Solar System Size vs Number of Powerwalls

[Jeremy3292]

Wondering what everyone's thoughts are on solar system sizing vs # of powerwalls. My utility has peak TOU periods in the summer (6-9pm) and winter (6-9am & pm) where I do not want to use any energy except from solar + powerwalls. I am unlikely to be generating much solar during the 6-9am winter and 6-9pm winter & summer peak TOU periods, so I would be relying on the powerwalls during those times. Currently debating 10-11kW of solar with either 2 or 3 powerwalls (house is all electric heating and cooling). Is 3 powerwalls too much for only 10-11 kW of solar?

The other thought I had is my utility nets all monthly energy usage in the various TOU periods, so in theory I could grid charge the powerwalls in the off peak TOU period to net against my solar generation (which is likely generated almost all during off peak) if my solar was unable to charge 3 powerwalls.

Thoughts?

 

[jjrandorin]

The other thought I had is my utility nets all monthly energy usage in the various TOU periods, so in theory I could grid charge the powerwalls in the off peak TOU period to net against my solar generation (which is likely generated almost all during off peak) if my solar was unable to charge 3 powerwalls.

I still havent figured out what does or does not allow one to have the "charge powerwalls from Grid" in their app. I have 8.65kWh of PV and (2) powerwalls. If I had 11kWh of PV I would have 3, but I am also in Southern California, where my 8.65kWh of PV is 13MWh a year.

 

[aesculus]

You have to add up your worst case house use scenario and that is the size of the PWs. Solar would subtract from that when the sun shines. So figure in the likelihood of it being overcast and how often that may happen. I would expect 2 PWs would be about right.

I have the same amount of solar and 2 PWs and it generally works OK except for long periods on 100F + days, then I wish I had 3 PWs. My peak periods are from 2-9PM, when I don't want to use any power from the grid.

 

[Jeremy3292]

I still havent figured out what does or does not allow one to have the "charge powerwalls from Grid" in their app. I have 8.65kWh of PV and (2) powerwalls. If I had 11kWh of PV I would have 3, but I am also in Southern California, where my 8.65kWh of PV is 13MWh a year.

According to Tesla it's due to your local utility company:

 

[jjrandorin]

Back before this was implemented in the Tesla app, there was always a LOT of finger pointing between Tesla reps and Utilities, when people were trying to figure out why we couldnt grid charge.

I have also seen people here with the same utility say they can or cant grid charge, and digging in and seeing it might have something to do with a persons installer.

Im aware that says that, in other words, but I am still not clear, and I would suggest no one "depend" on grid charging their powerwalls in the US, unless they are installing without PV, or get something in writing from someone saying it absolutely will be allowed on their account.

Shrug...

 

[Jeremy3292]

You have to add up your worst case house use scenario and that is the size of the PWs. Solar would subtract from that when the sun shines. So figure in the likelihood of it being overcast and how often that may happen. I would expect 2 PWs would be about right.

I have the same amount of solar and 2 PWs and it generally works OK except for long periods on 100F + days, then I wish I had 3 PWs. My peak periods are from 2-9PM, when I don't want to use any power from the grid.

That's good to know. You are likely getting some good solar output from 2-5pm I would imagine also.

My utility gives me good insight into my hourly usage. I average around 2 kWh per hour with peaks of 3-4 kWh per hour in the hot summer afternoon, or around 45 kWh a day. This excludes car charging of course.

In the winter the inverse is likely true: my peaks of 3-4 kWh an hour are at night. So during the two 3 hour peak periods with virtually no solar (6-9pm & 6-9am) I am probably using 18-24 kWh (3/4x6) if it's pretty cold out I would imagine.

 

[Jeremy3292]

Im aware that says that, in other words, but I am still not clear, and I would suggest no one "depend" on grid charging their powerwalls in the US, unless they are installing without PV, or get something in writing from someone saying it absolutely will be allowed on their account.

Fair enough. That is a helpful point to take into account also.

 

[Matt-FL]

Taking @jjrandorin's estimate of 13mWh, divided by 365 days, you are looking at 35.6kWh per day. Two powerwalls is 26kWh storage, If they were at 25% in the morning, then you'd need ~18kWh from solar to fully charge, leaving another 18kWh to run your house. Granted, you mentioned 10-12kW solar, so you'd be another 25% more production at 12, so roughly 45kWh per day. Anyways, that's how I did the math when planning my system. 20.4kW + 4 powerwalls. Sometimes I wish I had more solar, sometimes I wish I had more powerwalls. My only advice is to get the max you can comfortably afford now, going above what you use now, as it will only increase. You'll never look back and say 'I bought too much solar' or 'too many batteries'.

 

[jjrandorin]

Taking @jjrandorin's estimate of 13mWh, divided by 365 days, you are looking at 35.6kWh per day. Two powerwalls is 26kWh storage, If they were at 25% in the morning, then you'd need ~18kWh from solar to fully charge, leaving another 18kWh to run your house. Granted, you mentioned 10-12kW solar, so you'd be another 25% more production at 12, so roughly 45kWh per day. Anyways, that's how I did the math when planning my system. 20.4kW + 4 powerwalls. Sometimes I wish I had more solar, sometimes I wish I had more powerwalls. My only advice is to get the max you can comfortably afford now, going above what you use now, as it will only increase. You'll never look back and say 'I bought too much solar' or 'too many batteries'.

During the lowest production days in winter, my system generates about 25kWh a day. During a normal May / early June with mild temps, my system Generates about 54-55kWh a day. I wish I had more solar (about 4-5kW more), and I wish I had 1 more powerwall as well.

 

[aesculus]

That's good to know. You are likely getting some good solar output from 2-5pm I would imagine also.

I also failed to mention that I sell back all of my solar generated between 2-7PM to the utility, so none of it can be used by me. I have to drain the PWs during that period for my consumption.

So if you look at my usage I am definitely a utility hog, using off peak energy to charge my car etc, but can make it up financially during super expensive peak periods when I am a supplier. This can be a bit challenging on super hot days for long duration where I sometimes hit my PW reserve (20%) before peak is finished, but not that often.

I don't think in my scenario I could justify a 3rd PW just to avoid these few days a year where this happens.

 

[sunwarriors]

Now, I feel this question is less of a more of which one (solar/PW) should I get more of and is more of a how much $$ do you have and how much of an impact on you would it be if you got 2 vs. 3 PWs?

There's the 30% tax credit and some states have more credits and adding 1 more PW on a large full install tends to not add that much more $$ I feel, at least not enough to affect most folks here. It's what, $5k more?

If your solar is already maxed at 10-11 kW and you have the means/$$, get as much as you can. On long stretches of clouds, I've noticed you never have enough panels nor enough PWs neither. Large solar gives you during the summer hot days extra generation to turn on all the ACs which lower solar panel amounts will have trouble with. I can't charge at max amps due to not a big enough array during the day as well as run the AC.

Outside of future V2H possibilities that Tesla hasn't voiced yet which would make me limit/lower PWs/batteries purchased, I'd get as much as you can comfortably afford.

3 PWs is definitely not what almost anyone would call too much or excessive.

 

[jrweiss98020]

IF you are counting on running the whole house on the Powerwalls for any length of time, there is no such thing as "too many". The only limitation guidelines are for a MINIMUM of 1 Powerwall for each 7 kW of solar output. 3 PW for 11 kW solar are definitely NOT too many. I have 10 kW and 2 PW, and thinking of a 3rd PW. If I switch from gas to heat pumps for water and/or central heat, I will also max out my roof with more panels.

Bottom line: Buy what you can afford.

 

[h2ofun]

During the lowest production days in winter, my system generates about 25kWh a day. During a normal May / early June with mild temps, my system Generates about 54-55kWh a day. I wish I had more solar (about 4-5kW more), and I wish I had 1 more powerwall as well.

You are lucky. My lowest in winter has been 5kWh in a day, most 185kWh in best conditions.

 

[h2ofun]

IF you are counting on running the whole house on the Powerwalls for any length of time, there is no such thing as "too many". The only limitation guidelines are for a MINIMUM of 1 Powerwall for each 7 kW of solar output. 3 PW for 11 kW solar are definitely NOT too many. I have 10 kW and 2 PW, and thinking of a 3rd PW. If I switch from gas to heat pumps for water and/or central heat, I will also max out my roof with more panels.

Bottom line: Buy what you can afford.

I have 7 PW's. Have never used them. Total waste of money for most, IMO, unless you can "afford" them

 

[Jeremy3292]

Now, I feel this question is less of a more of which one (solar/PW) should I get more of and is more of a how much $$ do you have and how much of an impact on you would it be if you got 2 vs. 3 PWs?

There's the 30% tax credit and some states have more credits and adding 1 more PW on a large full install tends to not add that much more $$ I feel, at least not enough to affect most folks here. It's what, $5k more?

If your solar is already maxed at 10-11 kW and you have the means/$$, get as much as you can. On long stretches of clouds, I've noticed you never have enough panels nor enough PWs neither. Large solar gives you during the summer hot days extra generation to turn on all the ACs which lower solar panel amounts will have trouble with. I can't charge at max amps due to not a big enough array during the day as well as run the AC.

Outside of future V2H possibilities that Tesla hasn't voiced yet which would make me limit/lower PWs/batteries purchased, I'd get as much as you can comfortably afford.

3 PWs is definitely not what almost anyone would call too much or excessive.

SC has a 25% state tax credit as well, so I will only be paying only 45% of the cost of the system. The cash up front is more of the dealbreaker when adding panels and/or powerwalls than the net cost at the end once you file taxes and get the refunds. I also am thinking 10-11 kW is the max my roof can have solar panel wise based on my measurements and local setback rules, but I won't know for sure until Tesla does the design. So I think it really is 10-11 kW of solar for me and then either 2 or 3 powerwalls.

 

[Jeremy3292]

IF you are counting on running the whole house on the Powerwalls for any length of time, there is no such thing as "too many". The only limitation guidelines are for a MINIMUM of 1 Powerwall for each 7 kW of solar output. 3 PW for 11 kW solar are definitely NOT too many. I have 10 kW and 2 PW, and thinking of a 3rd PW. If I switch from gas to heat pumps for water and/or central heat, I will also max out my roof with more panels.

Bottom line: Buy what you can afford.

I think my roof will max out at 10-11kW and I do have all electric heating and cooling also.

 

[sunwarriors]

SC has a 25% state tax credit as well, so I will only be paying only 45% of the cost of the system. The cash up front is more of the dealbreaker when adding panels and/or powerwalls than the net cost at the end once you file taxes and get the refunds. I also am thinking 10-11 kW is the max my roof can have solar panel wise based on my measurements and local setback rules, but I won't know for sure until Tesla does the design. So I think it really is 10-11 kW of solar for me and then either 2 or 3 powerwalls.

Then the decision is easy. Get 4.

 

[ArizonaJon]

I have 10 kW of solar and 2 PWs. My peak times are almost the same as yours, Jeremy. During the summer in Tucson, AZ I use almost no grid power during the peak (3-7 pm). Solar and the batteries take care of me.
I considered putting in a 3rd PW for extra backup in case of a power failure but the cost didn't look that great. If I had the extra 25% tax credit you have, I probably would have added a 3rd PW. FYI I only let my PWs discharge to 60% unless there is a grid failure. My main reason for the PWs was backup for grid failures. We have CPAPs that I don't want to stop during the night. But it was also great a week ago when the power was out for 3 hours during the 110 degree part of the day.

 

[hmcgregoraz]

Personally I suspect your 10-11KW of solar is a on the low side, but if it's all your roof can fit...

For 10-11KW in SC, I suspect Tesla might try and get away with a single 7.6KW of inverter on a PowerWall+, which would throw away some peaks during the very highest production time of may/june range, but not a huge amount of production.

I would try to see if you can get closer to 14-16KW of panels, two 7.6KW inverters, and IMHO 3 power walls is the right choice.

We have 23.5KW of panel, 20KW of inverter, and 3x powerwall 2, and are also in Tucson AZ like ArizonaJon.

I have a 40% reserve for backup, as we are hitting summer storms, and to the most part 40% will get us thru to solar the next day, I may adjust that to 50 or 60% as it is a bit tight with the amount of Air conditioning use during the night. Last night from power wall output stop to solar take over we used 26.5KWh from the grid, and 3x power walls is basically 40.5KWh, so 40% is about 16KWh, which would not have been enough to cover all of the grid usage.

I did not have any major car charging yesterday (We have a Model X 90D, Model 3LR, and a Model Y AWD LR)...

The last 7 days are looking good:

-Harry

 

[Jeremy3292]

Personally I suspect your 10-11KW of solar is a on the low side, but if it's all your roof can fit...

For 10-11KW in SC, I suspect Tesla might try and get away with a single 7.6KW of inverter on a PowerWall+, which would throw away some peaks during the very highest production time of may/june range, but not a huge amount of production.

See picture below of house roof. I don't think I can fit more than 3 rows of 9 panels each or 27 panels total, which is approx 11 kW. I also have some piping on the roof which will likely knock me back down a panel or two to 10 kW. As the rear of my house faces almost directly south, panels on the front of the house are useless and my wife doesn't want any there anyways even if it was viable.

Tesla will for sure only give me one 7.6kW inverter with a 10-11 kW setup. I've read on here and Reddit you need to get to 13 kW or so before they will give you another inverter (above 1.7 DC/AC ratio). I'm ok with that though as like you said I shouldn't peak shave too much. Per PVWatts around 1.25 DC/AC is optimal for my location, but 1.3 or 1.4 doesn't change total output that much.