Can I use my Manual Transfer Switch Home Backup Setup for Solar Backup as well?

Primitivus

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Hello all,

I am trying to find out whether I can utilize (all, or most of) my existing transfer switch setup by swapping out the gas generator with a solar generator. The setup was installed by an electrician for the purpose of using it with a gas generator.

I have a power inlet box and transfer switch already in place for home backup which I'm currently using for my gas powered generator during power failures. The generator is attached to the inlet box via a 30-Amp 250-Volt Generator Power Cord (L14-30P to L14-30R)

Inlet box:
Amazon.com : Reliance Controls Generators Up to 7,500 Running Watts PB30 30-Amp NEMA 3R Power Inlet Box, Gray : Generator Cord Sets And Plugs : Patio, Lawn & Garden

Power cord:
Amazon.com: Champion 25-Foot 30-Amp 250-Volt Generator Power Cord for Manual Transfer Switch (L14-30P to L14-30R) : Everything Else

My hope is to utilize this setup with solar.

I believe this is a standard Manual-Transfer-Switch setup. This works by turning off Main and enabling the generator breaker in the main panel, and then enabling/disabling the breakers independently as needed. Note that the generator breaker is in the main panel box. There are no sub panels. This gives me full utilization of the main box as long as I stay within the constraints of the 30 Amp inlet box (to my understanding).

So is it possible to utilize the existing manual transfer switch by simply replacing the gas generator with solar? Could it be so simple as to find a cord that fits one end into the inlet box above, and the other into a solar generator, and add some panels? Or am I thinking too optimistically?

Do you have any recommendations for how you would go about this?

Thanks in advance for anyones time on this!
 
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LearningIt

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This sounds more like an interlock than a manual transfer switch.

Also, most solar generators seem to be single 120v and your L14 inlet is 240v split phase. You would need an adapter that bridges both legs to get 120v on both halves of the service and that is only safe without shared neutrals.
 

Supervstech

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The answer would be yes, but you don't have a transfer switch. You have a main panel interlock.
As mentioned above, two issues would be phasing if you use a 120v solar generator, and sizing.
If you build a solar system, you could use the interlock. But a better use for solar is in conjunction with the grid system.
Either in a grid feeding setup, or constant on critical needs sub panel, that uses solar and battery every day.
 

LearningIt

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The answer would be yes, but you don't have a transfer switch. You have a main panel interlock.
As mentioned above, two issues would be phasing if you use a 120v solar generator, and sizing.
If you build a solar system, you could use the interlock. But a better use for solar is in conjunction with the grid system.
Either in a grid feeding setup, or constant on critical needs sub panel, that uses solar and battery every day.
I concur. Because solar has high fixed and low marginal costs while ICE generators have the opposite, low fixed and high marginal (fuel) costs, ideally you would use solar all the time, with the grid. As I understand it, an issue is that grid tied solar is even more expensive due to the need for certification and, usually, professional installation.

I've casually seen that when you go to 240v and split phase solar systems, the costs rise significantly.
 

Primitivus

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I attached a picture of my panel. It says “mechanical interlock” right there :) Thanks for clearing that up.

I would love to do one of these recommendations (feed the grid or constant on critical needs sub panel) however a) I don’t think I’m capable to handle the install myself and b) I was hoping to utilize the setup already in place.

My primary goal is to handle “critical needs” in an off grid manner. If it makes sense to do grid fed or install a sub panel.. I could contact an electrician but the ones I’ve spoken with have virtually no solar knowledge so it’s been a difficult subject and solar installers are snakes, in my experience.

I know that my gas generator (Briggs & Stratton Q6500) has handled everything thrown at it from the house aside from the central AC and the Dryer which I can live without.

Q6500:
https://www.briggsandstratton.com/n...s/q6500-inverter-generator-with-co-guard.html

I wouldn’t even be against hitting the interlock switch everyday or night to use solar backup. Would probably end up not happening in reality though unless the power cuts.

Obviously your ideas above would be ideal but I’m not at a point where I understand how to do that and may never be… which led me to this (PRICEY) Bluetti AC300/B300 setup:

https://www.bluettipower.com/products/ac300-b300

which apparently is 240V capable with a Fusion Box Pro (and 2 AC300 units!?), so the cost of this setup is staggering in my opinion.

And then there’s the issue of actual setup should I attempt to utilize the interlock/inlet box. Obviously I wouldn’t want a solar generator to reside outside where the inlet box is located..
So your ideas are welcomed and appreciated but I’m still dead in the water.

Current obstacles:
  • Decide on interlock or sub panel for critical needs. Whether I use the interlock which seems like the easy way, or go with a recommended setup and hire an electrician for another panel install.
  • Overpaying for something I possibly don’t need (like the Bluetti) when building it out could be cheaper.
  • Getting around the outdoor inlet situation if I stick with using the interlock.
 

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Primitivus

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I concur. Because solar has high fixed and low marginal costs while ICE generators have the opposite, low fixed and high marginal (fuel) costs, ideally you would use solar all the time, with the grid. As I understand it, an issue is that grid tied solar is even more expensive due to the need for certification and, usually, professional installation.

I've casually seen that when you go to 240v and split phase solar systems, the costs rise significantly.

“I've casually seen that when you go to 240v and split phase solar systems, the costs rise significantly.”

This alone could rule it out altogether.. If it doesn’t make sense to utilize the interlock setup, I will avoid. I would prefer to seek out the most economical options!
 

Supervstech

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With what you have, the interlock isn't ideal.
I would recommend a critical needs subpanel, all 120V circuits, setup on a solar connection.
 

LearningIt

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I think whether you have an interlock or a "regular" or at least a manual transfer switch, you would have the same issue using solar full time. You would have to mechanically switch either the whole panel with the interlock or those (critical) circuits with the manual transfer switch. With a 120v solar system or a 120v gas generator you would need a cable adapter that bridges both legs to provide the same phase 120v on both. Those are fairly readily available even at the big box orange or blue home improvement stores. That is certainly do-able and not a big deal, but you would lose 240v circuits (maybe not a big deal for emergency power backup) and make sure you don't have any shared neutral circuits used at the same time.

A critical needs subpanel is a good option but requires that installation. There are inverter/chargers and AiO units mentioned here that I presume could be used on just that subpanel, and that panel could be limited to single 120v circuits if that is what your unit supports.

Another issue (as I understand and anyone feel free to correct me) is that some 120v inverters don't drive 120v AC on the line. They actually drive half voltage 60v on each of the line and the "neutral." This wouldn't affect most appliances since they just use the voltage difference but would affect some things like furnaces that check for 0v between neutral and ground. Then you have the floating vs bonded neutral issue both for operations and safety.

Another factor to consider is the power budget. Your ordinary inexpensive gas generator can produce 5000W running but 5000W of solar power would require a lot of panels and battery capacity. Do you need 5000W peak? More? Less? Without an A/C, maybe less is needed. How many Watt hours of energy do you need for overnight, non-sunny conditions? An "ordinary" 100 Ah 12v LiFePO4 battery would store over 1200Wh. A furnace blower motor could be anywhere from 100W to 400W, so that battery could power it (alone!) from 3 to 12 hours of running, but the blower should not have a 100% duty cycle. It's not constantly running. Refrigerators , maybe a few 100W each?

Yes, solar has basically 0 marginal cost and conveniently does not require refueling, but a Natural Gas generator (even a portable one) also does not require refueling and NG is very reliable (more than grid electricity, but not as much as solar?), if NG is available for you.

You kind of don't want to run the generator constantly when the house load does not require its full power, in order to conserve fuel and make your supply last longer (if gasoline or propane), but battery storage is expensive per kWh, whether with solar or a fuel generator.

One gallon of gasoline can probably run your generator for an hour, at $3/gallon -> $3/gallon*1 gallon/hour * 1 / 5000W = $ 0.0006/kWh, but that's only relevant if you are using 5000W. If you are using only 500W, you use basically the same amount of fuel.

Battery $400/battery * 1/1200Wh = $ 0.3 / kWh, but you can use the battery 1000s of cycles, $0.0003 /KWh/cycle (IF you use it 1000 cycles, not if you use it a few times during power outages!).

So, after all that, the answer is, "it depends."
 
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LearningIt

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The inverter (and generator) manufacturers don't seem to do a good job, that I have seen, documenting how their inverters work. Do they do split phase 60v/60v? Do they have a bonded neutral?

I see recommendations on inverters generally, but not recommendations specifically for inverters used with an interlock on a house circuit breaker panel, either 120v or 240v split phase.
 
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BMcL

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With what you have, the interlock isn't ideal.
I would recommend a critical needs subpanel, all 120V circuits, setup on a solar connection.
A critical needs subpanel is what I am planning, with about 10k solar panels feeding an 8kw outback radian.
I haven't made a final decision on the inverter. A friend is using a SunGoldPower 18kw low frequency model that's very economical ($4300).
I need split-phase to run my 220V heat pump.
 

Primitivus

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These replies have been invaluable, thank you all! For the purpose of this thread you all have answered my questions and concerns and I’ve now ruled out the idea of using the dedicated interlock for solar. Since I feel that I am now “in the same boat” as everyone else when it comes to solar home backup, I will need to research what the best approach is. Always open to recommendations. Thanks again
 

Primitivus

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A critical needs subpanel is what I am planning, with about 10k solar panels feeding an 8kw outback radian.
I haven't made a final decision on the inverter. A friend is using a SunGoldPower 18kw low frequency model that's very economical ($4300).
I need split-phase to run my 220V heat pump.
I’ll take a parts list when you’re ready :):)
 

BMcL

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I’ll take a parts list when you’re ready :):)
The SunGoldPower 18kw model is a beast that weighs about 220 lbs :)
And I haven't found anyone besides my electrician friend using one.
He has two Midnite Solar SCC's handling his solar panels.
Seems very reliable; he's been using it for several years. The over-capacity seems like it will help reliability ?
 

OffGridInTheCity

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I use an ATS + MTS for automatic daily switching between my off-grid solar power and the grid. Works great, nearing 3 years now - so 2000+ auto-switchovers. The MTS let's me choose the circuits to be part of the auto-switch. You could replace you're manual interlock with ATS'ing... The pic below handles 240v@50a, and you can do more than 1.
1638113981334.png
 

BMcL

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I use an ATS + MTS for automatic daily switching between my off-grid solar power and the grid. Works great, nearing 3 years now - so 2000+ auto-switchovers. You could replace you're manual interlock with ATS'ing... The pic below handles 240v@50a, and you can do more than 1.
In my area, it's much more expensive to tie onto the grid. Not to mention inspections and bureaucrats getting involved.
I'm intentionally staying off-grid, but every area has different rules.
 

OffGridInTheCity

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In my area, it's much more expensive to tie onto the grid. Not to mention inspections and bureaucrats getting involved.
I'm intentionally staying off-grid, but every area has different rules.
I agree about off-grid. This ATS + MTS setup is off-grid (no power going to the grid) and to code / all UL equipment. Doesn't require power company interaction. Its same as you can use for a traditional stand-by generator, its just the off-grid inverter is 'the generator'.
The ATS I use is like this but with surge protector - https://www.amazon.com/Progressive-...ve+50a+ATS&qid=1638114788&s=automotive&sr=1-6 (It's possible for surge on certain motors such as AC compressor due to mis-matched sine-waves during switch-over. I don't have this issue - but some might)
The MTS I use is this - https://www.homedepot.com/p/Relianc...ircuit-Manual-Transfer-Switch-A510C/206503336
 
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BMcL

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Here's a link to a SunGoldPower 18kw setup :

 

LearningIt

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These replies have been invaluable, thank you all! For the purpose of this thread you all have answered my questions and concerns and I’ve now ruled out the idea of using the dedicated interlock for solar. Since I feel that I am now “in the same boat” as everyone else when it comes to solar home backup, I will need to research what the best approach is. Always open to recommendations. Thanks again
I too have a 50A L14-50P inlet connected with an interlock to my single circuit breaker panel and an ICE generator to power it for backup during power outages.

My grid power prices are low enough, around $0.10 per kWh that solar to replace grid power has a very long payback time, over 10 years by my calculations and much more with battery backed solar, but I would like to have options in case of extended power outages. Over the last 10 years, I've twice had power outages that lasted 1-2 weeks. Even that frequency makes the high fixed cost of solar problematic unless I can take advantage of it daily.

To avoid the costs of grid-tied systems and since my POCO does not support TOU or net metering, I'm not interested in feeding back to the grid. My main options are something like an MPP feeding a subpanel and using it daily or something smaller just for emergency use with the interlock inlet but am undecided.
 

Paul_in _Berkeley

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These replies have been invaluable, thank you all! For the purpose of this thread you all have answered my questions and concerns and I’ve now ruled out the idea of using the dedicated interlock for solar. Since I feel that I am now “in the same boat” as everyone else when it comes to solar home backup, I will need to research what the best approach is. Always open to recommendations. Thanks again
We installed an Outback radian inverter, fed by batteries. The outback is smart enough to feed through utility power when it's available... you could configure it to run off solar when available, then switch seamlessly to utility when the solar fades. We have an interlocked breaker setup like you, and leave it in "generator" continuously. You may want to move the AC and dryer breakers to a utility only sub panel.

paul
 

mpickus

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I use an ATS + MTS for automatic daily switching between my off-grid solar power and the grid. Works great, nearing 3 years now - so 2000+ auto-switchovers. The MTS let's me choose the circuits to be part of the auto-switch. You could replace you're manual interlock with ATS'ing... The pic below handles 240v@50a, and you can do more than 1.
View attachment 73741
What is the make and model of the ATS shown in your setup?
 
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