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.