Looking at installing Powerwall system

[MichaelCo]

I’m looking at getting two or three Powerwalls. I have an existing solar installation, so just trying to understand the pieces and interconnections.

It looks like Tesla’s standard approach is to split out the “essential” circuits from lower priority circuits.

One of the scenarios, I’m trying to understand, is during normal operation, with the grid available, and during peak power period. I wouldn’t want to use grid power during this period, when there is enough power in the batteries. I know the circuits in the “essential” panel would be powered from the Powerwalls, but would the lower priority circuits be powered from the Powerwalls as well? Ideally, I would want them to be powered from batteries, and shed if grid power was lost.

What would be the downside if we chose not to separate out essential circuits, and had the entire home treated as essential. If we did this, and lost grid power, does the Powerwall system “trip” if the load exceeds the Powerwalls max output capacity? Or, does the Powerwall fail to deliver in a more unfriendly way - i.e., with low voltage.

Does the Powerwall system provide alerts (text), when grid goes down, and PowerWall is sourcing the power? I could see the possibility we wouldn’t know power is out, and we are pulling from the PowerWalls. And, happily using appliances we’d otherwise shut down

 

[power.saver]

All circuits can be powered by battery when the grid is up. Non-essential circuits will not be powered when the grid is down.

If you home load exceeds your PW capacity, the whole system will shut down (no power). So you want to exclude non-essential loads, especially large loads from the backup circuit. Things like pool pump, EV charger, and maybe even A/C if it is large. Or get enough PWs to cover all your loads.

There are alerts on the PW app when the grid fails and when it is restored.

 

[MichaelCo]

All circuits can be powered by battery when the grid is up. Non-essential circuits will not be powered when the grid is down.

If you home load exceeds your PW capacity, the whole system will shut down (no power). So you want to exclude non-essential loads, especially large loads from the backup circuit. Things like pool pump, EV charger, and maybe even A/C if it is large. Or get enough PWs to cover all your loads.

There are alerts on the PW app when the grid fails and when it is restored.

Thank you for quick response. This info could allow me to include some circuits that can be managed manually (by myself), where their total power load could exceed PWs capacity - For example, turning on well’s pressurizer pump, but doing this when oven range, or entrance gates are not being used.

I view that primarily as an inconvenience if I inadvertently have multiple large items running concurrently - I.e., if the only “hit” for exceeeding PWs max current capacity is tripping the PWs. I have a number of large loads I should be able to control the timing of their usage and limit their usage to one at a time - that could allow me to use the largest load in the calculations when determining number of PWs. The large loads I’m thinking about won’t be drawing down much of the battery storage, as they operate for short duration. However they could exceed max current output of PWs if all were running at same time.

Does that approach seem reasonable, or am I missing something?

 

[bob_p]

If you have a smart meter and access to reports from the meter on actual energy usage, recommend you pull down the data and look at your peak energy usage.

While planning for our solar/PW system, the initial strategy was to isolate the essential circuits and exclude the rest.

But after reviewing the actual usage, the strategy has shifted to powering all of the house (excluding the 2 Wall Connectors for our S & X). While we have 2 150A panels for the house (excluding EV charging), we're not using close to that amount of peak power.

This should be enough to provide us a reasonable amount of backup power for unplanned outages, and for anticipated outages (after a hurricane), we'll plan to manually turn off non-essential circuits/devices for sustained off-grid power (such as our pool pumps which apparently have been using about 30% of our power).

 

[MichaelCo]

All circuits can be powered by battery when the grid is up. Non-essential circuits will not be powered when the grid is down.

If you home load exceeds your PW capacity, the whole system will shut down (no power). So you want to exclude non-essential loads, especially large loads from the backup circuit. Things like pool pump, EV charger, and maybe even A/C if it is large. Or get enough PWs to cover all your loads.

There are alerts on the PW app when the grid fails and when it is restored.

If you have a smart meter and access to reports from the meter on actual energy usage, recommend you pull down the data and look at your peak energy usage.

While planning for our solar/PW system, the initial strategy was to isolate the essential circuits and exclude the rest.

But after reviewing the actual usage, the strategy has shifted to powering all of the house (excluding the 2 Wall Connectors for our S & X). While we have 2 150A panels for the house (excluding EV charging), we're not using close to that amount of peak power.

This should be enough to provide us a reasonable amount of backup power for unplanned outages, and for anticipated outages (after a hurricane), we'll plan to manually turn off non-essential circuits/devices for sustained off-grid power (such as our pool pumps which apparently have been using about 30% of our power).

If you have a smart meter and access to reports from the meter on actual energy usage, recommend you pull down the data and look at your peak energy usage.

While planning for our solar/PW system, the initial strategy was to isolate the essential circuits and exclude the rest.

But after reviewing the actual usage, the strategy has shifted to powering all of the house (excluding the 2 Wall Connectors for our S & X). While we have 2 150A panels for the house (excluding EV charging), we're not using close to that amount of peak power.

This should be enough to provide us a reasonable amount of backup power for unplanned outages, and for anticipated outages (after a hurricane), we'll plan to manually turn off non-essential circuits/devices for sustained off-grid power (such as our pool pumps which apparently have been using about 30% of our power).

Thank you for the information. Have you implemented the PWs, or still in planning stages.
We do have access to hourly meter data. I feel comfortable sizing the PWs from a reserve power standpoint, but couldn’t, with 100% confidence, know we wouldn’t exceed max current limit. There are a few things missing in the data we have. The smart meter data lacks resolution, and our solar production numbers aren’t being captured. I have a reasonable idea on solar #s (by month), but granularity is fairly coarse.
The surge currents, for motors like HVAC compressors, are also proving hard to come by. We have a geo thermal heat pump, which uses a compressor pump that is advertised as having near zero surge current with its soft start approach-getting actual numbers, though hasn’t happened.
One of the questions I have in to Tesla is the downside to powering the whole house, and manually managing the large circuits.
We live in California, and our area was hit by a planned power outage. We weren’t impacted, but it was enough to move me from “on the fence” to having power backup. Just want to do this without giving up too much garage floor space .

 

[bkp_duke]

I've done this, and I would be surprised if you need more than 3 powerwalls. In theory, I can exceed the capacity of the powerwalls by charging a car and running the AC or the oven at the same time, but in the two power outages that we have had so far we've known the power was out and were like "ok, we have plenty of battery power, but let's be conservative in our usage anyway".

3 powerwalls can consistently deliver 15 kW of power, so it's a good bit, but not insurmountable for power draw.

 

[MichaelCo]

Good to hear it’s in use!

We are hoping to get by with 2 PWs, as they could easily fit under a couple of windows in garage. A 3rd PW would be a little more effort to find a home. That would deliver 10 KW (+ 4 KW surge).

 

[bkp_duke]

Good to hear it’s in use!

We are hoping to get by with 2 PWs, as they could easily fit under a couple of windows in garage. A 3rd PW would be a little more effort to find a home. That would deliver 10 KW (+ 4 KW surge).

If you have more than one AC unit, you likely won't be able to do less than 3 PWs because the power draw during condenser start up is too high (even with slow start modules) - unless the units are VERY small.

Or you just do a partial backup and leave one AC unit off the backups.

 

[MichaelCo]

If you have more than one AC unit, you likely won't be able to do less than 3 PWs because the power draw during condenser start up is too high (even with slow start modules) - unless the units are VERY small.

Or you just do a partial backup and leave one AC unit off the backups.

Thank you for this input - we may just go with three to avoid the potential issue. It could be possible the startup currents are high, but these are variable speed compressors which are controlled by varying the frequency. There is some latency in the displayed status on the phone, but when I monitor the consumption and speeds, I see the start up speed starts at speed 2, and consumes ~150 watts. This isn't conclusive as the current monitoring aren't fast enough to capture transients, but the nature of these inverter compressors are to start with low torque, and speed.

Even when I set the thermostat 10 degrees below ambient temp, it still starts at this low speed and increases to the max speed of 9 (setting which I set as a max speed). Even at speed 9, the consumption is 1.2 KWH. Our average consumption over the 4 years in operation has been ~.5 KW for each hour of operation - in general, I see it fluctuating between .4 and .6. It's difficult to believe the surge currents would amount to much. The geo thermal unit is a different beast from anything we've had - just lack numbers.

 

[bkp_duke]

Thank you for this input - we may just go with three to avoid the potential issue. It could be possible the startup currents are high, but these are variable speed compressors which are controlled by varying the frequency. There is some latency in the displayed status on the phone, but when I monitor the consumption and speeds, I see the start up speed starts at speed 2, and consumes ~150 watts. This isn't conclusive as the current monitoring aren't fast enough to capture transients, but the nature of these inverter compressors are to start with low torque, and speed.

Even when I set the thermostat 10 degrees below ambient temp, it still starts at this low speed and increases to the max speed of 9 (setting which I set as a max speed). Even at speed 9, the consumption is 1.2 KWH. Our average consumption over the 4 years in operation has been ~.5 KW for each hour of operation - in general, I see it fluctuating between .4 and .6. It's difficult to believe the surge currents would amount to much. The geo thermal unit is a different beast from anything we've had - just lack numbers.

Easy question to answer. Look at the labels on the compressors outside for LRA (Locked Rotor Amperage), RLA (Rated Load Amperage), and FLA (Full Load Amps). LRA is going to be the biggest number and the determinant of the bulk of your power draw.

This website shows how to do the calculation for power draw and generator sizing, which would be appropriate to just plug in for your powerwalls.
Sizing a Generator to Start Air Conditioner and Motors

Remember also, Tesla is going to be conservative, because they have to warranty this. So if you barely squeak under on the calculation, they will probably say too bad, you need a 3rd powerwall because other loads could be running.

You mentioned geothermal, so I assume you are talking a geothermal heatpump, so you could very well have a very low amperage of max draw there, just don't know. (very jealous there, I hate my AC compressors, and I don't have enough land to install a geothermal HP)

 

[MichaelCo]

Easy question to answer. Look at the labels on the compressors outside for LRA (Locked Rotor Amperage), RLA (Rated Load Amperage), and FLA (Full Load Amps). LRA is going to be the biggest number and the determinant of the bulk of your power draw.

This website shows how to do the calculation for power draw and generator sizing, which would be appropriate to just plug in for your powerwalls.
Sizing a Generator to Start Air Conditioner and Motors

Remember also, Tesla is going to be conservative, because they have to warranty this. So if you barely squeak under on the calculation, they will probably say too bad, you need a 3rd powerwall because other loads could be running.

You mentioned geothermal, so I assume you are talking a geothermal heatpump, so you could very well have a very low amperage of max draw there, just don't know. (very jealous there, I hate my AC compressors, and I don't have enough land to install a geothermal HP)

Yes, it is a geo thermal heat pump. We installed horizontal loops, but you could always go vertical.
I take your point on Tesla’s warranty position. Thank you for that perspective. We’d have to have a mechanical failure to hit the LRA level, and FLA has a N/A listed. I could accept a PWs trip resulting from an heat pump equipment failure, but could believe some might ding Tesla for that situation.

 

[stevemc]

Yes, it is a geo thermal heat pump. We installed horizontal loops, but you could always go vertical.
I take your point on Tesla’s warranty position. Thank you for that perspective. We’d have to have a mechanical failure to hit the LRA level, and FLA has a N/A listed. I could accept a PWs trip resulting from an heat pump equipment failure, but could believe some might ding Tesla for that situation.

According to this site https://www.achrnews.com/articles/115601-the-professor-compressor-amperage-data FLA hasn't been used since 1972. RLA is now used.

Rated Load Amperage (RLA). As it relates to HVACR, Rated Load Amperage (RLA) is a mathematical calculation used to get Underwriters Laboratories (UL) approval for a certain compressor motor. It should not be confused with Full Load Amps (FLA). The term Full Load Amps (FLA) has not been used by compressor manufacturers since 1972, when UL changed the term to Rated Load Amps (RLA).

 

[bkp_duke]

My 2 yo AC compressors still show both RLA and FLA on the labels.

 

[Stretch2727]

Looking at the powerwalls as a back up power supply and some load shifting.

I have the same question as I really don't want to decide what are the essential loads. Charging the car or running our relatively small central AC maybe essential depending on our needs. I am thinking 2 power walls are sufficient. We have 150 amp service. Worst case peak load is when we run the AC (about 3kW) and charge the car (10Kwh). No where near the service level of the house. Obviously we would limit the charge rate of the car if we had to charge it. We have a 6.2kW solar system already installed. Has anyone not had the system broken into essential loads? Does Tesla allow this even if you could potentially overtax the powerwall?

 

[Ampster]

Does Tesla allow this even if you could potentially overtax the powerwall?

I think Tesla has a maximun number of 30 Amp circuits per Powerwall. Would it be easier to decide if you described them as critical loads or some other name? My inverter manufacturer calls them protected loads.

 

[Vines]

If you try to do whole home backup with 2 Powerwalls they may pushback, but with some diplomacy you cam probably convince them, especially if you have an easy setup with a separate meter and main service panel, and the existing subpanel with your loads in it can take a main breaker.

The limit is no branch circuit may be larger then what you can start with the Powerwalls. Per Tesla guidance no branch circuit breaker can be backed up larger than 30A x number of Powerwalls.

So as long as you don't have an 80A HPWC it should be fine to go whole home backup with 2 units only. However Tesla may push you to 3 units instead of 2 for whole home backup due to cookie cutter approach. There may be local regulations I am not aware of as well.

 

[Stretch2727]

If you try to do whole home backup with 2 Powerwalls they may pushback, but with some diplomacy you cam probably convince them, especially if you have an easy setup with a separate meter and main service panel, and the existing subpanel with your loads in it can take a main breaker.

The limit is no branch circuit may be larger then what you can start with the Powerwalls. Per Tesla guidance no branch circuit breaker can be backed up larger than 30A x number of Powerwalls.

So as long as you don't have an 80A HPWC it should be fine to go whole home backup with 2 units only. However Tesla may push you to 3 units instead of 2 for whole home backup due to cookie cutter approach. There may be local regulations I am not aware of as well.

My largest circuit is a 60A for the subpanel in the garage with a Tesla charger. The charger maxes out at 48A. There is another 60A circuit going to an attic subpanel, but the actual loads on this circuit are minor. The air handler for the AC is on this and probably draws 4-5 amps max.

I will put in the order and we will see where this goes.

 

[sorka]

I chose to backup the entire house except my car charger. I don't want to be charging the tesla off powerwalls when the power is out. It's a 100 amp circuit all by itself.

Even without that and with 2 ACs and electric cooking appliances, Tesla said I HAD to have 3 or more poweralls. I also had to upgrade my main service panel AND install a 200 amp distribution panel all for an extra $4200. There is no downside to having enough battery capacity and service panel capacity to power your entire house. It's just a matter of how much more you'll have to pay for it.

 

[aswami]

@sorka - what was the harm in including the car charger in your Powerwall backup? You could choose not to use it except in an absolute emergency.

When designing my 2 Powerwalls system, Tesla made me choose between backing up a 40A oven or a 40A cooktop, since I have separate oven and cooktop appliances. They wouldn't backup both. I hadn't expected that.

When they did the design, I did not have a Tesla wall charger. Now, I do. I have requested them to include the wall charger to the Powerwall backup when they install them next month. Waiting to hear back on that.

Hope there will not be an issue with that. As per Tesla website, 2 Powerwall system is supposed to support EV charging. Has anyone else had an issue getting backup for Tesla wall charger with 2 Powerwalls?

 

[sorka]

@sorka - what was the harm in including the car charger in your Powerwall backup? You could choose not to use it except in an absolute emergency.

I would have had to derate it to 32 amps. I'd just as soon leave at at 100 amps and use my nema 14-50 40 amp which IS backed up if I really had to charge off the PWs.