Is my schedule too complex?

[Fru T Bunn]

Hi.

I am on Octopus IOG for import and the flat 15p export rate.
My inverter is a Fox ESS H1 SERIES (G2) 5kW Hybrid Inverter.
My battery is a Fox EP11 10 kWh battery with a 90% depth of discharge

What I want to do in English is ...

• Charge the battery between 23:30 and 05:30, and not use the battery should I also charge the car.
• Feed in all my surplus between 05:30 to 18:00.
• From 18:00 to 19:00 I want to discharge to the grid leaving 50% for the evening.
• Then from 22:00 I want to dump any remaining battery to the grid but leaving 12%. I've set the minimum to 12% as I noticed it went below 10% when set at 10.

To achieve this I have set the following schedules:

Code: Select all

00:00-05:30 - Force Charge: Min SoC 10%; Max SoC 100%; FD SoC 10%; FD Pwr 0W
05:30-18:00 - Feed In: Min SoC 10%; Max SoC 10%; FD SoC 10%; FD Pwr 0W
18:00-19:00 - Force Discharge: Min SoC 10%; Max SoC 50%; FD SoC 50%; FD Pwr 5000W
19:00-22:00 - Self Use: Min SoC 10%; Max SoC 100%; FD SoC 10%; FD Pwr 0W
22:00-23:30 - Force Discharge: Min SoC 10%; Max SoC 10%; FD SoC 12%; FD Pwr 5000W
23:30-23:59 - Force Charge: Min SoC 10%; Max SoC 100%; FD SoC 10%; FD Pwr 0W

My only concern is doing pretty much a full charge and discharge of the battery in a 24hr period.

Is what I've input into the app correct? Would you consider this overkill?

Many thanks in advance.

Best regards
Andy

[Dave Foster]

The commercials will largely depend on how much you pay for the electricity (i'm guessing around 7p/kWh) vs the cost of using/replacing your batteries based on their lifetime of 10~15 years...

I tend to work on a physical cost of batteries at 8p/kWh and so the actual cost of using your batteries would be the 7p (but you'll lose 15% in the txfer to DC, storage and recovery from the batteries) so assume 8.24p / kWh to buy (and re-use), plus the 8p/kWh to buy your batteries in the first place. ~ your actual cost per kWh of power the batteries give up is 16.24p/kWh

Obviously you can look at that differently and say well i've bought the batteries and that cost is sunk, I just want to get the most back in which case your cost per kWh of power the batteries give up would just be 8.24p/kWh

Your batteries are warrantied for a certain energy throughput but they will obviously last longer than that, and with what you are doing you are effectively cycling the batteries every day; the main issue being for each cycle you will reduce the life of the battery (but as long as you are being rewarded for it no problem, if not don't do it)

As the battery ages each cycle will reduce the capacity available i.e. after (say) 2000 cycles your battery will only have 85% of it's capacity '85% SOH=state of health' and after 4000 cycles that might be (say) 75% and at some point you will decide the batteries need replacing hence the reason why it's a good idea to think of that cost as you use them.

So having said all of that, there is nothing wrong with the number of items in your schedule as long as you are happy with the commercials, but you'll need to make a few modifications to deal with overlaps.

A schedule starts at :00 seconds and ends at :59 seconds (whereas the old charge periods start and end at :00 seconds) - what that means is to avoid overlapping schedules you need to finish 1 minute before and then start bang on the next time schedule.

e.g. your 00:00-05:30 would be 00:00-05:29, then
05:30-17:59,
18:00-18:59,
19:00-21:59,
22:00-23:29,
the last item is correct.

On your Feed In, Self Use, Force Discharge schedules it is best to keep the MaxSoC at 100%

[Fru T Bunn]

@Dave - thank you for taking the time to reply. I apologise for not replying sooner.

Could you please let me know how you arrived at 8.24p/KwH please? I am working on 7p, and adding 15% to that comes out at 8.05p.

Many thanks again, and thanks also for setting up this community.

Andy

[Dave Foster]

You pay 7p per kWh from the grid, but that has firstly to be converted from 230V AC to DC (the batteries are DC) and there are ‘some’ losses in doing that, the actual number depends on your system, number of batteries and the DC voltage on the bus.
You then have a storage loss in the batteries so for every kWh you store because of chemical losses you will lose approx 3-5% (this is known as RTE or return on efficiency) these losses are seen as heat which is a good thing in colder months.
There is then the recovery of power where the batteries DC is converted to AC to be usable in the home, and finally there are small losses in running the inverter/BMS/Battery comms etc…
I have measured these on my system and the total loss is 15%, in practice they can be a bit lower or a bit higher but 15% seems to cover it for most systems.

What that means is that for every 1kWh of power I store from grid, the usable amount when the demand comes from the batteries is 0.85kWh and so 7p/0.85 would be ~ 8.24p/kWh for every kWh realised through the system (It’s only a small difference but if you are fully costing it should be taken into account).

[Fru T Bunn]

Thank you so much.