Help me understand a SolarCity proposal w/r/t Model S charging

40 Amps @ 240 Volts = 9,600 Watts = 9.6 kW

So, if you're charging your Model S in the middle of the day, you're still pulling power off the grid. Assuming you are setup with Net Metering, where you're paying your electric company for the net usage of electricity that month, in reality, it doesn't matter. If your panel has a 100 Amp breaker, you may have difficulty having enough capacity to get a 50 amp outlet (required for charging at 40 amps continuous). It all depends upon how many other loads you already have in the house.

I don't totally understand how all of this works so maybe someone who does can speak to this but maybe installing a 14-50 would still work. That way when you need faster charging you still have it.

Are they not upgrading the service to your house to 200A or something more? Could charging during the day when AC and other things are on lead to issues?

Code doesn't have anything to say about your main breaker in that way. It's up to you to think about what all stuff you turn on at the same time. Just schedule the car to start charging in the middle of the night and pull 40A, no problem.

HankLloydRight said:

On most days, my car won't be parked at my house overnight.. it will only be there during the day.

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None the less, you want a 14-50 installed, if only for future flexibility. The cost difference will be minimal, unless you need lots of wire. For short runs, the bulk of the charge is labor.

If you want to charge at a lower rate, fine, you can dial back the charge rate in the car. But don't force yourself to with a lower current outlet.

You can install a control unit at your meter with read in realtime the total consumtion of your house, including the production from the solar.array. Depending on the currently aviable capacitiy it can tell the Model S via the pilot signal, how much current it can draw. If you want, you can set the controll only to use the power from the solar.

We called this PV-controlled charging.

Best

Eberhard

zwede said:

Code doesn't have anything to say about your main breaker in that way. It's up to you to think about what all stuff you turn on at the same time. Just schedule the car to start charging in the middle of the night and pull 40A, no problem.

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Code definitely does tell you many loads you can run off a 100A (or any other size) main breaker and you determine the minimum main breaker size by running a load calculation on the house. All spelled out in the NEC, though of course, your local jurisdiction may vary in how strictly or not it follows those guidelines.

SolarCity appears to have sized the PV system as large as possible without having to upgrade the service. You are allowed to backfeed up to 20% if your panel's rating assuming your main breaker matches your panel rating which is usually the case. 20% of 100A = 20A. Like an EV charging circuit the maximum PV load is also derated by 20%, so you can have an inverter with a maximum output of 16A. At 240V that's 3840W. In this case, the inverter is rated at 3640W which would be 15A at 240V.

Your mileage may vary, but here is a sample of the load calcs from my house in San Diego right from the approved permit that show a 125 amp panel and an installation of a Blink 30 amp charging station (listed at 7200 VA in the load calculations). It's not about how much power your individual breakers may consume, but these standard NEC load calc formulas...I'm not sure how the solar system changes these calcs. Anyone else have that information?

RandyS said:

.I'm not sure how the solar system changes these calcs. Anyone else have that information?

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Solar doesn't affect your load calcs since you can't assume it'll be on.

drees said:

Code definitely does tell you many loads you can run off a 100A (or any other size) main breaker and you determine the minimum main breaker size by running a load calculation on the house. All spelled out in the NEC, though of course, your local jurisdiction may vary in how strictly or not it follows those guidelines.

SolarCity appears to have sized the PV system as large as possible without having to upgrade the service. You are allowed to backfeed up to 20% if your panel's rating assuming your main breaker matches your panel rating which is usually the case. 20% of 100A = 20A. Like an EV charging circuit the maximum PV load is also derated by 20%, so you can have an inverter with a maximum output of 16A. At 240V that's 3840W. In this case, the inverter is rated at 3640W which would be 15A at 240V.

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The "20% of panel rating" rule only applies to a backfed system, correct? I can't remember if it's 20% of the main breaker rating, or 20% of the bus bar rating (eg, I've seen panels where the main is 100A, but the bus bars are rated at 125A).

With a supply side connection, I think this rule no longer applies (I've seen several PV systems with supply side connections where the system output is > 20% of the main breaker). If the OP is looking for a larger system, this may be an option.

drees said:

Code definitely does tell you many loads you can run off a 100A (or any other size) main breaker and you determine the minimum main breaker size by running a load calculation on the house. All spelled out in the NEC, though of course, your local jurisdiction may vary in how strictly or not it follows those guidelines.

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What I was trying to say (unsuccessfully), was that adding a new outlet without upgrading the main breaker is not against code. I'm aware that a load calc is required to determine the initial service/main breaker. But maybe I'm wrong about that? I haven't studied that part of NEC.

zwede said:

What I was trying to say (unsuccessfully), was that adding a new outlet without upgrading the main breaker is not against code. I'm aware that a load calc is required to determine the initial service/main breaker. But maybe I'm wrong about that? I haven't studied that part of NEC.

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A load calc is typically required any time you want to add an additional branch circuit. Most home services typically have room for this and so most electricians don't worry when you tell them to add a 15A circuit. Adding anything bigger becomes a question.

The 20% of panel rating rule applies to any situation where you are feeding two sides of a panel with a power source - downstream and upstream. This is because you can end up drawing more current across that panel: main breaker size + backfeed breaker size. If the panel is rated for 200A, and your main breaker is rated at 200A, you may only feed up to 40A through a downstream breaker.

There is a solution if you need it, you can reduce your main breaker size or your feed breaker size so that you can't exceed more than 120% of the panel's rated capacity through circuit breakers. For example, if your panel is rated at 200A, you can reduce your main breaker size to 150A, and that would permit a backfeed of up to 90A. (120% of 200 = 240A; 150A main + 90A backfeed).

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tga said:

With a supply side connection, I think this rule no longer applies (I've seen several PV systems with supply side connections where the system output is > 20% of the main breaker). If the OP is looking for a larger system, this may be an option.

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This is correct, it depends on how the system is fed.

The OP's 100A service is likely enough for a load calc that assumed late-night charging, although it would probably depend upon the inspector's evaluation.

Here's what my solar output looks like in Southern IL (near St. Louis), on a 9 kW system (30x300W panels, 8 kW inverter), yesterday (a very clear day):



As you can see, during the peak hour (typically 1-2 pm), it's feeding ~30A to the grid. At different times, the current is different.

Thanks. I feel a bit more educated now.

tga said:

The "20% of panel rating" rule only applies to a backfed system, correct? I can't remember if it's 20% of the main breaker rating, or 20% of the bus bar rating (eg, I've seen panels where the main is 100A, but the bus bars are rated at 125A).

With a supply side connection, I think this rule no longer applies (I've seen several PV systems with supply side connections where the system output is > 20% of the main breaker). If the OP is looking for a larger system, this may be an option.

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It's 20% of the buss bar rating of the circuit breaker panel that is the limit. If you have a 100 Amp panel, you can feed it with a 100 Amp breaker from the Utility and a 20 Amp breaker from your grid tied solar system. The concern is having an over current situation in the buss bars on a nice sunny day with lots of loads running in your house. If you put the Utility feed at one end of the breaker panel and the solar feed at the other, you never get close to this because the currents never add, but 20% is the limit for the uninformed that but both utility and solar feed breakers at one end.

You could replace your 100 Amp panel without upgrading your utility service and have room for tons of solar. For example, replace the 100 Amp panel with a 225 Amp panel and keep the utility feed at 100 Amps through a 100 Amp breaker. Going all the way to 225 Amps would give you lots of margin for solar when you do upgrade the utility feed to 200 Amps. 120%*225-200=70; 70 Amps of solar is 16.8 kW AC or about 18.7 kW DC!

I'm a new model s owner and also looking into solar city. Here's my analysis so far:

http://teslaliving.wordpress.com/2014/04/19/solar-gains/

Right now waiting on a second engineering site visit.

I am really shocked, how much you have to pay for a solar array. In Germany, your price is about €1.200,-- per kWp +VAT (which you get back) or your 24kWp solar array would cost in Germny
€28.000,-- including installation ready to run. At current exchange rate for the Dollar is about $40.000,-- You really get heavily overcharged.

best Eberhard

tliving said:

I'm a new model s owner and also looking into solar city. Here's my analysis so far:

Solar Gains | Tesla Living

Right now waiting on a second engineering site visit.

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In your blog, you mention $144K for a 24kW DC system. That work out to $6/W installed, which is way more than current "real prices". I've seen systems installed recently for $3.25-$3.50/W, installed price. After the Fed tax rebates, that drops to $2.28-$2.80/W.

If you bought a 24kW system outright, my rough estimate of the numbers look like:

Depending on site orientation, shading, roof pitch, etc., that system could produce >26mWh/year, which equates to 26 SREC's (Solar Renewable Energy Credits). I don't know what SREC-2 is going to look like, but SREC-1's have traded around $225-$250 in 2013. So that means a 24kWh system could generate over $6,000 in SREC income each year, for 10 years, which pays off the after tax price of the system.

There are financing programs structured to use the Fed tax refund and SREC income to pay off the system (Sunpower offers a program through their dealers for systems using Sunpower panels). After 10 years, you own your solar system outright, and pay nothing for electricity. With a SolarCity PPA, you're still paying SolarCity.

I haven't done an in depth model to compare outright purchase vs a lease or PPA, but my gut feel is you come out much better in MA if you purchase because of the SREC income.

PM me if you want more details on my system and installation experience. I'm happy to let you come look at it, show you the numbers for my system, and refer you to my installer. I'll even let you have a free charge off my 14-50 (but I need a ride in return).