Taking generator out putting solar in

LewBailieLewBailie Registered Users Posts: 4
Good morning, Thanks for letting me join in. I decided to take my new Generac gp 17500 portable generator out to go solar. This is a back up system ,we are on the grid. When we installed the generator our electrician wired to manual transfer switches to go from the grid to the generator. All I had to do is run the cable to the plug outside the house and switch. My question is, can I connect from the inverter to a cable and connect directly to the plug and switch when I need it? I see that all the inverters use 2-3 110 plugs, can they all be tied together? I want to start with a 400W panel system and a 2000W inverter  with 2 golf cart batteries. I hope that can operate everything but the AC in case of a power outage. We live on the Maryland / Pennsylvania border.


  • BB.BB. Super Moderators, Administrators Posts: 30,050 admin
    In general, each device needs its own transfer switch. You cannot tie the generator's AC output directly together with the inverter's AC output together and run "one at a time"--If you do, it will generally damage or ruin one of the devices.

    You can find a lot of AC inverters that have their own internal AC transfer switch--But a 2,000 Watt size inverter will not mange the full output of your genset...

    You can get another manual transfer switch to wire your inverter into your main panel/generator setup--But there are lots of options to choose from.

    Are you going to try to run your whole house from the 2kWatt inverter (with most everything turned off except for a few things)? Or do you want to just connect a few circuits to your inverter's output (some lights, TV, computer, cell charger, washing machine, central furnace)?

    One other thing to check for--Some gensets require the installer to bring a 120 VAC circuit out to the genset to wire to the battery charger--A few folks have missed this, and end up with a dead battery in a few weeks/months.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LewBailieLewBailie Registered Users Posts: 4
    my plan was to eliminate the generator all together and use it's installed circuits, for solar I have the installed transfer switches that I would use to go solar backup using the circuits in the house I need.
  • BB.BB. Super Moderators, Administrators Posts: 30,050 admin
    Have you figured out how much power you will need? Just an Energy Star Refrigerator, LED lightning, washer, LED TV, Laptop, + well pump--And you are looking at ~3.3 kWH per day (100 kWH per month) for a "near normal" very efficient small home.

    The battery bank needed to run something like that (2 days of storage, 50% maximum discharge, 24 volt battery bank):
    • 3,300 WH per day * 1/0.85 inverter eff * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 647 AH @ 24 volts
    Or, roughly 12 x 6 Volt @ 220 AH batteries arranged in 4 serial * 3 parallel strings ~ 24 volt @ 660 AH (if done with golf cart batteries).

    To charge your battery bank, you need ~5% to 13% rate of charge (typical for solar panels). 10%+ if this is a full time off grid system:
    • 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,243 Watt array minimum (weekend/seasonal usage)
    • 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,486 Watt array nominal (full time)
    • 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,231 Watt array "cost effective" maximum
    And then there is sizing the array based on how much energy per day and hours of sun:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    Generally, either you have to use less energy in winter, or make up for the periods of poor sun/weather with occasional genset use... For the moment, toss the bottom three months and use February as your "break even" month:
    • 3,300 WH * 1/0.52 off grid system eff * 1/3.63 hours of average sun (February) = 1,748 Watt array minimum (Feb)
    So--Not knowing anything about your home or electrical needs--The above is a first cut at an off grid system (on the medium to smaller size). If you only powered lights, radio, cell charger, and RV water pump, plus laptop computer, you may be able to get down towards 1,000 WH per day--But that is still a larger system than you seem to be planning on right now.

    If you presently have utility power--Why would you want to go off grid? In most cases, grid power is cheaper than off grid solar power. And if you have a few days/week of no-power (ice storms, etc.)--A genset with some stored fuel is, usually, still cheaper than a full-on solar power system (you can have a very small solar power system <1,000 WH per day for LED lighting, laptop, radio, etc.--And use the genset a couple times a day for refrigerator/well pump/washing machine during power outages)

    Your thoughts?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LewBailieLewBailie Registered Users Posts: 4
    Bill , not going off grid, Just for  emergency instead of running a gen set when the power goes out. Maybe doing both and using the Gen. several times a day for the fridge,freezer and hot water might work. Thanks for your insight. Lew
  • BB.BB. Super Moderators, Administrators Posts: 30,050 admin
    Do you know how much solar/battery backed power you will need? We really try for a "balanced" system design. Inverters can be pretty cheap these days--But the battery banks are not.

    If you are looking to run a 2,000 Watt AC inverter near rated output (and up to 2x rated output for starting loads like refrigerator compressor and (small) well pump)--You would need a ~400 AH battery bank @ 24 volt bank, or a ~800 AH minimum 12 volt battery bank.

    If you are looking for 2x golf cart batteries (at 6 volts and ~200 AH), then your maximum "reliable" system would be about 500 Watt AC inverter--To give you an idea--For such a battery bank, a MonringStar 300 Watt 12VDC TSW inverter would be a better match for the batteries (plus the inverter has remote on/off and low power standby)--Much better for running a few LED lights, TV, and radio at night.

    A reasonable solar array for 2x golf cart batteries would be:
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 207 Watt array minimum (weekend, emergency, seasonal)
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 414 Watt array nominal (recommended)
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 539 Watt array "cost effective" maximum
    Run a 20-40 amp 12 VDC charger (use a Honda eu2000i genset at ~4-9 hours per 1 gallon of gasoline)--Your Generac probably uses around 1.6 to 3.2 gallons per hour (1/2 to full load). And Gasoline gensets are not very efficient below ~1/2 load (the fuel flow remains almost the same at 1/2 load or 1/10th load).

    A 220 AH battery bank will nicely support:
    • 220 AH * 12 volts * 0.85 AC inverter eff * 1/2 days of storage * 0.50 maximum discharge (longer battery life) = ~561 Watt*Hours of storage (for 2 nights).
    • 414 Watt array * 3.63 hours of sun (Feb) * 0.52 off grid system eff = 781 WH per average February day
    If you want more battery backed power, you could 2x or 3x times the Golf Cart battery system pretty nicely (and use a similarly larger AC inverter)--For any larger system, I would suggest to use higher voltage (24 or 48 VDC) and probably better than golf cart batteries (I don't suggest a bunch of golf cart batteries for larger systems if you can avoid it).

    Note that many (most?) people do kill their first battery bank to two as they learn to maintain/use their system ("cheap" G.C. batteries are a good choice for first time solar power folks).

    If you do need more AC power than a 1,600 Watt Honda can provide--Look at how much AC power your home really needs during an outage. A 17 kWatt genset will run everything--But a 4 kWatt genset will run your emergency/reduced/planned loads on about 1/4 the gasoline.

    Killers are starting loads... A standard refrigerator compressor may need >~1,000 VA to start--And a typical well pump may need a lot more (forcing you to use a larger genset).

    For smaller inverter and gensets, a simple relay based transfer switch is pretty handy:


    Understand that your golf cart batteries will need monthly check of water levels (unless you go with AGM--More expensive) and they will probably need to be replaced every 3-5 years.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LewBailieLewBailie Registered Users Posts: 4
    My17500 Generac does run everything in the house nicely, just looking for something that would run things in the house with out gas in an emergency.I can only store so much gasoline.

  • BB.BB. Super Moderators, Administrators Posts: 30,050 admin
    edited September 2016 #8
    That is the problem with any genset--If you have natural gas available, perhaps that would be an option. If you have propane, a large tank (or two) has been used by some folks here.

    But--It really depends on how much power you need. Solar off grid power is not cheap--And, very expensive initial costs (batteries, panels, electronics, labor). Where as generator installation costs tend to be relatively cheap, and it is the fuel costs (spread, hopefully over many years) that add up over time.

    In the end, any off grid system will be most cost effective if your needs (with lots of conservation) match your solution--I.e., do not oversize your system.

    A small system that can run an off grid, very energy efficient, home is around 3.3 kWH per day or 100 kWH per month (most people have 500-1,000 kWH per month power bills, or even higher if AC and electric heat/hot water/cooking). A 100 kWH per month system will cost something on the order of $10,000 to $20,000 to install (don't quote me--Just trying to get a quick understanding of how much this system is worth to you). More or less, that "free" solar power usually costs on the order of $1 to $2+ per kWH (with lots of effort, it is possible to get off grid solar down to ~$0.50 per kWH)--Or roughly 10x the cost of grid supplied power. And that is if the system is used >9 months a year.

    For emergency backup power--Off grid solar systems are not usually cost efficient. You can install a hybrid solar system (Grid Tied + Off Grid capable)--But, again, that is about the same costs as a pure off grid system (plus, you need to coordinate with your utility and building department, usually,  to install GT connected solar power).

    Without knowing your power needs--It is difficult for me to give you better answers from across the continent. Just to give you an idea of the comparison between a large battery bank and gasoline backup power...

    Here is a (pretty massive) 24 volt fork lift type battery (for not a large off grid system--You may use 2 or more of these batteries for a larger off grid system):

    Crown Industrial Battery - 24 Volts, 850 Amp-hours
    • Dimensions: 38.13"L x 13.44"W x 23.25"H
    • Weight: 1380 pounds
    How much energy does that store? Typically we recommend 50% discharge over two days of "no sun":
    • 850 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 4,335 WH per day
    • 4.335 kWH per day * 30 days per month = 130 kWH per month (enough power for a small off grid home/highly energy efficient)
    In terms of gasoline... A Honda eu2000i will run about 9 hours per 1 gallon of gasoline at 1/4 load:
    • 1,600 Watt rating * 1/4 load * 9 hours per 1 gallon of gas = 3,600 WH per gallon
    And that 1,380 lb battery will provide (per day) the energy in roughly:
    • 4,335 WH per day (battery) / 3,600 WH per gallon = 1.2 gallons of gasoline per day equivalent
    A solar array to support that battery bank (10% rate of charge recommended minimum for full time off grid system):
    • 850 AH * 29 volts charging * 1/0.77 charge controller+panel deratings * 0.10 rate of charge =  3,201 Watt array nominal
    And a roughly maximum rated AC inverter @ 24 volts of ~ 4.25 kWatt maximum (4,250 Watts) output (recommend a minimum of ~1,200-1,500 Watt AC inverter if you want to run your refrigerator, larger if you have a well pump).

    If you run your loads roughly 10 hours per day (5 hours in morning and 5 hours at night):
    • 4,335 WH per day / 10 hours =~433.5 Watt average load during those times
    So--A $1,000 Honda eu2000i + 12 gallons of fuel (~$36 of fuel plus fuel stabilizer, changed every 6-12 months) or $10,000 to $20,000 for an "equivalent" off grid solar power system (plus monthly maintenance/distilled water addition to battery bank).

    Since I do not know your needs--I really cannot suggest either of the above systems as "best for you"--Only you know your power needs (and ability to store fuel, etc.). Energy usage is a highly personal and complex set of decisions. What I may do will probably not work for you.

    To better understand your energy usage--A Kill-a-Watt type energy meter which you can plug in any of your small 120 VAC loads (refrigerator, washing machine computer, cell phone charger, etc.) and measure those loads will help. If you have larger loads (well pump, HVAC, electric stove, etc.), a Whole House energy monitor can be very helpful. And even just looking at your energy bill and saying what could you live on for 10 days at 1/10th of your average usage may give you some insight.

    Any more than 10 days may run into problems with your city services... What happens if there is no electricity in your region--No water pumping for town water, no sewage pumping (some places have turned off water to stop sewage overflows), etc...

    Having enough to last 3-10 days is much different than trying to survive months+ in a large metropolitan region. A genset+3x 5 gallon fuel cans is much easier to move around than a fixed off grid emergency system with ~1,300 lbs of batteries and 3kWatt solar array (10-20 relatively large solar panels).


    PS: Comparing a 17 kWH genset against a 1.6 kWatt genst... 16 gallons of fuel for 10 hours (at 1/2 power or 8.5 kWatts or less) vs 1.2 gallons of fuel for 10 hours at 400 Watts (1/4 load).

    A 1/2 day of big genset fuel will give you 24 hours per day on the small genset:
    • 16 gallons * 9 hours per day per gallon with small genset * 1/24 hours per day = 6 days of "enough power" to keep food and sanity around the home (plus a few quarts of motor oil to change every 25 hours).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,611 ✭✭✭✭✭
    If you add batteries (with your solar) you NEED to keep the generator to charge the batteries when the solar cannot (big storm, blew lawn chairs into all your PV panels, or glazed with 4" of ice, or 4 cloudy days of storm......)
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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