Batteries versus gas generator for home backup

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:

https://forum.solar-electric.com/discussion/11546/using-a-prius-as-a-generator/p1

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:

https://www.amazon.com/s?k=kill+a+watt+meter

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.

-Bill