Home Backup System Idea

I was considering installation of a whole house propane-powered generator, however, I have heard that these cost a lot to run during extended power outages. I am wondering if I should install a big battery bank that is grid-charged and will assume control when the grid power goes down and then (assuming the batteries would only power the house for 2 days) install a propane generator that only runs to recharge the batteries and then shuts down. It seems that this would be much more fuel efficient and would be able to support longer outages on our propane supply. Hoping someone that knows more then me can comment. I attached a picture showing what I am thinking. Thanks in advance for any feedback you can provide.

I think you would be better off running the house on your batteries until they go dead and then using the generator to power the house directly, rather than using it to charge your batteries. Any time you convert DC to AC there is a loss, and a loss when you convert back. If you use the generator to charge batteries, you convert from AC to DC for the batteries, and then back to AC for the house.

Yes, very doable, and I myself have most of the components already, though my system is totally off grid.

First and foremost, you MUST have a way to disconnect your system from the grid when a blackout occurs.  That's because your batteries could potentially backfeed power into the grid that's supposed to be dead, and put a repair lineman's life in danger.  The utilities take this very, very seriously, and basically you better have a system that is professionally installed by a certified electrician.  Typically, it's done with what's called a tranfer switch, or either/or breaker panel, which prevents the two from ever meeting.

What you need to do first is make an itemized list of what you want to power, what the peak power demands would be, and how many days you want the power to last.  Then it's just simple math to scale things to the size you want.

Let's say we take the example of my own cabin, where I consume about 3.0kWh (3000Wh) of power per day when nothing special is happening.  Let's plan on a two day power blackout, and have a 24V battery bank  that wont get depleted more than 50%.  That's because batteries that are totally drained die quickly.  A good rule of thumb is to scale everything 2X what you need.  Let's say the largest thing you want to run is a 1000W microwave.  

The math is (3000Wh X 2days X 2times scaling)/24V battery = 500Ah battery.  Not too big.  This level of power would be provided by two parallel strings of four 6V 250Ah Trojan T-105 batteries (140$ each).  You would need eight of them to make two parallel strings, which would be written as 4S2P.  You would need a generator that could charge at 1/8th of Amphour capacity (C) which works out to be 500Ah/8 = 62.5A.  For a generator you'd need 62.5A X 25V charging X 2fold margin = 3125W, so call that a 3500W generator.

The core of your system would be the inverter.  I have two, the Schneider XW+6848 (3100$) and the Conext 4024 (1500$).  Both have ACout to power your home, and both have ACin for accepting generator/grid power.  The more sophisticated 6848 has two ACin terminal sets, one for grid, and one for generator.  The smaller 4024 has just one set of ACin terminals, so you'll need a switch to select either grid or generator.  So, if you wanted to use the 48V 6848, you'd wire your eight batteries in a single series string to make a 48V battery instead of a 24V one for the 4024.

So, depending on how you want to configure things you are looking at least 3000-5000$ worth of hardware/batteries, and that does not even begin to include the electrician/inspection costs.  You are on your own here, because I never had to get any permits for my totally off-grid system, so I have no clue what those are going to cost you?

As mentioned above start with a generator back up panel. That is your first step whether you end up with a whole house generator OR an inverter battery system. I would suggest a manual transfer switch not an automatic. That is a permited and inspected item which I would suggest you have professionally done.

Thanks for your comments and solutions. My goal is not to save money on the system cost, it is to be able to continue living normally for long periods of power outages. We are at 3,800' elevation and occasional storms may drop 3'± of snow and disrupt grid power for weeks (from tress falling on power lines). A lot of neighbors complain that their generators eat through their propane reserves and the propane company can't keep up with the demand. So if I only run the propane-powered generator to recharge batteries and then shut it down I believe I would be able to run much longer (assuming that I avoid consumptive behaviors). Main uses for power during an outage would be for the well pump, the propane heater fan, and a few LED lights. The drawing shows "DC" which is the disconnect to the power company so my system won't back-feed to the grid.    

Jeffrey Ross wrote:Thanks for your comments and solutions. My goal is not to save money on the system cost, it is to be able to continue living normally for long periods of power outages. We are at 3,800' elevation and occasional storms may drop 3'± of snow and disrupt grid power for weeks (from tress falling on power lines). A lot of neighbors complain that their generators eat through their propane reserves and the propane company can't keep up with the demand. So if I only run the propane-powered generator to recharge batteries and then shut it down I believe I would be able to run much longer (assuming that I avoid consumptive behaviors). Main uses for power during an outage would be for the well pump, the propane heater fan, and a few LED lights. The drawing shows "DC" which is the disconnect to the power company so my system won't back-feed to the grid.    

That is a pretty short list of loads, a good fit for your described plan. You're right that you should get many more days from your propane; if the generator only consumed propane to output power, the conversion losses would get you, but of course it is going to use fuel the entire time it runs, even when only putting out 10w.

An extra large pressure tank can help shift the load of pumping water to when the generator is on.