Prioritising battery discharge over PV self consumption

[Dillsue]

I'm looking to install batteries to charge on discounted off peak to then supply the house during peak time. We have significant PV that we get paid to export at 15p/unit. On our current Octopus Go tariff we could charge the battery at 8.5p/unit so it will help our £ROI if we can let all our PV go to export and run the house on off peak rates via the battery.

 

I can't see a way to stop us using the PV when it's available so is there a way to force house loads to use the battery and not draw on the PV??

[Bramco]

Electricity is like water, so flows around any system, so you'd have to somehow separate the house from the PV. I'm sure someone will be along soon with an answer to that.

 

But why don't you simply wire things up normally, so the house will use whatever is available from the batteries or the PV, or the grid.  Then set things up, so that you dump what's left in the batteries at the end of the day before the cheap rate - this gets you the 15p export and if you've had any excess PV during the day, over and above house use, then you'll have had 15p for that.  Then in the cheap rate simply fill you boots (batteries) before you start the cycle again.  This is what we do and it's sort of self regulating.  Your only issue is working out when to start dumping (exporting) the excess from the batteries to the grid if you start too early then you'll be using the grid before the cheap rate for the house and if you start too late then you'll be missing out on your 15ps worth of export.

 

We worked out roughly how much time it took to dump 1% of the batteries to the grid. IIRC it was about 2.5 minutes.  So we have an algorithm that starts checking the SoC of the batteries at 8:30 and checks every 2.5 minutes. When the SoC is larger than the amount of time left to discharge it, we start discharging and keep checking every 2.5 minutes until the start of the cheap rate.  We very rarely use any standard rate units and export the maximum we can.

 

Also, it would it be worth getting on to the Intelligent OG which is 7.5p?

[RobLe]

That depends in the inverter generally.  A way to definitely achieve this no matter what inverter, is to have separate inverters for PV and battery, the battery one connecting to the main CU and it's CT sense coil clipped to the CU feed.  Any PV then connecting via an inverter to a henley block before this main CU.

We have a sunsynk; this is a hybrid inverter, and defintely cannot do as you wish if it has both batt and PV connected to it.  The software it comes with is designed for sunny places with a grid with frequent power cuts; so it is designed to always first charge the battery, then export.

Other hybrid inverters may by default allow PV export first.

 

[JohnMo]

The normal run of things

 

You charge on cheap rate, battery depletion starts on expensive tariff. As PV generates it go to charge battery first, then when full, any excess not be used by house, goes to export.

 

Any variation from this is robbing Peter to pay Paul.

[Dillsue]

2 hours ago, JohnMo said:

The normal run of things

 

You charge on cheap rate, battery depletion starts on expensive tariff. As PV generates it go to charge battery first, then when full, any excess not be used by house, goes to export.

 

Any variation from this is robbing Peter to pay Paul.

If I charge the battery from PV then I'm charging at 15p/unit(export rate) rather than 8.5p/unit(off peak rate). If I power the house from PV rather than drawing off peak battery charge then it's the same....15p to self use PV and 8.5p to use the battery.

 

I appreciate your description is the normal PV/battery set up but I'm just trying to see if it's possible to force export of all PV generation and use the battery until it's empty then use any available PV followed by the grid if there's no PV.

[Dillsue]

3 hours ago, RobLe said:

That depends in the inverter generally.  A way to definitely achieve this no matter what inverter, is to have separate inverters for PV and battery, the battery one connecting to the main CU and it's CT sense coil clipped to the CU feed.  Any PV then connecting via an inverter to a henley block before this main CU.

The battery inverter will be a separate inverter to the 2 x PV inverters so that's OK. The physical layout is problematic as both PV systems connect to consumer units at the back of the house whereas the ASHP that will be the biggest battery load is fed from a CU at the front by the DNO incomer. I'd need to get a chunky cable from the back to the front to take the PV to the Henley block. That's doable if this is a solution.

 

So PV needs to be connected nearest DNO incomer then CT connected between PV connection and house load?? Battery connected anywhere on house side of CT?? Have you done this....be awesome if thats a solution??

[Dillsue]

3 hours ago, Bramco said:

But why don't you simply wire things up normally, so the house will use whatever is available from the batteries or the PV, or the grid.  Then set things up, so that you dump what's left in the batteries at the end of the day before the cheap rate - this gets you the 15p export and if you've had any excess PV during the day, over and above house use, then you'll have had 15p for that.  Then in the cheap rate simply fill you boots (batteries) before you start the cycle again.  This is what we do and it's sort of self regulating.  Your only issue is working out when to start dumping (exporting) the excess from the batteries to the grid if you start too early then you'll be using the grid before the cheap rate for the house and if you start too late then you'll be missing out on your 15ps worth of export.

 

We worked out roughly how much time it took to dump 1% of the batteries to the grid. IIRC it was about 2.5 minutes.  So we have an algorithm that starts checking the SoC of the batteries at 8:30 and checks every 2.5 minutes. When the SoC is larger than the amount of time left to discharge it, we start discharging and keep checking every 2.5 minutes until the start of the cheap rate.  We very rarely use any standard rate units and export the maximum we can.

 

Also, it would it be worth getting on to the Intelligent OG which is 7.5p?

I believe IOG needs an Octopus approved EV charger which we don't have so we're stuck with the basic Go.

 

Assuming you do all those calculations automatically with a bespoke system then that level of automation is beyond where I want to go with this, but....... If the battery inverter supports it, I suppose I could configure the inverter to dump what's in the battery to export after we've gone to bed??

 

Any suggestions for inverters that might do this??

[JohnMo]

A GivEnergy AIO comes with a default setting of ECO, this can be switched on or off. 

 

Turning Eco Off requires you to set specific Timed Charge, Timed Discharge, or Timed Export schedules.

 

So set eco off, you can set the charge during a cheap period and set a timer discharge this would discharge to house until flat or time window has elapsed. It would ignore PV so it would all be exported. Once battery is flat it would remain so, ignoring any PV until next charge window.

 

You can also schedule a discharge to grid.

 

This an AC coupled battery 

Edited 2 hours ago by JohnMo