Interaction with controlled load (offpeak hot water).

[Sylvia Else]

Where I live, there is the option of having water heated using off-peak electricity overnight, where the exact timing is determined by the distributor.

For some reason, the distributor today decided to operate my offpeak water heating after 7am. This resulted in my powerwall discharging to supply some of the current required for heating, and then recharging itself from mid-priced electricity. This is less than optimal.

Given the configuration (current advanced, cost saving), I can understand why the system discharged to supply the power for the heating, though not why it would then recharge itself from the grid.

Either way, it's not a sensible strategy. Unfortunately, the system has no way of distinguishing between controlled load power, and other power. I think I'll have to get my installer to change the position of the current transformer so that the controlled load current is excluded.

Something that others might want to consider.

 

[cwied]

I think this is actually a weakness of the current cost-saving algorithm, not an issue with how your installation is designed. The algorithm should be smart enough not to discharge during a given rate level only to recharge at the same level later. If the cost-saving were implemented correctly, it shouldn't matter what the load is, only what the current rate is.

If the heater came on during peak rates, you would want the Powerwall to offset that load as much as possible, right?

I expect eventually Tesla will update the algorithm to be smart enough so it doesn't run into these sub-optimal edge cases.

 

[Sylvia Else]

I think this is actually a weakness of the current cost-saving algorithm, not an issue with how your installation is designed. The algorithm should be smart enough not to discharge during a given rate level only to recharge at the same level later. If the cost-saving were implemented correctly, it shouldn't matter what the load is, only what the current rate is.

I agree that discharging and recharging at the same rate is silly. I think it recharged because it was below the reserve level - it's not clear why it discharged below that level.

The controlled load is charged at the controlled load rate, regardless of when it comes on, so it doesn't fit in with the TOU concept. The controlled load rate is lower than any of the TOU bands, and indeed lower than the solar feed in rate, so it never makes sense to deplete the battery to feed the controlled load.

Usually, this wouldn't be an issue, because the controlled load is operated only at night. What happened in this case is unclear.

 

[cwied]

Oh I see - I didn't understand that the controlled load was separately metered. It sounds like your electric billing gets a lot more complicated than it does around here. I guess Tesla may be behind the curve on doing their research on all of the different billing arrangements available around the world.

 

[Sylvia Else]

Oh I see - I didn't understand that the controlled load was separately metered. It sounds like your electric billing gets a lot more complicated than it does around here. I guess Tesla may be behind the curve on doing their research on all of the different billing arrangements available around the world.

It may relate to the very cheap coal that Australia has. Using electricity to heat water at night allowed more coal fired baseload without having to reduce output overnight (coal fired stations really don't like to change their output).

With the changing mix of generation, plus no new coal fired generation, this approach may cease to be economic, and the option will disappear. Indeed, it may already be uneconomic, and be a kind of loss-leader.

Still, it's nice to have while it exists, particularly as converting my hot water heating to gas on my property would involve getting gas pipes installed from the street - not a cheap exercise.