Solar PV & Tesla Powerwall.

[Dodgy]

Well there you go - I know it might be the last time I see the sun for a week or so, but just after 11am, the battery ticked up to 100% and 2kW started flowing out to the grid. We are actually a power station...

 

[Deleted member 126969]

That’s what bothered me about pre-charging. I’d be miffed if I paid to charge it up overnight then the sun came out and I ended up feeding the grid. If we had net metering it would at least be fair and neutral on cost.

Clearly, the crystal ball function isn’t quite right yet

 

[Dodgy]

What’s bothering me is that I don’t know how the machine learning works in the system, so it’s quite possible it won’t charge up to 100% tonight when I know I’ll need the power tomorrow. Who decided this thing needed an autopilot when some simple manual controls would do the job much better?

 

[Deleted member 126969]

I’ve not even looked at the pre-charge option since it appeared - pointless given I’m not on E7 but I did wonder how it was going to work. A good weather forecast being the main prerequisite I’d think. I guess it’ll average out as a good thing on the whole, just have to live with the odd blunder when we get a sudden weather shift.

 

[Dodgy]

I’ve not even looked at the pre-charge option since it appeared - pointless given I’m not on E7 but I did wonder how it was going to work. A good weather forecast being the main prerequisite I’d think. I guess it’ll average out as a good thing on the whole, just have to live with the odd blunder when we get a sudden weather shift.

I think I did read something about using weather data, but as a future enhancement rather than a current feature. It’s connected to the internet and there are loads of weather APIs out there, so it’s just a software thing.

There are manual overrides, because there was a story about a message Tesla pushed out to a load of Powerwalls to fill to 100% when there was a storm coming - in Florida, I think. They just haven’t made the override available to owners. [emoji848]

 

[Dodgy]

The good news is that the Powerwall hasn't decided to part charge since the unexpected export incident. The bad news is that there doesn’t appear to be any daylight forecast for the next two weeks...

 

[Deleted member 126969]

Not great weather at the moment is it. Had 13.7kW Thursday, 2.5 yesterday and 0.9 today!

I use the Weatherpro app for forecasts, it gives sunshine hours as well.

 

[Deleted member 126969]

Sorry, slip of the finger, obviously figures should have been kWh, not kW. Left it too long to edit.

 

[Dodgy]

Not great weather at the moment is it. Had 13.7kW Thursday, 2.5 yesterday and 0.9 today!

That’s odd. You generated much more than me on Thursday, but not the other two days. I wondered if that was because the cloud cover we had here from 11am. I can’t show you the Tesla App chart from Thursday so I thought I’d dig out the Solaredge chart - which doesn’t show any dip at all due to the cloud cover. The Tesla chart made it look like we lost about 20% of our production to cloud cover, but here’s the Solaredge chart. No sign of the cloud cover:

I use the Weatherpro app for forecasts, it gives sunshine hours as well.

Nice app. It was a bit optimistic for sunshine this morning, but at least it makes an attempt to predict minutes of sun. Interesting to see it uses Meteogroup data that’s also used by the BBC, but the BBC doesn’t bother with the sunshine prediction.

 

[Dodgy]

Got the wrong day. It was Tuesday that clouded over... [emoji849]

 

[Dodgy]

Experiment time.

Today I’ve shipped 2.1kW out to the grid. Not ideal. Tomorrow’s forecast is for over 5 hours of sunshine - possibly the biggest day of the year so far. If I leave the settings as they are, I’ll probably buy 5kWh too much power tonight. This is a whole 35p of energy - serious stuff!

If I switch off the overnight load, I might be lucky to get to 40% state of charge before the evening drain kicks in. That’s 56p wasted by buying energy at daytime rates instead of nighttime. Woah!

So I’m going to tell the system I only have 2.5 hours of off-peak energy tonight. Max charge rate is 3.4kW. I might be lucky and E7 plus solar might get me to 100% SoC without exporting too much - if the system accepts my strange instructions.

I can’t believe I’m giving this so much attention...

 

[Deleted member 126969]

I know, it turns one into a right obsessive! Good luck with the experiment - tweaking available hours of off-peak seems a sensible way to guide the behaviour of the thing.

 

[Dodgy]

Apparently, max charge rate is not 3.4kW. That’s just the rate it charges when it has plenty of time to fill the battery. Max charge rate is actually 5kW, just like the discharge rate.

At least I’ve only charged to 85%. A bit of tweaking required for tomorrow...

 

[Dodgy]

As it happens, my battery topped out at 96% today - more by luck than judgement. Best day so far for savings, but I expect I’ll get a few of those in the next 6 months...

 

[Dodgy]

Some more stats on the breakdown of returns:

December
FIT 12%
Overnight charging 63%
Using solar electricity 25%

January
FIT 14%
Overnight charging 56%
Using solar electricity 30%

January was 30% better than December.
Running the Powerwall cost 12% of the benefit.

February started brilliantly, but looks like it’s stuck in the doldrums now.

 

[Deleted member 126969]

I don't have anything like that level of analysis on my setup(!), but while looking for something else, I came across the paperwork from the installers with the projections - first full year I was ahead both in monetary terms as well as generation. Hopefully that will continue.

 

[Dodgy]

I have a new thing to get my head around tonight - for the first time we’re not going to empty the battery.

I’m guessing that when the car starts to charge at 1am it’s going to use the Powerwall rather than the grid, which is stupid. Battery to battery transfer...

At the moment, the Powerwall is half-full and we’re not cooking tonight, so it won’t be much different by the time we hit the sack. Ideally I’d like to charge the car from the grid and take on a couple of kWh into the battery to run the underfloor heating in the morning. As far as I know, it’s not possible...

 

[Deleted member 126969]

Ah, yes that’s a quandary isn’t it. So, what does the PW do during low rate charging activities normally? Presumably it charges and you draw from the grid to power the house? Can you schedule the car to charge at the same time as the PW?

 

[Dodgy]

Normally, it charges during off-peak hours - at either 3.7kW if I set the whole night as off-peak or 5kW if I make off-peak less than 3 hours. The car won’t charge from the Powerwall during off-peak hours, but the Powerwall automatically charges during off-peak and I really don’t need it to tonight.

So I either charge the car from the Powerwall or over-charge the Powerwall (and end up exporting some of the electricity I drew from the grid overnight). I could switch the overnight charging off, but then I will definitely empty the Powerwall into the car and end up using quite a lot of daytime electricity tomorrow.

Would it be that hard to create some manual controls?

 

[Dodgy]

OK - so my brain has recovered from that lapse. I can just set the car and the Powerwall to charge at the same time. If the car battery was bigger I think I’d have a problem, but it ain’t.