Powerwall Expected Charging Behaviour

[pheo]

Hi all,

Got a recently installed PW2 plus 3.2kw solar array. I’m on a time of use tarriff with Octopus Energy with a cheap rate between 23:30 and 05:30 (7.5p Vs 40.7p), I’m located in the UK.

I’m trying to get my head around what the expected behaviour from the powerwall when in time based charging mode.

Last night the powerwall only charged itself to 73%. It then stopped and by the time I woke up and checked the system it was down to approx 55% as it’s running the house from 05:30.

Todays weather is strongly sunny.

Is the PW even in time based mode trying to adjust its charge for the weather? Ie expecting good solar output so will not charge as much?

Or was it some kind of mistake or error? It was quite cold last night, I considered perhaps this caused a problem, although it still had plenty of time to charge at a reduced rate.

At the moment I am trying to ensure no house peak electricity usage by load shifting, hence using time mode. In the summer I expect to switch to Self Powered mode which I presume is broadly the opposite Ie PW chargers in the day, and discharges at night.

 

[scdoubleu]

That's normal behaviour for time-based mode. The theory is that it'll learn your normal daily energy use pattern and charge overnight according to the following day's solar forecast.

In practice, it's not 100% reliable, but gets it mostly right more often than not.

 

[pheo]

That's normal behaviour for time-based mode. The theory is that it'll learn your normal daily energy use pattern and charge overnight according to the following day's solar forecast.

In practice, it's not 100% reliable, but gets it mostly right more often than not.

Perfect thanks - we’ve only had it installed since beginning of Jan, so i guess not had any predictably good days.

Wasn’t sure if you needed to be in self powered mode. So thanks for confirming!

 

[swedge]

I am in USA, California, so mine may be different than UK. My relevant settings are 20% reserve, Time-Based with grid charging and export everything enabled.

Being in Calif, most days our solar produces enough to charge to 100%, and it usually does not charge from the grid at all. But it does appear to anticipate cloudy weather and grid charge at night if it thinks it needs to. Our higher kWh pricing starts at 3pm, and if we are using too much power during peak solar, PW will start grid charging soon enough to get up to 100% by 3. It's exact behavior varies from day to day and often charges is spurts and at seemingly randoms rates, but it never runs down to the reserve before the 4-9pm peak price ends.

Sometimes when very strong winds are forecast which might result in a grid failure early in the day, I will adjust the reserve to 100% which makes it grid charge right away.

 

[JohnRatsey]

Time-based control tries to minimise your overall cost. If the export rate is lower than the off-peak charging rate then the algorithm tries to figure out how much off-peak charging is needed to ensure that export is minimised and the battery doesn't get drained before the next off-peak period. Prior to Tesla incorporating the solar generation forecast into the algorithm a year or two ago it would get it wrong every time the weather changed as it assumed that 3 sunny days would be followed by another sunny day.

We are now into the time of year when the potential solar generation is increasing daily (see SolarEdge for several years of my generation data) and it's a good test of the algorithm. In mid winter it's a safe bet to fill the battery every night while in summer it's rarely necessary to do off-peak charging.

You can, if necessary, guide the charging algorithm by adjusting the unit rates for peak, off-peak and export.

 

[kickboxingboy]

Just before I email tesla, this is how my PW has been working quite happy in the UK on go.
My power wall has always starting charging at offpeak time and once charge goes to standby until peak time when it’s starts discharging.
Due to updated export amounts
Offpeak
7.5p kw im 15pkw ex
Peak
30p kw im 15pkw ex
Prior I was only getting 4p export.
I updated the rates and was expecting the same charge behaviour , fully charge in peak and excess solar Would be exported.
Anyway I was up early (call it power wall terror) and noticed I was not changing all the way till 30 min before off peak ended (approx 12kw charge needed) and in the end I changed the export rates back to 4p and it started charging.

I told know what it was trying to do, I could see no logic or cost saving in what it was doing.
Anyone any ideas

Normally it charges to 100% every day was expecting the same. All be it a bit less for sunny days for some headroom. And that logic to export as much solar would be the max cost saving .
That and or exporting battery beyond house use as well.

 

[swedge]

Offpeak
7.5p kw im 15pkw ex

Perhaps setting the sell price above the buy price confused your PW. If you had both Export Everything (EE) and Grid Charging enabled, optimal behavior would be to charge to full and export to the reserve over and over again. Pure rate arbitrage. 100% profit on every kWh you import. Your utility would not appreciate this, so probably the PW is programed not to do it. So, perhaps it disables Grid Charging when the export (sell) rate is higher than the buy, maybe, just a guess here.

But I gather you don't have EE, and maybe do enable Grid Charging?

Also interesting is setting the sell price the same during peak and off-peak. This tells PW there is no advantage to export during peak. Then it should try to charge off peak and discharge during peak to match the house consumption, I would think.

Still, the expected behavior would depend on several factors including the timing of the rates relative to the timing of the sun, the reserve setting, the typical consumption and it's timing, and the likely solar production. That said, I agree that failing to charge is odd behavior.