Looking for an Automatic "special" Transfer Switch

SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
90% of the time in my little workshop I only have the need for small lights and charging power tool batteries etc.

Im planning to put a small solar system on my workshop to handle my normal needs.

However sometimes I need to use my welder or air compressor
(one of my welders is split phase also, maybe this should just be hard wired to the grid)

Is there an automatic transfer switch that anyone knows of that:
A. ) senses battery life
B. ) will Automatically switch power to grid IF the power draw is above say 2KW? and able to handle 15KW?
(ideally this would be a programable parameter) 
C. ) maybe a split phase option too

Thanks for your help!


  • BB.BB. Super Moderators, Administrators Posts: 32,923 admin
    The issue is a "small AC inverter" system will have internals designed for small loads. If you want it to "transfer" large loads in a fraction of a second... Then part of the inverter needs to be 10x heavier. It just does not usually make any sense.

    So, the other method, is to have a "backup AC circuit" driven by a smallish solar power+AC inverter into a separate branch circuit (with Orange outlets, for example).(300 watts is usually not bad, you can go as high as 1,200 to 1,800 Watts on a 12 volt battery bus, but then you really need a large AH battery bank and solar array to keep the battery bank "happy"--And it is no longer a nice/simple/cheap/cost effective system. Just plug your small loads into that circuit.

    Then, if you wish, you can do the cutover pretty easily (automatic or manual transfer from solar AC to mains)--If you want. 

    Automation--I have made a good living out of automation and redundant systems (auto failover communications systems, etc.).

    In practice, I try to dissuade people from doing automation/failover... While it can be done, and successfully, many times it just ends up with creating more paths to failure (similar with auto-start genset systems--Just more complicated than most people need, or can self self maintain).

    For me, I would design the complete Solar Panel+Battery Charger+Battery Bank+AC inverter+separate branch circuit to work bench for small loads. A 300 Watt AC inverter and 2-6x 6 volt @ 200 AH "golf cart" Batteries should work pretty well...

    Then you can look at automation. For example, we use ~11.5 volts on an FLA battery bank as the 50% cutoff "under load" level. And suggest that as the "failover point" (using FLA battery voltage to figure out state of charge, is not really very accurate). Get an AC inverter that has a programmable "battery cutoff point"--Normally it is 10.5 Volts (battery dead)... 11.5 volts under load, a bit easier on the battery bank.

    Then you can put a voltage sensing relay that changes over from AC inverter as a source, to AC mains if the AC inverter shuts down.

    Of course, there are battery monitoring systems/devices that can give you a better estimate on state of charge, and even those that will measure the battery current directly and figure out AH in and AH out. Some even have RS 232 or even Bluetooth communications and external alarm contacts.

    And look at the AC inverter carefully, besides being able to reprogram the battery disconnect voltage, there is the "search mode"--Inverter goes into a low power state, checking ~1x per second for a 6 Watt or greater load before turning on. For smaller off grid systems, AC inverter "tare losses" can be a significant power hit (smaller inverter ~6 Watt Tare losses--power used just "turned on and no loads"--To 20-40 Watts or more for kWatt class inverters--40 watts * 24 hours per day = 960 Watts just to run a large inverter 24 hours per day--As much as a very energy efficient refrigerator).

    Just to point you to a quite, efficient, PSW/TSW 120 VAC 60 Hz, 300 Watt, 12 VDC input inverter that has the basic requirements:


    And some basic models of Battery Monitoring systems, from voltage monitoring only, to full AH measuring units:


    And if this was a daily use power system, I would suggest a 10% rate of charge (up to 13-20% or so typical maximum):
    • 200 AH @ 14.5 volts (2x "golf cart" batteries) * 1/0.77 panel+solar charger deratings * 0.10 rate of charge = ~377 Watts of solar panels per 2x "golf cart" batteries.
    Nominally, if daily use, suggest 2 days storage and 50% maximum discharge for better battery life. Charging during day and using batteries at night:
    • 200 AH * 12 volts * 1/2 days storage * 0.50 max discharge * 0.85 AC inverter eff = 510 WH per night (per 2x golf cart batteries)
    And, the maximum power I would pull (reliably over time/temperature/age/state of charge) would be C/8 (discharge to 50% in 4 hours):
    • 200 AH * 12 volts * 0.85 AC inverter eff * 1/8 hour discharge rate = 255 Watts (surge to 2x or 525 Watts)
    Again, this is per "2x golf cart" batteries.

    And the the harvest for a 10% rate of charge array for a fixed array tilted to ~45 degrees in Pontiac Mich:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 47° angle from vertical:
    (For best year-round performance)


    Or, the "average" for April harvest would be:
    • 377 Watt "nominal" array (for 2x GC batts) * 0.52 off grid system eff * 4.04 hours of sun per day (April) = 902 WH of 120 VAC power per day (April).
    So you can divide your 902 WH "daytime" harvest between daytime use and (ideally) ~510 WH per evening--Or whatever works for you.

    All the above is about designing a "balanced" system based on your energy needs. Numbers are not "exact" (solar never is), and may or may not meet your needs--Just a stake in the ground to start a conversation (yes, there will be math  :) ).

    What I try and stop is a 400 Watt solar array with a 3 kWatt AC inverter on a 150 AH @ 12 volt battery bank... It really does not end up well most of the time.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
    such informed and good information! I'm taking some time to ingest it. (I do love math, I'm not a wizard, but I am always up for it!)
  • SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
    From talking to a few folks and Bill's info, it seems like I should alter my plan, maybe it's best that I try to be safe and learn a bit by going small scale on my old School Bus-RV that my wife and I do our summer trips in.

    Then we can go bigger with the workshop and combining that with the house over time.

    The arrows in orange are what we have now, basically, normal power from grid -> house -> workshop -> RV.

    The green are what I have learned (I Think!) is a mostly normal setup.

    I thought since the RV/Bus is parked most of the time maybe I should put it's panels to use and charge the main battery bank with the excess?

    Then *I think I am finding some "hybrid inverter/chargers" that do charge control, inverter, grid tie, and auto transfer switch all in one. 

    First I think I need to figure out the "balance" (Wattages of all the Hardware?) based on the power needs I have taken from our 2019 electric bill (has by the hour KW use!) and a kill-o-watt meter for the RV's usage

    Bill, your info on sunlight up here in Michigan is really helpful, I'm sitting down with it and attempting to wrap my head around it properly, thanks!

    Then I'm going to research and try to find the proper parts to accomplish this. 

    Any suggestions on suggested hardware and/or wattages or if my plan is super wacky and I should do something else are MUCH appreciated.


  • SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
    edited April 2020 #5
    I think I accidentally deleted my last post or maybe a moderator deleted it? sorry If I did it wrong!

    From the info I have been gathering and Bill, thank you for your info! I think its smart for me to do a bit of learning by going small scale on our RV (actually an old junky school bus, so it's great for work benching!) 

    So re-thinking about keeping some high draw appliances (welder, sauna, air compressor) just on grid power:

    I've seen these all in 1 "hybrid inverter charger, grid interactive controllers" those look pretty simple/nice, maybe now that I have some numbers on our power uses I can get one of those and just feed back into the grid to "pay for" the time that I'm welding or using the sauna on grid power, maybe I wouldn't need a huge system then?

    Or should I just get a hybrid controller that *can* handle those big loads too potentially go totally off grid one day?

    I like to get a "full picture" idea of where I'm going though, so I'm trying to still basically plan the fully realized system.
    So far here is the setup that I think will work? 

    (I already have a pure sine 2kw inverter running off 4 barely functioning old lead marine batteries (will replace) on the RV, and a 5kw inverter that *can* run off the alternator, but I just use that for while driving in the summer and running an AC)

    I really appreciate any advice and suggestions on hardware I should look at, or the wattages I need!

    I calculated average KW used for our home and workshop combined as well as used a Kill-o-watt meter to get an average and idea of needs the RV has.

    I'm going to try and understand the sun info that Bill shared, as well as read more on how to size the system.

    Thanks y'all!

    (orange arrows are what I currently have hooked up, and green arrows are what (I think!) a normal setup would be)

  • BB.BB. Super Moderators, Administrators Posts: 32,923 admin
    edited April 2020 #6

    I did not delete any of your posts... You got caught in the Spam Filter (new posters, if they have 'too many' Links or other "magic" issues)... After 5+ posts, generally no more Spam filter issues.

    I have released your post.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
  • SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
    So starting of on figuring out what Hardware I need for "Step 1" on the RV setup:

    (if anyone reads this and sees anything they think is weird or isn't what they would do, I'm all ears!)

    -my average use is 1.5 KW/day, I'll round that up to 2Kw
    -with 2 days of autonomy,
    -multiply days by KW->I'd need 4000 watt hrs of Battery
    -if I run 12v batteries,
    -divide battery kWh by 12v,
    -I'd need 333 usable amps
    -so 4 100ah lithium batteries or 8 FLA
    -looking at the table Bill put above, Detroit has about average of 5 sun hrs in summer
    -so I divide 4000 watt hr by 5 hrs
    -smallest solar array that "could" charge would be 800watts
    -so I'll try to round up to at least 1000 watts to be safe
    -Searching NAWS I find the cheapest/watt panels are the 325watt REC Solar
    -so if I get 4 of those
    -I'll potentially get 1300 watts
    -then if I divide that by 12 volts 
    -I'd need at least 108 amp charge controller, so a 125 to be safe

    So I think like you were warning me, Bill
    this quickly becomes a very expensive system! so maybe first I need to work backwards and find where all my power is running away to, and build a smaller system. 


  • SchrockstudioSchrockstudio Registered Users Posts: 13 ✭✭
    I realized I took my own topic off topic. sorry...
    Haven't updated much as I'm trying to see what power I can reduce on the RV.
    (seems mostly fridge, I took out the mini fridge and am testing turning down the thermostat on a mini freezer to see if that will work(more insulation=more efficiency?))

    Bill, after reading a lot more on the forums and reading through manuals, your first post is making a lot more sense to me now. I do agree that probably most cost effective and least paths to failure would be just small system for my small loads.
  • BB.BB. Super Moderators, Administrators Posts: 32,923 admin

    Yep... Even though we are a solar power forum, much of our work here is getting people to conserve energy and aim at a small system that just meets their needs--And we try to be very practical too--Many times solar does not really meet people's needs if they have any sort of reasonable utility power nearby.

    And if the power needs are more or less just for emergency use, a nice genset and some stored fuel ends up being more cost effective.

    At this point, your path of measuring your loads and conservation is the right approach.

    Have fun,
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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