Using grid when batteries are full or quite full ?

[twowheelsgood]

Hi,

System installed a month ago and I have recently changed over the Octopus Pulse, so cheap rate charging is from 0200 to 0500.

Scheduler is set and the batteries are charging to full capacity at the end of the charging period.

I have;

16 panels producing 7kWp
FOXESS 5kw Hybrid 1 Phase Inverter
2 off FOXESS 4.8kWh batteries

I have noticed that when the electric showers are used, the draw is almost exclusively from the grid and not the batteries.The peak of the pull from the grid is 5.3kW but most of the oull is below 5kW which I assume is due to the inverter limit.

Anyone advise why the battery isnt the primary source of current ? I can understand if theres a 'start up time' for the batteries being able to supply, but I would have thought the grid draw would be backed off until the batteries have kicked in.

Is there a setting I am missing somewhere ?

Many Thanks

[Dave Foster]

Assuming your batteries are not in a charge period and you are set to self use, the batteries should be used to meet the load.

If your inverter is the H1 (G2) the max battery discharge power is 40A (the G1 only 35A) and as you only have 2 batteries the battery voltage will be ~90V so the max power the batteries can meet will be approx 3.6kW - you haven’t mentioned what power the shower is but i’m guessing it’s 8.5 to 9kW which is why 5.3kW is coming from the grid.

You could add more batteries to increase your max power output, a 3rd battery would bring you up to approx 5kW which would match your inverter output.

[twowheelsgood]

Thank you - a very concise answer - much appreciated.

So would it be your belief that the batteries are being used in the first instance and its the balance thats coming from the grid and showing up - or that unless you can get close to 100% from the battery/inverter, the grid will come first ?

I will check the shower type and power usage.

I guess this is 'one of those things they dont tell you about' when sizing your system. I for one had never imagines electric showers were that hugely hungry for power ! I had assumed in my payback calculations that electric showers are powered from the batteries and therefore effectively free in the summer, but that appears to not be the case.

[Dave Foster]

So would it be your belief that the batteries are being used in the first instance and its the balance thats coming from the grid and showing up - or that unless you can get close to 100% from the battery/inverter, the grid will come first ?

I will check the shower type and power usage.

I guess this is 'one of those things they dont tell you about' when sizing your system. I for one had never imagines electric showers were that hugely hungry for power ! I had assumed in my payback calculations that electric showers are powered from the batteries and therefore effectively free in the summer, but that appears to not be the case.

Yes the batteries are being used but at their maximum power and you are seeing the balance then come from the grid.

Electric showers have large heaters in them anywhere from 7kW to 11kW and it would take a sizeable inverter and battery combination to meet that load. It’s not ideal drawing from the grid but a 5kw load for 6 minutes is only around 0.5kWh import.

[twowheelsgood]

Thank you again. I found the manual for the electric showers and they are indeed 9kW showers, which explains all.

i played around with some numbers and even doubling the battery storage wouldnt get to the 9kW and the constraunt appears to be the 40A inverter figure. I also looked at the data sheets for the FoxESS range and even the bigger inverters appears to have the same 40A limit. If I am reading them correctly of course.

As an academic exercise, can the inverters be run in parallel thereby giving 80A current or is that just too much for conventional cabling ?

My solution may be to leave the electric showers as they are and when the en suite is refurbished ( its a start from scratch job`), then to stick with the hot water tank fed shower setup. I already have an EDDI box which dumps excess power into the immersion heater in the hot water tank, so that we have free hot water rather than lose the excess solar production at off peak times. And then only use the electric showers when for some reason, the hot water tank is insufficient.

But as you say, the usage of the shower is old 1kWh of power so not the end of the world. but just annoying as unplanned for. I suspect if I had known beforehand, I wouldnt have made a different decision as its too extreme to try and run 9kW showers off batteries all the time.

Thanks again - very informative.

[Dave Foster]

Thank you again. I found the manual for the electric showers and they are indeed 9kW showers, which explains all.

i played around with some numbers and even doubling the battery storage wouldnt get to the 9kW and the constraunt appears to be the 40A inverter figure. I also looked at the data sheets for the FoxESS range and even the bigger inverters appears to have the same 40A limit. If I am reading them correctly of course.

As an academic exercise, can the inverters be run in parallel thereby giving 80A current or is that just too much for conventional cabling ?

My solution may be to leave the electric showers as they are and when the en suite is refurbished ( its a start from scratch job`), then to stick with the hot water tank fed shower setup. I already have an EDDI box which dumps excess power into the immersion heater in the hot water tank, so that we have free hot water rather than lose the excess solar production at off peak times. And then only use the electric showers when for some reason, the hot water tank is insufficient.

But as you say, the usage of the shower is old 1kWh of power so not the end of the world. but just annoying as unplanned for. I suspect if I had known beforehand, I wouldnt have made a different decision as its too extreme to try and run 9kW showers off batteries all the time.

Thanks again - very informative.

Yes on the parallel option - you can add a second H1 inverter ( both must be the later Gen2) and they run in parallel - if you added a similar 2 battery stack to that you would be at (nominal 90V @ 40A = 3.6kW * 2 = 7.2kW) which is getting closer but you would need 3 batteries on each to get where you need to be (134V * 40A * 2 = 10.7kW)

An alternative is the KH10 inverter (which can manage 50A discharge) and add 3 more batteries (5 in total) you would be able to meet the demand from the shower (nominal 224V @ 50A = 11.2kW).

Obviously bigger inverters would require you to go back round the G99 approval which might limit the size you can have.

The 40A limit is a hard inverter limit for the H1 and you can't do much to control the amps but by adding extra batteries you can increase the volts which will be required on whichever inverter you had - So even on your existing H1 a single extra battery would increase your capability to at least run at the full inverters output (nominal 134V @ 40A = 5.3kW) and the import gap would be smaller.