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New Construction: Experience of getting new solar roof + powerwalls

BrettS

Active Member
Mar 28, 2017
2,109
2,512
Orlando, FL
Sounds like the tiered plans we used to have here. There were 3 or 4 tiers and the price per kWh went as your consumption increased and you moved up tiers. Now, most of those plans are being converted to Time of Use.

Yeah, except here we just have the two tiers... the first 1000kWh and everything else. And the difference between the two tiers is really only a couple of cents. It’s been this way for at least the 11 or 13 years that I’ve lived in Florida.
 

BrettS

Active Member
Mar 28, 2017
2,109
2,512
Orlando, FL
What do people with powerwalls do now with TOU rates?

You can tell the system when your peak times are, so assuming you have enough capacity in your powerwalls the system will ensure that you only use power from solar or the powerwalls during the peak times. That way you won’t have to pay the high peak rates.
 

getakey

Active Member
Jan 28, 2020
1,130
368
95762
What do people with powerwalls do now with TOU rates?

I have 3 PWs on PG&E TOU plan as I showed above. The PWs make it possble to rate arbitrage. That is I charge them with solar during Partial PEak and then run the house off them during Peak. I can usually make it completely through PEak (2pm to 9pm) unless it gets over 95 degrees with 33% reserve. Over 95 degrees, I still make it to almost 8pm
 

getakey

Active Member
Jan 28, 2020
1,130
368
95762
market price option?
Solar that gets sent back to grid is at same rate I buy from grid. Thus while powering house from PWs, I getting credited at peak rate ($.54/kWh) sent to grid. So I "pocket" the Delta from Partial to Peak rate
 

cridinger82

Member
Aug 5, 2020
66
31
Hollywood, Florida
Here are the tou details for fpl..

FPL’s “peak” hours are listed below. All other hours are considered “off-peak.”

  • Nov. 1 - March 31: Monday through Friday between 6 a.m. to 10 a.m., and 6 p.m. to 10 p.m. excluding Thanksgiving Day, Christmas Day and New Year’s Day
  • April 1 – Oct. 31: Monday through Friday during the hours from noon to 9 p.m., excluding Memorial Day, Independence Day and Labor Day
 

ScottFLA

Member
Nov 1, 2019
45
100
Tampa, FL
I touched on this in another post, but I’ll say it again here. The powerwalls won’t add any extra power to your system..

Brett - if I can bother you with one more question without acting like you're my consultant. When you say powerwalls won't add any power, I don't understand. I do understand now thanks to your input that there are no off-peak rates, but if powerwalls won't add power, why have them? This is my very layperson's assumption: Let's say it's sunny and I generate more power from the solar roof than my house uses that day, and it's stored in the powerwalls. Then wouldn't my house use the power stored in the powerwalls during the evening? And if so, while that's not generating power, is that not saving money from power I'd be using from the grid? I guess I'm just at a loss for what the powerwalls are even for if I'm not generating enough kwh from the roof to store excess to the PWs to draw from? TIA.
 

Southpasfan

Member
Jun 2, 2019
371
489
Pasadena
Scott,

Brett's point is that its sometimes hard to realize, math-wise, that what Powerwalls do in a system (other than being a back up when the grid is down, which is important) is enable you to use all the power the panels generate, but not more than they generate (with one exception).

So, yes, because systems generate power during the morning and early afternoon when the sun is up, and because people usually don't use most energy during those times, indeed, you can charge the powewalls in the morning and discharge them in the early evening.

But that's not "additional power" - its power from your panels from earlier in the same day.

Powerwalls can only be charged from the grid in something called "Storm Watch" mode which Telsa activates when a storm may hit an area.

There are a bunch of assumptions in there. though. For example, if you have a system which is small in comparison to your usage, such system might not be in a position to easily charge the powerwalls fully each morning.
 
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BrettS

Active Member
Mar 28, 2017
2,109
2,512
Orlando, FL
Brett - if I can bother you with one more question without acting like you're my consultant. When you say powerwalls won't add any power, I don't understand. I do understand now thanks to your input that there are no off-peak rates, but if powerwalls won't add power, why have them? This is my very layperson's assumption: Let's say it's sunny and I generate more power from the solar roof than my house uses that day, and it's stored in the powerwalls. Then wouldn't my house use the power stored in the powerwalls during the evening? And if so, while that's not generating power, is that not saving money from power I'd be using from the grid? I guess I'm just at a loss for what the powerwalls are even for if I'm not generating enough kwh from the roof to store excess to the PWs to draw from? TIA.

Lol, I don’t mind answering questions and neither do any of the other helpful people on this board. Feel free to ask all the questions that you want.

In florida the big reason to have powerwalls is for backup power when the grid is down. Not only will they power your house in the event of a grid failure during the night, but they will also allow your solar system to keep operating if there is a grid failure during the day. It sounds kind of surprising, but with a solar only install (no powerwall) installation if the grid goes down the solar system will also shut down due to regulations and safety concerns. So even though you might have solar panels and it’s a bright sunny day, if the grid is down your house you will have no power. However when you have powerwalls you will also have a transfer switch that allows your house and solar system to safely operate in the event of a power failure.

As far as storing energy for use during the night, they can do that, but because we have 1:1 net metering in florida it’s not really necessary. If you don’t have powerwalls, or just operate with your powerwalls in backup only mode, like I do (which is where the powerwalls stay fully charged all the time, waiting for a power outage) then any excess power that is produced by your solar system will be fed into the grid (and used by your neighbors), but you will get credit for any power you feed into the grid Iike that.

So, for example, my house used 50kWh of power the other day. My solar system also generated 50kWh of power that day. Obviously the solar power was only generated during the day when the sun was out, but my house uses power 24 hours a day. So of the 50kWh that my solar system generated 20kWh was directly used by my house during the day. The other 30kWh was fed back into the grid. During the night and evening when the sun was down my house pulled 30kWh from the grid, but because I get credit for the 30kWh that I fed into the grid my net usage for that day was 0kWh and I will not pay for any power that day.

The powerwalls can be used for this as well. Instead of running in backup only mode I could switch the powerwalls to self powered mode. In that case my system would have drawn 30kWh from the powerwalls during the night and evening and then the extra 30kWh that my solar system generated would be used to recharge the powerwalls. The net result would still be that I used 0kWh from the grid.

However, there are some disadvantages to using the powerwalls for this. First, there are some power losses due to heat and energy conversion. They are rated for 90% round trip efficiency, so that means that for every 100kWh you put into the powerwalls you only get 90kWh out. However with 1:1 net metering, like we have in florida, for every 100kWh you feed into the grid you get credit to pull a full 100kWh back out of the grid. Additionally, by using the powerwalls to routinely power your house that means that there will be times when they have a low charge. If the power fails at that point then they will have less energy to provide backup power for your house.

Because of these reasons it makes more sense to set the powerwalls to backup only mode and use the grid to ‘store’ your excess power.

However, like I said, the powerwalls won’t generate extra power. If your house is using 50kWh per day and your solar system is only generating 30kWh per day then whether you have powerwalls or not you will still be short 20kWh and need to buy that from the grid.
 
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darhall993

Member
Jan 24, 2019
129
119
Sandy Springs, GA
Lol, I don’t mind answering questions and neither do any of the other helpful people on this board. Feel free to ask all the questions that you want.

In florida the big reason to have powerwalls is for backup power when the grid is down. Not only will they power your house in the event of a grid failure during the night, but they will also allow your solar system to keep operating if there is a grid failure during the day. It sounds kind of surprising, but with a solar only install (no powerwall) installation if the grid goes down the solar system will also shut down due to regulations and safety concerns. So even though you might have solar panels and it’s a bright sunny day, if the grid is down your house you will have no power. However when you have powerwalls you will also have a transfer switch that allows your house and solar system to safely operate in the event of a power failure.

As far as storing energy for use during the night, they can do that, but because we have 1:1 net metering in florida it’s not really necessary. If you don’t have powerwalls, or just operate with your powerwalls in backup only mode, like I do (which is where the powerwalls stay fully charged all the time, waiting for a power outage) then any excess power that is produced by your solar system will be fed into the grid (and used by your neighbors), but you will get credit for any power you feed into the grid Iike that.

So, for example, my house used 50kWh of power the other day. My solar system also generated 50kWh of power that day. Obviously the solar power was only generated during the day when the sun was out, but my house uses power 24 hours a day. So of the 50kWh that my solar system generated 20kWh was directly used by my house during the day. The other 30kWh was fed back into the grid. During the night and evening when the sun was down my house pulled 30kWh from the grid, but because I get credit for the 30kWh that I fed into the grid my net usage for that day was 0kWh and I will not pay for any power that day.

The powerwalls can be used for this as well. Instead of running in backup only mode I could switch the powerwalls to self powered mode. In that case my system would have drawn 30kWh from the powerwalls during the night and evening and then the extra 30kWh that my solar system generated would be used to recharge the powerwalls. The net result would still be that I used 0kWh from the grid.

However, there are some disadvantages to using the powerwalls for this. First, there are some power losses due to heat and energy conversion. They are rated for 90% round trip efficiency, so that means that for every 100kWh you put into the powerwalls you only get 90kWh out. However with 1:1 net metering, like we have in florida, for every 100kWh you feed into the grid you get credit to pull a full 100kWh back out of the grid. Additionally, by using the powerwalls to routinely power your house that means that there will be times when they have a low charge. If the power fails at that point then they will have less energy to provide backup power for your house.

Because of these reasons it makes more sense to set the powerwalls to backup only mode and use the grid to ‘store’ your excess power.

However, like I said, the powerwalls won’t generate extra power. If your house is using 50kWh per day and your solar system is only generating 30kWh per day then whether you have powerwalls or not you will still be short 20kWh and need to buy that from the grid.
Bretts is spot on with the setup for Florida and 1:1 net metering, I would do the same thing. The Powerwall+Solar is very flexible, so for me I pay 14cents/kwh when I take power from the grid but only get 6cents when I feed back to the grid, so mine is setup for Self-Powered mode and I have plenty of excess capacity for charging the Tesla. My bills are typically $15-20 dollars credit in summer and very close to break even in the winter.
 
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