Would you please help me understand this better? Isn't there a way for the utility to shut off the back flow into the grid, while still allowing you to use your own solar generation? It seems counterintuitive that during prolonged outages (like the recent Texas fiasco), people cannot use their solar generation at least during the day time, even if they don't have Powerwalls.
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Moving to new house.. solar system sizing
- Thread starter tfan2018
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You need something to route and control flow of energy between the sources (solar, grid, powerwalls) and sinks (house, grid, powerwall). That is the job of the Tesla Energy Gateway (TEG) that comes with Powerwall installs. Normally when the grid is up and there is excess power it can be routed to the Powerwalls and/or the grid. But, when the grid is down and the Powerwalls are full, there is no place for the excess solar power to go so the TEG shuts down the inverters. Then as the house uses power and the Powerwalls drain down, the TEG can start the inverters to recharge the Powerwalls. This cycle can repeat over and over to keep the house powered for days without the grid.
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Ctein
Member
Dear Aswami,
That one surprised me, too, when we got our solar system. It took a little thought to see that it couldn't be any other way.
For one thing, how is the utility supposed to be able to shut off the backflow when your grid goes down? It has no way to communicate with your home system, because... the grid is down! Likely future grid infrastructure improvements will have a way for the grid to talk to your home system, but for now we're grafting distributed generation onto a system that was never designed for such a thing.
A bigger problem is that without some kind of an intermediate reservoir of electricity, there's a high probability something in your house will get damaged. What happens, for example, if a cloud passes over your house and the solar output suddenly drops below your draw? Not anything good. Lots of electronics get very unhappy with large power fluctuations or brownouts.
Even with a PowerWall — unless you've bought a whole buncha them — the installers are only going to wire a limited number of circuits in your home into the wall to make sure your house doesn't attempt to draw more power than the battery is rated to provide.
pax / Ctein
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A blanket statement that 4 kw is extremely small, is just false. I have 4.4 KW and have an annual surplus now which is enough for about 10000 mi/year for an EV.
There is an online tool called PVWatts which I don't think anyone has mentioned. You enter the data for your system including location, size, orientation and it gives you monthly solar production.
If you're producing more than your consuming, and and not getting paid a good rate for it, then it's oversized and isn't a good financial choice.
The OP needs to do an estimate of current consumption, in kWh not $, add something for near term EV changes, and an estimate of solar production from PVWatts. From that you can make a smart decision.
You missed my point. Every person I have talked with that has solar, and same for me, is if one has excess production, its great because one can keep their home warmer in the winter, but the big one is cooler in the summer. So, I stand by my comment. Needs grow. And with NEM3 probably going to make solar less attractive, now is the time to add. I know I am looking at getting a bid to see what is the largest extra I can add. Trust me, if I end up with too much, wife will love the house staying at 70 degrees in the hot summer.
It didn't read like a blanket statement to me. Key qualifiers were: charge EV at home, in CA, not all gas appliances, house > 800 square feet.
OK, maybe not a blanket statement, but an easily misinterpreted generalization. I have all electric, including heat, 2300 sq ft, EV in the budget, and 4.4 kw is enough. I am not in CA, I have more solar gain, but a much higher heat load.
Man, you must have an amazing insulated house. I am 3300 sq feet, 2.5 stores, high ceilings and all electric. I have 9 mini split heads in my house. Running just 5 of them at 64 degrees has just the heaters using 30 to 40 kwh per day. How much production in winter do you get? When cloudy, I get 3kwh, and I have a 15kw system. I get lots of just 20khw days in winter.
since I will be required to use TOU, shouldn’t I look at both $ and kWh? Since the utility wants to generate solar during off peak and charge peak during the evening when there’s no solar
I would have said yes in the past. I have west-facing panels, which are situated for the most return on my investment (also the only direction my gable roof permitted besides east). But the utilities are adjusting TOU to more accurately reflect demand curves. I used to be in peak at noon or 1pm, depending on the day. I could sell at peak for a good stretch of the day. Now my peak doesn't start until 4pm, and carries over until midnight. As solar continues to be installed, rates will become cheaper during high solar gain hours due to excess supply.
At this point, I would focus more on production than I would on trying to match up with current TOU rates. Those are subject to change, and if you're always able to produce at least what you consume, you're less likely to find yourself performing a time of day juggling act.
Ctein
Member
Dear tfan,
Unless you know your daily power cycle very well, it's impossible to pin that down much. As ohmman pointed out, the TOU's are going to be regularly adjusted to minimize the demand curves on the utilities. You can certainly take measures to minimize your electric bill but in terms of optimizing your solar installation, it's hopeless.
A good general guideline for your economic sweet spot is that your system should handle about 80% of your annual needs. Unfortunately, you don't know what your annual needs are! You're going to have to wing it. But unless you're obsessed with trying to go totally off grid or want to be super-green about it, don't install based on your maximum usage.
Don't worry about all this talk about NEM changes. They're going to happen. Over the life of your system, they will continue to happen. The current rate structure is biased heavily to encourage solar installation (yay for us!). It's not economically sustainable, so over time you can expect to see more of the costs shifted onto us. But even if the utilities got everything they wanted in NEM3 (they're only going to get a fraction of that), your system would still pay for itself. It'd just take 12-15 years instead of 8-10.
Here in Daly City the schedule I am currently on has off-peak rates from midnight until 3 PM of about $0.17 per kilowatt hour. Between 3 PM and midnight, the electricity costs 2 to 3 times as much, depending on the exact time of day and year. Obviously I don't want to be charging the car at maximum peak rates — that's as expensive (or more) than gasoline!
I'm self-employed, so this works out just fine for me. Over the 15 months that I've had my Model X, my average daily driving has been about 25 miles. Most days not driving and then the periodic long drives. I take advantage of the free Supercharging when I'm doing a long drive (and I have the spare time) — why not! — but I don't rely on it heavily. I've read that supercharging degrades the battery faster. I don't know if that is still true with the latest version of the software and battery preconditioning and not taking it above 90% except when starting out on long drives. I may be excessively and unnecessarily cautious. Someone may want to correct me on this.
At home, I don't even have a 220 line installed in the garage yet. I've been trickle-charging off of the 110. Adds about 1% of capacity per hour of connect time, 15% total per day over off-peak hours. That, combined with the occasional supercharging, has been sufficient. I'm going to get a 220 V line installed in the next month for the convenience when I need to fully charge in a handful of hours. But I don't know how much I'll use it! Because...
The game I'm currently playing with myself is seeing how cheap I make it. I plug in the car in the morning when I get up and unplug it at 3 PM (or later now, as my panels are still generating net power). I put on 6% charge a day (15-20 miles) using electricity that is only costing me five cents per kilowatt hour (what PG&E/San Mateo Clean would be paying me for that excess generation).
I figure altogether I'm spending about 1-1.5 cents per mile on juice. It's a silly game, but it amuses me.
- pax \ Ctein
[ Please excuse any word-salad. Dragon Dictate in training! ]
Zeronet
Member
My new house was built by a fanatical energy builder. The entire house is rated at using only 22% of the power for a house its size. It has foot-think insulated concrete walls. I originally had only a 3 kW system, but I upgraded to a 6 kW system because my utility was going to "Grandfather" existing "net-metering" accounts for 20 years, but new accounts would be charged far more. I would have gone for 10 kW if my garage roof had been bigger. Neighbors of mine with big houses have 15+ kW. Now I produce enough energy to heat, cool, and light the house as well as charge my Tesla with extra for another Tesla and some pond pumps.
The efficiencies for solar are entirely dependent upon what your power company will allow. "Net-metering" is the optimum, since they give you kWh credits for extra you produce during the day or in the summer. But all else being equal, installing solar has certain fixed expenses, especially your $1500 to $2500 inverter. I had to replace my inverter when I doubled my array. So, all else being equal, I'd recommend you install as much as your budget and roof will allow on the first try.
Yep, this is why I added another 11.4K SE inverter, since never wanted the inverter to limit me. So now looking at adding more panels, the infrastructure is all set for as big as my house will fit, PGE will approve, and my budget will allow. At 15K now, would love to hit 20K
The grid does have some limited control of your (new) PV system by pushing voltage out of the established norms, perhaps they would play with frequency but I doubt it. This whole protocol is called Rule 21 SA here in California. There are others, for instance in Hawai'i that have other protocols.
The long and short of it is that new inverters do respond to signals from the grid and can partially curtail your PV production, turn off your PV production entirely and such.
I am not totally sure that the grid operators are currently using this, but I know there is new tech in the new inverters that allows grid operators to limit the production of distributed generation as they need for grid stability.
The long and short of it is that new inverters do respond to signals from the grid and can partially curtail your PV production, turn off your PV production entirely and such.
I am not totally sure that the grid operators are currently using this, but I know there is new tech in the new inverters that allows grid operators to limit the production of distributed generation as they need for grid stability.
It looks like they might be updating things now - pricing is definitely odd at the moment, and changing the number of PWs in MD from 1 to 2 somehow loses all of the incentives. But overall, the price of a PW appears to be much lower at the moment.
Also looks like they have added simple/medium/complex options on solar roof.
Hm it seems a price increase on solar panels is happening. I can no longer see the $11k option for 8.16kw
Not super insulated, just IECC 2013 Code (I think), which is R60 attic, and an extra 1" outside the studs. No vaulted ceilings, 1 story. I do get heating help from passive solar. Avg production for the last year was 650 kWh / mo. Dec was 440, Jan was 520. Yesterday we got snow & I went skiing instead of scraping snow off the panels & I got 2.5.
Yep, you're probly right about that. It also depends on the net metering details you have, and as ohmman pointed out the TOU details can change. Do you get paid full retail TOU rates for your surplus?
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