Solar + Batteries = import from grid

[hiddenhippo]

I finally got approval from UK Power Networks to install a RHI-5K-48ES and 15kw batteries. The installation went smoothly, as I used a local certified electrician. For back reference, I already had solar installed, using a grid-tied inverter but no batteries, however, the system worked brilliantly and I calculated that ~70% of generation went back to the grid. So I kept the wiring from the old installation and installed the hybrid + batteries.

To date I've been very impressed, we're nearly nett zero from the grid, which brings me to my point. Using the LIVE display on the inverter (and graphs on solis-cloud + app), I always have a trickle of import from the grid, anywhere from 1w through to 100w, with an average of around 40w. This trickle is in spite of the following a) solar generating more than the house demand, b) solar + battery generating more than the house demand. This might be completely normal, but I thought I'd check, and having Googled I can't find anything that indicates whether this is normal or not.

I know that a picture speaks a thousand words, so attached is an image of my installation; ignore the Growatt, this is disabled and no longer used. In summary, we've got the Solis on the right hand side with it's AC going to the Solar AC Isolator, to the solar meter and then to the solar consumer unit. To the right of the solar consumer unit is an Acrel with a CT clamp on the output of the grid meter. The solar consumer unit and the grid meter then connect to some henley blocks before going through the wall into the house consumer unit. The only "odd" thing (for no knowledgeable reason) is that the LIVE and NEUTRAL feed for the Acrel meter come from the solar consumer unit rather than the from the house consumer unit - this might not be an issue though.

Ultimately, is it normal for there to be a constant feed in from the grid even though solar and/or batteries can supply the demand of the house? Equally, the demand from the house can be low/high, it matters not, there will always be some level of trickle import. I've included a picture of the solis-cloud web application and drawn some black lines to show the flow of current. The house demands 223w, the solar panels are generating 535w, yet there is a 28w draw from the grid. I just find this slightly confusing. The solis is configured as self-use with no timers for export/import. Charge from grid is disabled etc.

Any help would be greatly appreciated.

 

[johnb2713]

Inverters in grid tied mode operate by adjusting their output voltage. If their voltage is higher than the grid they take the load / export to grid, if their voltage is lower they shed the load / import from the grid. Modern inverter are digital and dont have infinite steps of voltage change on their output which in many circumstances will mean they cant get to zero import / export. So when it make an adjustment one step may change from 15w export to 30w import. My Growatt inverter used to constantly stay on the import side line until the export was enabled, now it hovers either side of the line. Another possibility is the inverter needs to keep a little import running to stay synchronised to the grid, I dont know how Solis achieve this but it is a possibility. Final possibility, is your import/export sensed with a meter of CT? If it's a CT, have a look at the cable routing and maybe consider screened cable.

 

[hiddenhippo]

Thanks for the reply John. It makes sense that inverters might have step ranges, and unless the difference between load demand, battery and import happens to fall exactly on a step increment, there will be either a potential for draw on the grid, or export to it. Having spent a number of hours (probably) looking at why the import value from the grid fluctuates, and why sometimes it's zero but most times it's floating between a range, I suppose makes some sense, knowing what you've now said. Ignorantly I'd have assumed the inverter would have small increment ranges, but possibly not, or as you also noted, it needs to import to maintain synchronisation, but why the import value would range wildly in order to maintain sync, not so sure, unless it is again a factor of the "steps."

From this I could enable the export of energy back to the grid (which is currently configured as "no, don't do that") and see whether that makes any changes.

The import/export is sensed with a CT clamp; no adjustments to the original cable length. Just clipped onto the LIVE. In the image I provided you'll see a black and white cable pair in the blue circle I drew - these are the CT clamp cables. You think some shielding would possibly help - I guess they could "pick up noise" potentially, if too close to other cables - it does sit close to the mains neutral.

 

[hiddenhippo]

I've spent a little while tweaking the inverter settings, and the findings might be beneficial for other Solis user who run into a similar problem.

In short, I enabled export to grid, limiting to 100w (there was no lower setting). Having done this I'm more likely to see a small amount of energy being pushed onto the grid, for example, about 9 watts. Importing from the grid (whilst having surplus energy production) is hardly seen anymore.


For some crude numbers.

Previously (with the old inverter settings) I'd be importing 28w throughout the day, so lets say that's 672w over 24 hours; over 31 days ~20kw @ 0.51p = ~£10; basically costing me £10 per month.

With the export setting enabled, I'm returning 9w throughout the day, so lets say that's 216w over 24 hours; over 31 days ~6.70kw @ 0.51 = ~£3.40; basically costing me £3.40 per month. In reality I'd be paid ~£1 for exporting this (15p kwh), however, I could be storing the energy, which has an equivalent import of £3.40 - £1.00 = £2.40

Ultimately, enabling the export, whilst still not allowing me to hit nett zero, results in better value for me.