Tie small solar panel string directly into AC unit

WhittakerJ
WhittakerJ Solar Expert Posts: 67 ✭✭
All,

I live in Mesa, AZ where as you know the sun is plentiful. I put a 5kw grid-tie system on my last house. I have recently moved and I'm not looking to make a huge investment at this point in time on my new house. I have a question/concept that I'm curious if some professionals can comment on.

Since my A/C unit takes up the majority of my electricity in the summer I was wondering if I could put a simple string and attach it to my A/C. The idea would be to simply buy say 4-5 250w panels. This would give me about a 1kw system, not very big by any means. I would like to then purchase the Enphase inverters and convert each one from DC to AC on the spot. My idea is to then add an additional AC disconnect to my A/C unit and simply wire this string into the A/C. My theory here is...

1. I can create a cost effective string of panels.
2. The breaker on my electric box should be able to handle this electricity as it powers the A/C unit already. (Not sure if this is a correct statement)
3. With the new AC disconnect box that I add to my A/C unit I can power off the panels when I need maintenance on the A/C unit.
4. I have a 5 ton A/C unit. So I'm assuming the cabling running to it would also be able to power a small string of panels like this.
5. The actual power flowing back into my power box would be limited when the A/C unit is running because the A/C unit is pulling its power from the string first. (Not sure if this is a correct statement)

I look forward to any comments as to if this is a good or really bad idea. The idea comes from some products I've seen pop-up lately where you can just simply take a solar panel convert it to AC and plug it into an outlet. Thanks in advance for your time.

Comments

  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Tie small solar panel string directly into AC unit

    4-5 250W panels are not enough to run 5 ton air conditioning.

    If you install Enphase inverters grid tied (the only way), then the energy from them, along with the power from the grid, will be used to run your A/C.

    The loads on your house do not use electricity in any particular order (such as A/C first), but rather all at once. So this consideration shouldn't affect any breaker size.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit

    NorthGuy,


    Thanks for the response. I realize that the panels will not completely replace my A/C's power requirements. I was planning on leaving it tied into my house AC lines still. Is it not a correct assumption that the 1kw of electricity produced by the panels would still be fed into my AC line and reduce my power usage from the utility company? I just assumed if I tied in at the A/C the lines would already be larger and therefore would be able to support a small 1kw system.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    Is it not a correct assumption that the 1kw of electricity produced by the panels would still be fed into my AC line and reduce my power usage from the utility company? I just assumed if I tied in at the A/C the lines would already be larger and therefore would be able to support a small 1kw system.

    They will offset the AC consumption when it cycles on. When it cycles off, it will feed other loads or the grid.

    I'm not an expert in codes, but electrically it doesn't matter if you tie it at A/C or at the main panel.
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: Tie small solar panel string directly into AC unit

    The only way to add the Enphase units is to add them to your grid tie system you have now. I too am considering an Enphase based addition, a simple breaker add to my AC combiner which has 2 PVP 5200 inverters on it now. I can add up to 15-20 amps AC output of solar to this setup safely, and still meet code I think. I recently upgraded the wiring from the AC combiner to my breaker panel with #4 wiring upgraded from #6. I am near the limit on my back-feed with a 60amp panel breaker and a derated main breaker to 175 amps. I will probably have to derate the main again to 150amp and upgrade the back-feed breaker to 80 amps to meet the NEC code on the 200 amp panel. The only loads there are my EV charging stations and the newish buildings sub-panel feed.

    The only reason to use Enphase is I will have a shading issue at the location I am making the addition.

    How long have you had your 5Kw setup? Are you on a TOU plan? if it is APS remember that on-peak generation is only allowed against on-peak usage. I still have not bought any on-peak generation this year at all, but if I add to my system I will definitely change metering plans. I am sort of waiting to see what happens with APS and the nasty deal they are trying to ram through the ACC (PUC).
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    NorthGuy wrote: »
    They will offset the AC consumption when it cycles on. When it cycles off, it will feed other loads or the grid.

    I'm not an expert in codes, but electrically it doesn't matter if you tie it at A/C or at the main panel.

    Ok, that is exactly how I understand it will work. I guess my question is now. How can I determine how many panels and enphase inverters are safe to string together and connect to my air conditioner's AC distribution box without overloading the cables in the wall or the breaker that is routed to the air conditioner?
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit

    I now rent the house so I haven't had the setup to analyze in over 2 years. It was replacing roughly 1/3 of my electricity each year from my calculations before moving out. I'm on SRP and I was on the 3-6TOU program. I feel for APS customers especially with how they're now trying to charge for grid-tie systems.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    Ok, that is exactly how I understand it will work. I guess my question is now. How can I determine how many panels and enphase inverters are safe to string together and connect to my air conditioner's AC distribution box without overloading the cables in the wall or the breaker that is routed to the air conditioner?

    You have to determine what will be a shared current path. In this case it is not much of a problem as there is a single, known load to supply; the A/C. Nothing else should be on the circuit 'down stream' of where the inverter output connects to the grid supply. That will be the wiring with the potential for grid Amps + inverter Amps. To be absolutely safe, this wiring should be protected from that point onwards by another breaker suitable to the wire size.

    Controlling the GTI's for purposes of offsetting a particular load only (with no grid sell back) is not that difficult in principal. You need to have the load control a relay to engage the GTI output to the wiring only when the load is on.

    I am reasonably certain that there would be code/inspection issues with this in most jurisdictions if only because bureaucrats love to be obstinate. :roll:
  • SkiDoo55
    SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    Re: Tie small solar panel string directly into AC unit

    One question on controlling micro-inverters with a relay to run only when A/C is running, wouldn't the 5 minute "connect" delay of the inverter string be a issue to over come? Controlling the connection as you said would be easy as you said just a parallel connection to a contactor circuit to second contactor.

    Just a thought other than local code issues.

    Really enjoy the information you provide in all of your posts.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tie small solar panel string directly into AC unit
    SkiDoo55 wrote: »
    One question on controlling micro-inverters with a relay to run only when A/C is running, wouldn't the 5 minute "connect" delay of the inverter string be a issue to over come? Controlling the connection as you said would be easy as you said just a parallel connection to a contactor circuit to second contactor.

    Just a thought other than local code issues.

    Really enjoy the information you provide in all of your posts.

    Yes it would be an issue; in order for it to be worthwhile the load would need to run long enough for the GTI to sync and start providing power. This probably would not be a big problem with an A/C in Summer heat. :D

    The other thing is it would have to be isolated to the motor/compressor load and not connected to any of the control circuit or else it would be on all the time.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    Yes it would be an issue; in order for it to be worthwhile the load would need to run long enough for the GTI to sync and start providing power. This probably would not be a big problem with an A/C in Summer heat. :D

    The other thing is it would have to be isolated to the motor/compressor load and not connected to any of the control circuit or else it would be on all the time.

    What would be wrong with tying it in directly to the line coming from the electric box and allowing it to be on at all times?

    Do the enphase converters take 5 minutes to start up and start providing power?
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    You have to determine what will be a shared current path. In this case it is not much of a problem as there is a single, known load to supply; the A/C. Nothing else should be on the circuit 'down stream' of where the inverter output connects to the grid supply. That will be the wiring with the potential for grid Amps + inverter Amps. To be absolutely safe, this wiring should be protected from that point onwards by another breaker suitable to the wire size.

    Controlling the GTI's for purposes of offsetting a particular load only (with no grid sell back) is not that difficult in principal. You need to have the load control a relay to engage the GTI output to the wiring only when the load is on.

    I am reasonably certain that there would be code/inspection issues with this in most jurisdictions if only because bureaucrats love to be obstinate. :roll:

    I see you are only recommending to do this if I have a relay only powering on the GTI when the A/C is turned on. What would be the downside of just tying the GTI into the AC shutoff box located on my A/C? I realize it would send power back down the line into my house. But wouldn't this be good as it would simply be powering other home devices when the A/C was turned off? Or is this not safe?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tie small solar panel string directly into AC unit

    No, I am not recommending it only for that instance. I'm saying if you want to do it only for that instance it can be done.

    Some people have utilities that do not allow sell-back. As such they want to find a way to offset their heavy power items such as A/C but can not have a GTI running all the time because it will push power all the time it can and there needs to be a place for it to go: if the loads aren't heavy enough it goes to the utility.

    If your utility will allow sell-back you just wire up the GTI directly to the service panel and let the power go where it's needed. Much, much simpler. You only have to stay within the restrictions for back-feeding.

    The start-up time on GTI's will vary with the particular unit so check their specs. It is there to make sure the inverter is in sync with utility power before it ramps up.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    No, I am not recommending it only for that instance. I'm saying if you want to do it only for that instance it can be done.

    Some people have utilities that do not allow sell-back. As such they want to find a way to offset their heavy power items such as A/C but can not have a GTI running all the time because it will push power all the time it can and there needs to be a place for it to go: if the loads aren't heavy enough it goes to the utility.

    If your utility will allow sell-back you just wire up the GTI directly to the service panel and let the power go where it's needed. Much, much simpler. You only have to stay within the restrictions for back-feeding.

    The start-up time on GTI's will vary with the particular unit so check their specs. It is there to make sure the inverter is in sync with utility power before it ramps up.

    Ah, ok. I'm not necessarily concerned with the power going back into the grid. I don't want to build anything huge may be a 1kw solar array. I can't imagine at any given time my house is using less electricity than this. That being said I'm assuming any other load in my house would automatically consume that electricity if the A/C were off right?

    The only thing I'm trying to do differently here is use the A/C power lines to my electric box to save on my infrastructure costs. I'm just trying to determine if it safe to do this. I figure why not tie into the A/C.

    I guess my main question is this. How can I determine the largest array size that I can create and safely send power back down the lines that my central A/C unit uses to pull electricity from my electric box? My thought on this was if the A/C is on great it will automatically take what it needs from the solar array that I just tied into it. However, if the A/C is off other devices in my house will simply pull this power from the solar array. I'm honestly not trying to design a system that is so large that I expect any type of excess going back into the grid. So all this being said how big of an array could I build before the 220V lines that supply my A/C would no longer safely handle? Or if there are any other things that I should be concerned with?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    Ah, ok. I'm not necessarily concerned with the power going back into the grid. I don't want to build anything huge may be a 1kw solar array. I can't imagine at any given time my house is using less electricity than this. That being said I'm assuming any other load in my house would automatically consume that electricity if the A/C were off right?

    Correct. Only power not used by household loads will go back to the utility.
    The only thing I'm trying to do differently here is use the A/C power lines to my electric box to save on my infrastructure costs. I'm just trying to determine if it safe to do this. I figure why not tie into the A/C.

    This would technically be in violation of code, because that requires a dedicated line from the GTI to the main service panel. Electrically it would work so long as current limits of the wiring are observed. This gets tricky in the wiring between there the GTI connects to the service wiring and the load. Even if this is at the A/C unit itself there is still a small amount of circuit that has the potential (however unlikely) of carrying the combined maximums of both power sources.
    I guess my main question is this. How can I determine the largest array size that I can create and safely send power back down the lines that my central A/C unit uses to pull electricity from my electric box? My thought on this was if the A/C is on great it will automatically take what it needs from the solar array that I just tied into it. However, if the A/C is off other devices in my house will simply pull this power from the solar array. I'm honestly not trying to design a system that is so large that I expect any type of excess going back into the grid. So all this being said how big of an array could I build before the 220V lines that supply my A/C would no longer safely handle? Or if there are any other things that I should be concerned with?

    This is actually quite easy to determine: how large is the circuit breaker handling the A/C?
    The current flow will be like this: when the GTI is active and the load isn't, then the GTI's full output will flow along the wires to the main service panel. When the GTI is active and the load is, current will flow from the GTI to the load. If the GTI's output is insufficient to meet the load needs the difference will be made up by power from the utility.

    So long as the GTI's output is equal to or less than the current the A/C draws most of the wiring would be safe. For best safety the join should be made in a sub panel and an additional circuit breaker used to protect any wiring from that point to the A/C.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    Correct. Only power not used by household loads will go back to the utility.



    This would technically be in violation of code, because that requires a dedicated line from the GTI to the main service panel. Electrically it would work so long as current limits of the wiring are observed. This gets tricky in the wiring between there the GTI connects to the service wiring and the load. Even if this is at the A/C unit itself there is still a small amount of circuit that has the potential (however unlikely) of carrying the combined maximums of both power sources.



    This is actually quite easy to determine: how large is the circuit breaker handling the A/C?
    The current flow will be like this: when the GTI is active and the load isn't, then the GTI's full output will flow along the wires to the main service panel. When the GTI is active and the load is, current will flow from the GTI to the load. If the GTI's output is insufficient to meet the load needs the difference will be made up by power from the utility.

    So long as the GTI's output is equal to or less than the current the A/C draws most of the wiring would be safe. For best safety the join should be made in a sub panel and an additional circuit breaker used to protect any wiring from that point to the A/C.

    It's a double throw breaker and each leg is labeled 50amps I'm not sure if this means it's 50 or 100.

    Based on this what size of an array could I safely use?
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    I look forward to any comments as to if this is a good or really bad idea.

    It will work, but will not be code compliant or legal with any US utility I'm aware of. It would have to be completely guerrilla, which means you run the risk of "getting caught" at which point you'd have to pull out all your additions and wire it in to your panel in a compliant fashion (i.e. external disconnect, dedicated circuit etc.)

    Question - why not start out doing it that way? It's not much more expensive and it doesn't matter where the power comes from.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    It will work, but will not be code compliant or legal with any US utility I'm aware of. It would have to be completely guerrilla, which means you run the risk of "getting caught" at which point you'd have to pull out all your additions and wire it in to your panel in a compliant fashion (i.e. external disconnect, dedicated circuit etc.)

    Question - why not start out doing it that way? It's not much more expensive and it doesn't matter where the power comes from.

    Yea, I suppose you make a good point. I just didn't want to hassle with running the wire through my attic and wall and fishing it to my electrical box. I was honestly just thinking of a quick way to get some solar panels wired into my house with out much additional expense. It also seemed to me to be logically pretty safe. But again I'm not an electrician.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    It's a double throw breaker and each leg is labeled 50amps I'm not sure if this means it's 50 or 100.

    Based on this what size of an array could I safely use?

    It's a double pole (common trip) breaker because it is 240 VAC. 50 Amp breaker, so it is probably 8 AWG wire to the A/C. That is a fairly hefty line: about 9.6 kW. It would have no trouble handling 1 kW.

    However, as bill von novak (and I) said this would be illegal for code.

    Whether or not the inspector would allow a change to a sub panel off this line and then connecting A/C and GTI to that via two breakers in that panel ... maybe.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    It's a double pole (common trip) breaker because it is 240 VAC. 50 Amp breaker, so it is probably 8 AWG wire to the A/C. That is a fairly hefty line: about 9.6 kW. It would have no trouble handling 1 kW.

    However, as bill von novak (and I) said this would be illegal for code.

    Whether or not the inspector would allow a change to a sub panel off this line and then connecting A/C and GTI to that via two breakers in that panel ... maybe.

    So you're saying it may be possible the line is thick enough to support both and as long as the maximum voltage of both did not exceed the wire there is a chance it could be approved by an inspector? How would one go about figuring this out?

    So back to what you had said before about using a relay in the A/C to turn the GTI on or off depending on the A/C's status... Does this technically mean now the GTI and solar panel array are part of the A/C unit since they cannot turn on without the A/C being on? I live in AZ and the A/C is practically running at all times anyways. Is there a commercial relay product that exists for this?
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    So you're saying it may be possible the line is thick enough to support both and as long as the maximum voltage of both did not exceed the wire there is a chance it could be approved by an inspector? How would one go about figuring this out?

    So back to what you had said before about using a relay in the A/C to turn the GTI on or off depending on the A/C's status... Does this technically mean now the GTI and solar panel array are part of the A/C unit since they cannot turn on without the A/C being on? I live in AZ and the A/C is practically running at all times anyways. Is there a commercial relay product that exists for this?

    I would bet this would not pass any kind of permitting, inspection or interconnect agreement. Just do it the right way, it is not much extra cost.
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    Yea, I suppose you make a good point. I just didn't want to hassle with running the wire through my attic and wall and fishing it to my electrical box. I was honestly just thinking of a quick way to get some solar panels wired into my house with out much additional expense. It also seemed to me to be logically pretty safe. But again I'm not an electrician.

    It can be done safely IF you are comfortable doing everything yourself and have the experience/knowledge to do it safely. Note that you still can't skimp on the mounts, which for me is generally the biggest chunk of work. (This is a safety issue; if your mounts don't hold up to wind, flying panels can be lethal.) However if you don't think you meet those criteria it's going to be easier to do it "right" - because then you will be able to contract someone else to do a little/a lot/all the work.
  • WhittakerJ
    WhittakerJ Solar Expert Posts: 67 ✭✭
    Re: Tie small solar panel string directly into AC unit
    It can be done safely IF you are comfortable doing everything yourself and have the experience/knowledge to do it safely. Note that you still can't skimp on the mounts, which for me is generally the biggest chunk of work. (This is a safety issue; if your mounts don't hold up to wind, flying panels can be lethal.) However if you don't think you meet those criteria it's going to be easier to do it "right" - because then you will be able to contract someone else to do a little/a lot/all the work.

    Yea, I'm not too concerned with the mounts. I seen how they did it on my other house and they look pretty simple to install.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tie small solar panel string directly into AC unit
    WhittakerJ wrote: »
    So you're saying it may be possible the line is thick enough to support both and as long as the maximum voltage of both did not exceed the wire there is a chance it could be approved by an inspector? How would one go about figuring this out?

    Okay, terminology. The Voltage is not an issue as that is fixed at 240 VAC. The current should not be an issue on the line from the A/C & GTI to the main panel as current can only flow in one direction. Thus that line would only ever: A). carry full current to supply the A/C when the GTI does not produce; B). carry partial current to supplement the GTI to provide power for the A/C or; C). carry full output of the GTI back to the main service panel to feed the other loads or the grid when the GTI is producing but the A/C is off.
    So back to what you had said before about using a relay in the A/C to turn the GTI on or off depending on the A/C's status... Does this technically mean now the GTI and solar panel array are part of the A/C unit since they cannot turn on without the A/C being on? I live in AZ and the A/C is practically running at all times anyways. Is there a commercial relay product that exists for this?

    I would bet no inspector would see it that way. They get antsy about these things.
    It would only be an opinion, but with the GTI unable to feed back to the grid and only connecting to the A/C when that is active it should not be a concern for the utility or inspector.

    But I sure wouldn't sign a document to that effect, and it is something of a waste of investment in panels & inverter.