PW+ Grid Charging with 100amp Service

[twsmit]

Just pulled the trigger with Tesla on 12.4kWh PV w/ 2x PW+ and GW 2.0. I have underground 100amp service in NorCal with PG&E. The cost to trench is prohibitive so leaving original service equipment alone. As one might guess, 100amps is adequate but doesn't leave a lot of headroom.

My question is if playing the TOU arbitrage game and grid charging my PW is there risk of using all my service amps and tripping the main breaker?

My understanding is each PW+ can pull ~32 amps each to recharge. So ~64 amps from the grid worst case scenario. If I fire up my AC, electric oven on top of the PWs etc... does the GW automatically slow down the rate of grid charging? Presumably the equipment is programmed to know I only have 100amp service?

Thanks in advance! I hadn't originally contemplated the possibility of the PW being a major draw from the grid, but with Storm Watch and time based grid charging seems a possibility they may start drawing heavily from the grid at the most inopportune time.

 

[mongo]

Right, thank you. That's disallowed by the book if the >80A is a continuous load (since 100% rated breakers are not seen in residential work and don't exist at the 100A size). And I guess PW charging and EVSE usage are both considered continuous. So the PW should be reconfigured to either (a) limit the current through the 100A breaker to 80A or (b) limit the current through the service to 80% of its rating.

On the practical front, my comments in the penultimate paragraph of my last post still apply.


Depends on whether the installation added CTs on the 70A breaker or not.


So, when you have multiple service disconnects like this, the service conductors are protected only by a load calculation, not by OCPD. That is, the sum of the service OCPD ratings is allowed to exceed the service rating as it does in this case (it's not a 170A service).

Then if the GW is blind to the 70A A/C load (no CTs), I would say the thing to do is to determine the service rating, subtract the A/C's MCA nameplate rating (which will be much less than the 70A breaker size, although when looking at the nameplate, confirm that "MOCP" or "Max OCPD" is at least 70A), and set the GW to limit import to 80% of the result (or 80A, if that is less)

If the GW isn't blind to the A/C load, then just set the G/W to limit the total import to 80% of the service rating.


As a motor-type load, feeder and OCPD sizing includes a 125% factor on the largest motor full load current. That 125% factor is not a continuous use factor per se (because it applies only to the largest motor), but a starting surge current factor (on the theory that simultaneous starting is unlikely). As a result it would also cover the possibility of continuous use, though, for any single motor at a time.

But for a motor in good working order to actually be a continuous electrical load, it would need to be no more efficient than implied by the motor full load current tables in the NEC, and mechanically loaded at its full HP rating for over 3 hours.

Cheers, Wayne

Engineering is fun

Just realized, a Powerwall can't be a continuous load at full power due to storage capacity.
14kWh total storage (13.5 kWh usable), 5kW max contiuous power. Full/ empty in under 3 hours.
At 90% power, it could last 3 hours. At 80%: 3 hr 23 minutes.
5kW = 21 amps, two units 42 amps.

 

[wwhitney]

Engineering is fun

Just realized, a Powerwall can't be a continuous load at full power due to storage capacity.
14kWh total storage (13.5 kWh usable), 5kW max contiuous power. Full/ empty in under 3 hours.
At 90% power, it could last 3 hours. At 80%: 3 hr 23 minutes.

There's a charging efficiency < 1, so those worst case times are a bit longer. IIRC the round-trip efficiency is 90%, so plausibly the charging efficiency is 95%.

Cheers, Wayne

 

[mongo]

There's a charging efficiency < 1, so those worst case times are a bit longer. IIRC the round-trip efficiency is 90%, so plausibly the charging efficiency is 95%.

Cheers, Wayne

Yeppers. Which stretches timing slightly. The 90% spec is at 3.3kW so I just shrugged and calculated that at 90% one way efficency it's still only 3 hours.
Seems PW is only slightly bordering contiuous load, and then only if completely drained initially.

 

[twsmit]

Thanks everyone. I have CTs on my AC so the PW can see the entire site load.

My solution for now is to limited site import to 80A. It’s a conservative number but should be adequate for my needs.

I have confirmed that I can reproduce the results consistently.

When the following conditions are met my import will jump to >80amps. Only happens with a big load like the EVSE not when the house is “idle”.

- EVSE 40amps / 9.6kW charging
- Low state of charge 5-10%
- PW below 20% reserve
- Time based Control mode
- Grid charging enabled

(I have not tested many variants of the above, more or less this is my default preferred setting and I can get results consistently)

In this configuration as soon as the EVSE starts changing the PW will jump from ~3 kW charge to >10kW

With the 80amp / 19.2kW import limit my total grid import will now peg at 19.2kW and stay there. My car charges at full speed and the PW is throttled. I guess it’s working now.

 

[mongo]

Thanks everyone. I have CTs on my AC so the PW can see the entire site load.

My solution for now is to limited site import to 80A. It’s a conservative number but should be adequate for my needs.

I have confirmed that I can reproduce the results consistently.

When the following conditions are met my import will jump to >80amps. Only happens with a big load like the EVSE not when the house is “idle”.

- EVSE 40amps / 9.6kW charging
- Low state of charge 5-10%
- PW below 20% reserve
- Time based Control mode
- Grid charging enabled

(I have not tested many variants of the above, more or less this is my default preferred setting and I can get results consistently)

In this configuration as soon as the EVSE starts changing the PW will jump from ~3 kW charge to >10kW

With the 80amp / 19.2kW import limit my total grid import will now peg at 19.2kW and stay there. My car charges at full speed and the PW is throttled. I guess it’s working now.

Thanks for update! Sounds like a well configured system.
(I'm still fuzzy on the 125 vs 100 Amp service/ meter size though )