self use query

[peter138]

Hi All
Newbie here
Can I specify a new Fox system to do the following:
No solar
Large batteries circa 30kWH
Large inverter circa 10kW
Charge batteries on off peak economy 7 daily (max 7 hours)
Use batteries rather than grid all day (17 hours)
BUT
Occasional power draw would be greater than 10kW i.e. more than the inverter can supply (normal use @ 2kW plus new ASHP @ 4kW plus occasional induction hob @ up to 8kW)
When inverter power is exceeded, can I just draw excess power from grid (automatically)
I ask because the data sheet and user manual refer to self use being limited by system CAPACITY, rather than my query regarding system POWER
The wiring diagrams seem to me to imply that the inverter is connected in parallel to the grid, so I believe this would serve my purpose
If the inverter is in parallel, how does it limit/switch off grid supply?

Thanks in advance

[Dave Foster]

Yes it can do all of that, if single phase you would be looking at the KH10.5

Batteries can be added to get over 40kWh capacity (either EQ4800 or EP11)

The inverter is wired in parallel with the grid, it is grid tied and works by measuring the load at the incoming grid feed and maintaining a balance to achieve zero grid import but if you were to demand more anything above the inverters peak output will be taken from grid.

[peter138]

Many thanks for the swift response Dave

Next questions:

Is there a recommended SOC range e.g. max 90%, min 20% gives a range of 70%, so that I need to upsize my capacity by 1/70% =~ 140%, and then would inverter/battery efficiency losses consume another 5-10%? Is there a cheat sheet to size batteries from known energy usage?

Separately, since the HV batteries are connected essentially serially, is it not harmful to then push up to 50A constantly through each battery and maybe also through each and every cell. In an alternative low voltage setup, say 48V, there would be high degree of parallelism in configuration with much lower currents through each battery/cell, with the inverter having to do more grunt work, so is this not a better setup?

[WyndStryke]

Generally people just use the default range 10% - 100% (since it is 90% DoD). The warranty will cover you for 6000 full 10%-100% cycles (EC/EQ) for 12 years, or 4000 cycles (EP) for 10 years with the extended warranty, and you should have at least 80% capacity remaining after doing that.

If you are very worried about battery degradation then you can use a narrower range if you wish (for example, 20% - 80%). However, if you restrict the range that you use, you still need to periodically hit 100% and 10% so that SoC calibration is not lost (lets say weekly for 100% and monthly for 10%).


I'm currently mainly sticking to 15% - 87% with periodic trips to 10% and 100%, and doing about 1.3 full cycles daily.


Other factors that affect battery degradation are cell temperature extremes, high charging speeds, and obviously the number of cycles. So if it is particularly hot, I have an automation which reduces my overnight charge speeds, and I also have a fan which will start up if my inverter approaches 45c, or the battery cells exceed about 35c.

All of this is above&beyond what is needed. Most people don't worry about it.

But one thing I would suggest is that you take account of the expected battery degradation when you are working out how much battery capacity you need. After 10 years the capacity will deteriorate to some extent, it should still be better than 80%, so I'd say allow a little extra space so that you don't run out a a few years time.

[Dave Foster]

Separately, since the HV batteries are connected essentially serially, is it not harmful to then push up to 50A constantly through each battery and maybe also through each and every cell. In an alternative low voltage setup, say 48V, there would be high degree of parallelism in configuration with much lower currents through each battery/cell, with the inverter having to do more grunt work, so is this not a better setup?

I think @wyndstryke has covered the battery SoC, but just to add with LV batteries you typically get higher currents but with the EP11 they have a nominal voltage of 384V and so at 10kW they are running at 26A and similar with the EQ batteries in a full stack.