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Tesla Solar and Powerwall for a large home?
- Thread starter blueinsofla
- Start date
reddy
Active Member
That’s actually very low. Why are you getting a 28kw solar system if your utilization is so low ?
Is the daily output of 28 kWh for the full day?
Actually 5kw max output or so? If so, two powerwalls will hold a full days output and should be plenty.
woferry
Member
Something doesn't seem to add-up here, unless the first quote has the units wrong. Do you mean 7kW *per* hour, or do you really mean 7kWh over 12 hours (which would be ~0.583kW per hour)?
My typical consumption is around 600W/hour, so overnight I run 7-8kWh (and a single PW is fine for that), but my monthly electric bill comes in around 300kWh (hundred), not 4-5 thousand. So either your daytime usage is a LOT more than your overnight usage, or something seems wrong with the above numbers.
If it's right, then 30 days with half of the day at 7kWh (210kWh/mo) means the other half of each day is over 126kWh/day (using the 4000kWh/month figure, almost 160kWh/day @ 5000kWh/mo). Or over 10-13kW/hour on average from 7am-7pm. That's more than 2 PW's worth, not even considering any peaks which might go well above that. Seems to me that there would be a decent chunk of the morning and late afternoon where even a solar system that big wouldn't be producing enough for the draw, so you'd be draining the batteries pretty hard before the sun even set, or before the sun gets high enough in the morning to get above the house consumption so that it can actually start charging the batteries again rather than continuing to drain them. Personally, I'd try to figure out where all that power is going and whether I could do anything to reduce it first, or to at least identify the things to not back-up.
I am fairly confident blueinsofla meant 7kw per hour between 7pm and 7am.
Has anyone discussed running two 200A main disconnect panels fed from a single 400A meter base? That way you could run 2 PW per panel. Downside is splitting the generation, battery, and loads into two separate groups.
Maybe this would be the best depiction of my monthly energy consumption...looks like July 2019 is going to be the highest we have had since building this home 2 years ago...
Funny you should ask! A third-party solar company just proposed this to me this morning as the most viable solution for my home...I'm definitely far from the level of knowledge required to completely understand what all of this means, but am intrigued. Would love to hear other thoughts on this from all the smart people posting here (definitely not me!)...
On some versions of 400A meters, there are dual output lugs. These could then feed 2 200A mobile home feeder panels. These panels have some number of breakers plus full load feed through lugs. Those lugs would then go to the Gateway which then feeds a 200A panel with your backed up loads, PW, and solar. This requires spliting the PW and solar if you want both panels backed up.
You can also skip the mobile home panel and just use a 200Amp disconnect, but that loses the non-backed up circuits.
Overall though it is probably easier to keep the meter/main panel combo you have now and add a 200A subpanel fed through the gateway from the main (assuming you can get a 200A branch breaker for your main panel). Then you can move the important loads to the new backed up panel along with all your solar and PW capacity. This keeps all your generation and storage together.
Hybrid is dropping the 400A main breaker to 200A, then running a 200A disconnect from the meter to the Gateway and a new 200A panel with the solar and PW.
The Gateway can also be set up as a 200A disconnect.
Wow - this is mostly Greek to me, but I'm glad there's a few different alternatives to make this happen. It's ironic to me that the Tesla Energy only proposed a workaround involving the installation of a second electric meter by my utility. The utility said "no can do" but a local solar company here in South Florida offered exactly what you proposed. Thanks!
Jigglypuff
Member
I have a 2,000 ACSF house and I am under contract with Tesla for 11.7kW solar and 3 Powerwalls. I had the same approach to my system design, just a smaller house. Tesla originally suggested 1 powerwall for partial back-up, we upgraded to 2 for whole house back up. I built a spreadsheet to calculate how much I would need vs produce and the result was that I would still far short, even if I had more solar due to the capacity of the powerwalls. Its a balancing act. I know that during an outage, I will need to cook on gas grill and not the range and avoid using the dryer on cloudy days. I also can't charge my car. One benefit to lots of powerwalls is that most of your energy draw from the grid will be at night and FPL offers a residential TOU (Time of Use) metering with net metering so you will pay $0.07/kWhr instead of over $0.11/kWhr. You might not be able to get to the nighttime only draw unless you upsize the system, it just depends what your average daily consumption level is at.
Keep in mind that going over 11.7kW DC (10kW AC) solar production requires added permit fee ($400) and insurance. It's probably not big deal due to the size of your install, but something to keep in mind. One installer told me that the $1M insurance policy would run $40/mo if you didn't already have a policy with excess liability coverage.
One thing you could do for initial guestimate on number of powerwalls is take a year of power usage in kW (FPL website give you history month by month in energy dashboard) and then divide by 365 to get your average daily usage. Then take the Tesla solar yearly production estimate and do the same. For reference I will produce 48kWhr per day and use 65kWhr per day. Going into hourly specific production and usage from my spreadsheet, on a good day in July (6am to 6am) I will produce 46kWhr and use 60kWhr and almost max out the powerwalls during the day. The issue I see with your system size is that you will top off the powerwalls early in the day and run out of juice before 8pm. You probably need to decrease solar and increase powerwalls for it to work as you are intending. Ideally I would probably have 15kw of solar and 4 powerwalls to run the house as if nothing had changed. I would think you need to go to 6 or 8 powerwalls unless you can drastically cut back to live within a portion of your home. If that's the case, you might be able to reduce solar and focus on the most efficient location (South facing roofs).
The other thing to consider is that the Powerwalls each only provide 20 amps of power so in order to get enough power to provide even half of your main panel capacity you need 10 powerwalls. It all comes down to which loads you can ignore in a power outage.
I asked local install for back-up power and the cost was more than Tesla. I think about double, but I don't have the figures anymore. This installer was able to match Tesla's solar rate of $2.65/watt and claimed to have better system due to US panel and microinverters.
Keep in mind that going over 11.7kW DC (10kW AC) solar production requires added permit fee ($400) and insurance. It's probably not big deal due to the size of your install, but something to keep in mind. One installer told me that the $1M insurance policy would run $40/mo if you didn't already have a policy with excess liability coverage.
One thing you could do for initial guestimate on number of powerwalls is take a year of power usage in kW (FPL website give you history month by month in energy dashboard) and then divide by 365 to get your average daily usage. Then take the Tesla solar yearly production estimate and do the same. For reference I will produce 48kWhr per day and use 65kWhr per day. Going into hourly specific production and usage from my spreadsheet, on a good day in July (6am to 6am) I will produce 46kWhr and use 60kWhr and almost max out the powerwalls during the day. The issue I see with your system size is that you will top off the powerwalls early in the day and run out of juice before 8pm. You probably need to decrease solar and increase powerwalls for it to work as you are intending. Ideally I would probably have 15kw of solar and 4 powerwalls to run the house as if nothing had changed. I would think you need to go to 6 or 8 powerwalls unless you can drastically cut back to live within a portion of your home. If that's the case, you might be able to reduce solar and focus on the most efficient location (South facing roofs).
The other thing to consider is that the Powerwalls each only provide 20 amps of power so in order to get enough power to provide even half of your main panel capacity you need 10 powerwalls. It all comes down to which loads you can ignore in a power outage.
I asked local install for back-up power and the cost was more than Tesla. I think about double, but I don't have the figures anymore. This installer was able to match Tesla's solar rate of $2.65/watt and claimed to have better system due to US panel and microinverters.
So I have a 2000 square feet home, 2 Powerwalls, and a 12 kw solar system (9.8 inverter). I recently ran a test of running off-grid. Here is what I can share:
1. On a sunny day the 2 Powerwall capacity is not enough to soak up all of excess power. You can manually manage the excess power by plugging in and charging the car, lowering AC temperature, and manually managing the water heater temperature.
2. The useful capacity of Powerwalls is a bit lower than 13.5 (especially in off-grid mode). They don't charge past 95%, and they start turning themselves off below 10%. So you are looking at closer to 11kw.
3. For night time, you need to have enough energy to last from about 6pm until 8am. For my house, 2 PWs are sufficient, but it does require being mindful of your energy use and not using oven, car charger, dishwasher after 6pm. My usage between 6pm and 8am is about 16-20 kwh. So 2 Powerwalls go from about 95% (if the previous day was sunny) to about 20-25% by 8am. This is for July.
4. Very cloudy days can create a problem. By very cloudy I mean severe overcast that covers most of the sky and rain. Partly-cloudy is not a problem. If you have two bad days in a row, that can lead to rationing of power and AC may have to be turned off at night.
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