic Socket
This involves connecting just one socket to the EPS output of the inverter. In the event of a power cut, this socket will become live after a few seconds. This is the most straight forward method and can power some lights, fridge, etc. from an extension cord.
With a reasonable level of DIY knowledge, if you have ever wired a socket or plug in the past, it’s likely you can hook up a socket to the EPS port.
Just remember, the socket earth MUST BE CONNECTED to an earth rod. If you have a “TT system”, it’s likely you’re already set to go but if not, at a minimum this socket must be connected to a ground spike.
You should follow the instructions in the manual in how to do this, but the outcome can be seen in this video posted on the Facebook group.
https://www.facebook.com/1250590769/vid ... 978714899/
Level 2 – Automatic Circuits
Like Level 1 above, but in this case, the EPS output is connected to a single circuit or a group of circuits. This is either through part of an existing consumer unit or a separate consumer unit on its own. This method is automatic, and the power will kick in after a few seconds in the event of a power cut.
This would power just the essential circuits such as lights, fridge and one or more rings, for example. This ensures that high load circuits such as EV chargers and cookers are unpowered, ensuring the EPS does not overload due to the cooker being on or the car being on charge.
This would almost certainly involve accessing the main consumer unit and thus would not be a DIY job. You would need to be qualified or hire a qualified electrician to set this up.
Level 1 & 2 are the only levels that allow you to utilise the UPS (Uninterruptable Power Supply) mode. Read more on this mode in the settings section later in this article.
This is where the EPS output is routed into your main consumer unit. In a power cut, your entire home will be powered via the EPS.
However, there are a few requirements and caveats with this method. You’ll need additional hardware installed and you need to be careful about how much power is being drawn while in EPS mode.
You will need a “changeover switch” that, when pulled, will isolate your home from the grid and in turn, energise the EPS.
This is a manual action and in a power cut, you must pull this switch before any power is supplied from the batteries.
You’ll also need to be careful about what you turn on when in EPS mode. The EPS can supply up to 5kw of power (approx. 20A in the UK). If you draw more than this (say you have the oven on and then switch on the kettle and microwave), the inverter will overload and shut down requiring it to be reset in order to continue working again.
One way around this is to pull the circuit breaker for the circuits you don’t want powered before you throw the “changeover switch”. I usually pull the breaker for the EV charger, cooker and outbuilding. This will make it unlikely that I put anything high powered on that would overload the EPS.
Ironically, this is likely to be cheaper to implement than Level 2 because a simple changeover switch can be bought for as little as £40. An electrician just needs to wire the EPS into this switch and in theory doesn’t need to touch the consumer unit.
https://www.electrical4less.co.uk/produ ... er-switch/
As mentioned at the beginning of the article, if you're earth is supplied by the grid, not only will you need an earth rod installed, your electrician will have to ensure that the EPS neutral is connected to local earth rod when in EPS mode and simultaneously disconnects the grid earth. This applies to both Level 3 and Level 4 (below).
A more detailed diagram and explanation can be found here:
https://github.com/TonyM1958/HA-FoxESS- ... ypass-mode
Level 4 – Whole House Automatic
This is in principle the same as Level 3 above.
The main difference is that the “changeover switch” is an automatic one which senses the voltage on the grid/EPS and switches automatically.
This is perhaps “overkill” for most households. One area where this could be useful is if there is essential medical equipment or systems that require powering while you are out of the house or cannot physically get to the “changeover switch” to manually activate the EPS.
It could be argued that Level 2 can be utilised instead in this situation where only the essential circuits are connected to the EPS rather than the whole house.
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2) What battery settings should I use when considering the EPS?
The FoxESS inverters have two settings involving the “Minimum Charge Level” of the batteries;
• “Min SoC” and
• “Min SoC (On Grid)”.
Min SoC – The minimum battery level the inverter will allow the batteries to fall to in any situation.
Min SoC (On Grid) – This is the minimum battery level while there is grid power being supplied to the home.
- MinSoC.jpg (6.7 KiB) Viewed 37863 times
If you don’t have EPS set up, then most will have both of these set to 10%. This is the lowest the batteries will go to in order to protect them from damage.
If you have EPS enabled, then you may want to set Min Soc (On Grid) a little higher in order to create a buffer to allow a backup of power to use in the event of a power outage.
To be clear, this will ensure that you have the minimum power in reserve to run the EPS for a set amount of time that you are comfortable with. In many cases, especially in the summer, you'll likely have well in excess of the minimum. Having the battery buffer in place is likely to be more important to you in the winter when solar production is lower, and your battery is frequently empty by the end of the day.
Let's look at an example...
You have Min Soc set to 10%.
You have Min SoC (On Grid) set to 20%.
In a normal environment with grid power on, the battery level will not drop below 20%.
This allows a permanent 10% buffer at a minimum which you can use in a power cut. While the power is out, the inverter will continue to supply power until the battery level drops to 10% and then will stop providing power.
How much of a buffer you keep will entirely depend on how much battery capacity you have and how much power you want to retain in case of a power cut.
Let’s say you have 20 kWh worth of capacity, then a 10% buffer will mean up to 2 kWh in reserve. A typical baseload of a home can be anywhere between 100-300w so this reserve would only last 6 to 20 hours. You could lower the baseload by turning off standby appliances, non-essential lights, perhaps a freezer than can last a period of time without power, for example. This would extend the time this buffer would last.
To extend the amount of time the EPS would last, you would either need to increase your battery capacity or increase your buffer by setting your Min SoC (On Grid) to a higher %.
For info, I have 17.92 kWh (7 x HV2600) worth of battery capacity and I have a 10% buffer allowing me up to 1.8kWh which roughly equates to about 8 hours of EPS time for me at my baseload usage. I have Level 3 set up which means I need to pull the “changeover switch”. Before I do, I pull the circuits I don’t need such as cooker and outbuilding (includes EV charger) and reduce my baseload to as low as I can.
3) What are the various EPS settings on the inverter and how should they be set for your chosen EPS setup?
There are 4 main settings to consider:
1) EPS Output - Enable/Disable
2) Bypass Relay - Internal/External
3) EPS OL AutoRec - Enable/Disable
4) Grounding - Enable/Disable
5) UPS Output - Enable/Disable
EPS Output
This is an obvious one. If you are setting up your EPS or already have it set up then this setting should be set to Enable.
If not, or you simple do not want to make use of the EPS whatsoever then simply set this to Disable.
Bypass Relay
There are two ways in which the EPS bypass relay can be set - there is an internal relay, or you can install an external relay.
When would you use which option? Well, it would depend on what level EPS you have installed.
Internal - This is to be used for Level 1 and Level 2.
External - This is to be used for Level 3 and Level 4.
What is a relay? In short, a relay is a switch which moves the power output from normal AC output to EPS during a power cut.
How does the inverter know if there is power cut?
Well, you may have noticed on your IHD that occasionally, there is small amount of power being imported during normal operation.
In a nutshell, if the inverter can no longer import the small amount of grid power, it assumes there is a power cut and activates EPS mode.
In levels 1/2, there is no need for an external switch (manual/automatic changeover switch) and therefore, the internal relay is needed and relied on to switch the power to the EPS output in a power cut.
As soon as the inverter senses a cut in grid power, it will switch to the EPS output until the grid power is restored. This happens automatically and requires no input from the user unless you accidentally overload the EPS. In which case, you'll need to reset the inverter (or not, see the next bit below on autorecovery).
Why does this all matter? Well, as there is no path for the EPS power to feed back into the grid (it has its own socket or circuits) there is no risk to people working on the power lines. Thus, there doesn't need to be an external disconnection between the internal system and the grid. When power is restored, the relay will switch back automatically.
In levels 3/4, you are required to install an external changeover switch (external relay). When the grid fails, in levels 3 & 4, we have to be sure that there is a physical disconnection of the system from the grid before the EPS will energise. As the EPS output is powering the whole house through the main consumer unit, the changeover switch acts as a physical gateway between the grid and the internal system. This ensures that there is no possible path back to the grid via the EPS output which would put workers on the power line at risk.
EPS OL AutoRec
When the EPS is overloaded - say you've put too many appliances on at the same time (doh!) - the EPS and/or the external breaker for the EPS will cut out for safety.
If the external breaker for the EPS trips, then of course you will need to manually reset this.
If the EPS on the inverter cuts out due to an overload, then by default you will need to reset the inverter manually. However, by setting "EPS OL AutoRec" (EPS Overload Auto Recovery) to Enable, after a short while, the EPS will reset itself automatically following an overload.
It's important to consider which to choose. If the EPS has overloaded, you may want to check that whatever caused the overload in the first place won't happen again.
Grounding
If your entire home has its earth protection provided by an earth rod (typically called a TT system, for example. See link at the beginning of this article), then it’s likely your main consumer unit, and subsequently, your inverter will be grounded to this. Under normal operation and in a power cut, your inverter will not provide any errors related to grounding and any operation of the inverter will be safe (Grid or EPS).
Some homes however do not have an earth rod, and instead earth protection is supplied by the grid (sometimes called TN-S or TN-C-S). In a power cut, this earth protection will likely be cut and this leaves your system vulnerable and could result in damage to your inverter or you and any visitors to your home if the system has not been earthed properly.
In levels 3/4, you realistically have no choice but to add an earth rod to your home to connect all of your circuits and inverter to the ground. I’m not aware of any alternatives to this although that’s not to say they aren’t any. In 99.9% of cases though, unless you live on a rock, you’ll be able to install one and it is the most cost effective method. In this case, you would set the Grounding option to Disable.
Why would you ever need to set this to Enable?
If you decide to install level 1 or 2, you will still have to install an earth rod. In most cases, a good electrician will still connect every circuit (including the main consumer unit, inverter and the socket or circuit(s)) connected to EPS) to the earth rod. However, if this is not practical for any reason and you decide to install an earth rod and only connect it to the EPS supplied socket or circuit(s), then in a power cut the inverter will throw up a grounding error. Your EPS supplied socket or circuit(s) will be protected but the error will cause the inverter to shut off.
This is where you would set the Grounding option to Enable.
This will stop the error occurring. You are effectively manually telling the inverter that the EPS sockets are grounded and to stop the grounding error from triggering. For the sake of everyone’s safety though, especially if you are installing Level 1 on a DIY basis, please ensure the socket is grounded before switching this to Enable.
UPS Output
The UPS output (Uninterruptable Power Supply) is essentially an enhancement of the EPS output in that it keeps the EPS output active during the switchover from Grid to EPS and vice versa.
UPS is usually defined as the ability to switch quickly enough to stop electronics such as PC's, servers, etc. from temporarily powering down during a power cycle between Grid and EPS. This is usually defined as a switching time of less than 20ms.
Whether this is enabled or not, it is rendered useless for whole house levels 3 & 4 as it is clearly impossible to pull a manual switch within 20ms and an automatic changeover switch will not guarantee to switch quickly enough.
Thus, if you require true UPS capability for server rooms, etc. then you either need to provide dedicated UPS units where necessary or follow levels 1 & 2.
4) FAQs
Do any of the EPS setups need DNO permission in the UK?
No, at the time of writing this article, none of the EPS setups require specific permission from your DNO.
There are standards to be met of course by the installer or electrician. This includes ensuring there is means for the system to isolate itself from the grid to ensure EPS power doesn’t feed back into the grid and endanger powerline workers in a power cut. This is assumed to be met in the eyes of the DNO.
More will be added over time as questions are asked outside what has already been covered...
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This information is a combination of my understanding of the manuals, my research on EPS methods and the numerous conversations on the Facebook group.
If you believe any of this to be incorrect or there are additions to be made, please let me know!
I hope this mini-guide helps with your own thinking & plans for your own EPS set up.
If anyone has any helpful additions/modifications to this article, please post it here and feel free to ask questions if you need any help.
