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backup battery power system help needed for Sonoma cabin

ribof

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Jun 26, 2021
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Like everyone else in california these days we need a backup power option for fire-danger days when PG&E shuts off the power. I'd like to spend as little as possible since these power shutoffs don't happen too often. At a minimum I'd like to be able to run our 3/4hp well pump (240v), fridge, router and a few lights. We don't have AC and we can live without the electric hot water heater and stove for a few days.

I'm imagining putting together a system like this:

lithium batteries, maybe like two of these 3840WH options: https://www.amazon.com/CHINS-LiFePO4-Battery-2000-5000-Off-Grid/dp/B08N53GF81

A 240V split phase inverter charger (which one? I assume it needs to be UL listed?)

Some type of transfer panel (auto or manual?)

When the power's out we'd love to be able to recharge the batteries via solar but we're deep in a valley of redwoods and get little sun anywhere. Maybe in the future it'll make sense to try to put in solar. So for now instead I was thinking of hooking up a 1000w inverter to our car and use it as a generator to recharge the batteries (I've done this before in a pinch to power the fridge and it works). It never gets below 32 degrees at the cabin so we don't have to worry about low temperature charging.

What am I missing - does this system make sense? Any help would be appreciated!

I found this video of someone who set up a battery system to run his well pump so it looks like it is possible to do cheaply:
 
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Welcome to the forum.

The biggest concern is the Locked Rotor Amps (LRA) or surge current of your pump. They can easily be 5X the run current, and it's generally for a non-trivial period of time that most high frequency (lightweight cheap ones) inverters can't handle unless sized for.

Split phase inverters tend to be pretty expensive, particularly with low frequency inverters that actually have a meaningful surge capability (2-3X rated continuous for 20-30 seconds).

Step 1) figure out what you actually need to power your pump and for how long.
Step 2) list all other items you wish to power and for how long.
Step 3) Figure out how many Wh you actually need.

FWIW, the Power Jack inverters he showed are notorious for being junk.
 
No solar? my honest answer would be by a 24 volt or 48 volt low frequency inverter/charger in the 4-6kw range and then size a lithium battery bank and BMS and build your own. plenty of info on this forum and several "safe" vendors to choose from. to finish it off I would buy a 2-3kw inverter genset and use it to charge your batteries if the power stays off more than a couple of days in an emergency. They sell ones that you can power off of propane or natural gas so you do not have to worry about fuel spoilage and cleaning the carb etc.

the inverter charger would be setup for lithium charging (listen to the recommendations on this forum about usable range of voltages and charging voltages (3.0-3.4 usable range and max of 3.45 per cell). As many cells as needed to get the amount of storage you need. The inverter would charge the cells whenever power is present and then auto switch to inverter if power drops out. many units have their own internal relay switches but a manual or auto switched mini panel that powers only the emergency loads (your above mentioned water pump, fridge, power for laptops and cells, etc.) when power is present the better inverter chargers pass through power to power those loads, and use the excess to keep the batteries topped off. when the power drops, they then provide power from the batteries

good luck :)
 
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