Batteries versus gas generator for home backup

eoleniceolenic Registered Users Posts: 2
I have been using the 12V battery in my Prius (used only for cabin electronics and not starting) with a 1kW pure sine wave inverter, and heavy-duty extension cords to backup a refrigerator, a computer and some lights during power outages, which we have several times a year.  I know that putting the 12V through extra cycles will shorten its lifetime, but I've been using this system for 10 years without incident. 

As I have aged, the extension cords get harder to drag, I am considering buying two 200ah deep cycle agm batteries (130 pounds each) connected in parallel, a 2kW inverter, and a battery charger/tender so I don't have to drag extension cords outside and also to go easier on the Prius battery.  The nice thing about the Prius is that the gas engine only runs for a minute or so every 15 minutes in order to charge the 12V, so I can go several days using this backup system on a full tank (10 gallons) in the car.  I estimate that the battery system outlined above would power my frig, a TV, a modem, a Verizon set top box, a laptop and several lights for almost 3 days.  I have a 9240kW grid-connected solar system.  Batteries would be independent of that system, for now.

My questions (at last) are:

1.  Since the battery backup system would cost in the neighborhood of $1500 or more, should I simply continue using the Prius, or perhaps replace the Prius 12V with a deep cycle battery?
2.  Would it make more sense to get a gas generator, which might cost $500-$1000, need to be run continuously during an outage, make lots of noise, be kept outside and require intensive maintenance both the generator engine and the gasoline?
3.  From what I have been reading in this blog, $300 200ah batteries may not be as good as they claim.  Are more expensive batteries better?
4.  I also have questions about which inverters and charger/maintainers to use.


  • BB.BB. Super Moderators, Administrators Posts: 32,329 admin
    Welcome to the forum Eolenic,

    If you are happy with the Prius--I am not sure you can do much better. The 12 volt battery as an consumable (replace every 3 or so years), is probably the cheapest out of pocket solution--Given that your Prius system is already working. As I understand, the Traction battery powers an AC inverter that keeps the 12 volt battery charged--So as long as there is sufficient energy in the traction battery, the 12 volt battery probably only handles the short term surge current and gets stabilized by the TB.

    Here is an older thread about the details of using a Prius as a genset:

    A battery is just storage--So you would need solar or genset or something else to recharge the battery every day.

    A place to start... Get a Kill-a-Watt meter and figure out how much energy you use in a day:

    If your 1 kWatt sine wave inverter is running the refrigerator--That is pretty good. Nominally a 1,000 Watt inverter (and smallish 12 volt battery) are not heavy enough to start a full size refrigerator/freezer compressor. If this is a backup/smaller refrigerator and is working--That is neat.

    More or less, we can do some estimates with what you have typed so far... Let say you are using 1,000 WH (1 kWH) for the fridge, and 500 WH for everything else for 1,500 WH per day.

    And lets guess that the Prius has a 1.3 kWH Tracsion Battery (not the ePrius). And uses around 50% of battery capacity, and you run the engine every 15 minutes... If the engine can charge at a 1,000 Watt rate, it should be producing:
    • 4 minutes * 1/60 minutes per hour * 24 hours per day * 1,000 Watts = 1.6 kWH per day
    So, that sounds pretty close for a first pass. Lets say you want the new off grid battery bank to store enough energy for 2 days, and 50% maximum discharge. The 12 volt battery bank would be:
    • 1,500 WH per day (@120 VAC) * 1/0.85 AC inverter eff * 1/0.90 AGM battery eff * 1/12 volt battery * 2 day storage * 1/0.50 max discharge (for longer life) = 654 AH @ 12 volt battery bank
    If you wanted to only have 1 day storage, then 1/2 that size battery bank (and 50% max discharge) = 327 AH @ 12 volt AGM bank.

    Your proposed 400 AH @ 12 volt battery bank is in that range. And a 400 AH @ 12 volt AGM battery bank should support your 1,000 Watt inverter nicely (no larger).

    Now fuel wise, a relativley efficient gasoline genset will run around 3,000 to 4,000 WH at 50%+ load per gallon of gasoline. An inverter-genset like the Honda eu2000i will run roughly:
    • 1,600 Watts * 0.25 load * ~9 hour runtime * 1/1 gallon of gasoline = 3,600 WH per 1 gallon of fuel
    Now this raises the question of how much fuel per day you use... You say a couple days, or 5 gallons per day. Others have said they run ~5 days on a tank of gas *10 gallons or 2 gallons per day.

    A Honda eu2000i will run about 18 hours on 2 gallons of gas with your loads (estimate). For nights, you could run the lights and TV at night for 8-12 hours (overnight) and just run the genset during the day (and recharge the battery bank).

    I don't think you are "over cycling" either the traction battery or the 12 volt vehicle chassis battery... Just stop and go driving probably cycles the TB more often during a couple hour drive than your off grid loads.

    Use a smaller battery bank and AC inverter (like a 300 Watt TSW inverter) for the evening/nighttime loads and the genset/Prius for daytime loads (and refrigerator).

    If you do have a second battery bank (400 AH or even smaller for nighttime only use), you will have to get an AC Battery Charger (typically 10% to 20% of battery bank capacity--I.e., 20% * 200 AH = 40 Amp battery charger). good quality chargers are not cheap, and a 30-50 Amp @ 12 volt battery charger would be about max for an eu2000i or your 1,000 Watt Prius inverter system (more work needed to figure out what will work for your needs). Large Battery Banks take a lot of current to charge--Larger genset, larger AC charger, larger AC inverter if used, larger solar array).

    Anyway--Just some thoughts about possible options.

    My 2 cents are that a genset is probably a better solution for usage a handful of times a year for a day or so at a time. A smaller nighttime off grid battery system may be good for your needs. And a small/quiet inverter-generator is not a bad solution.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,513 ✭✭✭✭✭
    If you have been happy with the setup for 10 years, I say stick with it.   Try dragging a generator around, it's lots heavier than a cord.
    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

    gen: ,

  • eoleniceolenic Registered Users Posts: 2
    Thanks to all for the helpful comments.  I'll stick with Prius power.
Sign In or Register to comment.