I inherited 16 x 100Ah 12V batteries and all the interconnect cabling from a decomissioned IT back-up power solution that we had running at work. The batteries are all in good shape and are rated for PV use. The intent was to have a workshop across the street on an adjacent property to my house. There are no electrical services there now and to establish electrical services would be too much money for my simple use. I want to do a low cost alternative off-grid solution, less than $2000.
I assumed that the 400watt panels would essentially trickle charge the battery bank when not in use, and the generator would supplement the rest of the charging when there was actual heavy battery draw. Leaving the PV panels aside for now, I was assuming I could buy a cheap generator like the Champion Model #76555 3000W GENERATOR; then get an AGS like the Xantrex’ Automatic Generator Start (part#84-2064-00) to start the generator automatically when the battery volt readout was low and stopped the generator when the readout was OK; and finally get a Inverter / Charger like the Nature Power 38326 Pure Sine Wave Inverter with 55-Amp Charger, 2000-watt to hook into the panels and generator. The theory is that this would give me a cheap solution for off-grid power and up to 5 hours of battery power doing standard workshop duties without draining the batteries more than 25% of their capacity. As soon as I reached the low battery threshold the Generator would kick in and recharge/supplement. Alternately, if I was only occasionally using lights and draining the batteries slightly before shutting the workshop off the PV system would trickle charge and replace the lost charge. Therefore, I assumed that I don’t need a lot of panel wattage to keep the system at a good state of charge. I don’t need 240VAC and I won’t exceed a 15amp circuit at any time. PV Panels will go on a separate shingled wood shed near by with full southern exposure (Can be built in any direction needed). I am assuming roof mounted panels.
What do you recommend and what components am I still missing? Is my logic sound?
Cheap generators have (very approximately) a useful lifespan of 700-1200 hours. They are all designed for intermittent use. If you use it every day, 6 hours per day, that's only 4-7 months of use before it craps out.
Also, while they have improved as a group, cheap generators tend to be pretty loud, and dB ratings are often a little optimistic.
The less expensive generators are often less fuel efficient, and fuel costs can quickly outstrip grid utility prices, so make sure you do the math. The "free" batteries may end up cost more.
I totally get the awesome feeling of being independent of the grid, so that's worth something too.
Diesel generators tend to be more durable than gas, and diesel is a lot less flammable for what that's worth...
1800 rpm gennies are almost universally quieter and much longer lasting than 3600 rpm generators, and more expensive...
Even with this, in the long run most reliable offgrid systems will cost more than a grid connection. Not a universal truth, but something needs to be out of the ordinary for it not to be true. I think your logic is generally sound, there are just a lot of little complications and gotchas. This list is probably not comprehensive.
On top of what Troy has already said, a diesel generator has the bonus that the fuel is less likely to go stale. You could also look into propane. I just hate gasoline, TBH.
How was the backup system set up for voltage? If it was a high voltage system (a pair of 96V banks, for example) your interconnect wiring might not be up to the strain of a 12V system. Something to check.
A 55A charger puts out 660W; given charge inefficiencies, less will actually go in to the bank. Meanwhile you'll take more out of the bank than nominal ratings due to inverter inefficiency. It will take quite a while to recharge these batteries at that rate; depending on your usage patterns, you might end up running the generator for longer to recharge the batteries, than if you simply ran your tools off the generator and forewent the battery bank!
To get good life and service from your batteries, which I am just blithely assuming are lead-acid of some sort, charging has to be done right. I would think a bank this size would benefit from a charger more powerful than 55A. I would be wanting a charger capable of doing an equalize charge, and intelligent enough to periodically provide absorption voltage while the battery is sitting in 'float' after being fully charged, to prevent stratification. However, I'm far from an expert on lead-acid; I just don't like em much.
The missing piece from your list is a charge controller for the solar panels, that inverter/charger doesn't cover that. Since you'd then be charging from 2 separate sources you need to figure out how they are going to play together; if you depend on the generator powered charger for bulk charging and equalization, You still need the solar charger to be reasonably smart in order to keep the batteries healthy long term. You don't need a lot of solar to provide a maintenance charge, but to provide absorption voltage periodically it needs to be a decent size, with a good controller. Maybe more trouble than it's worth with free batteries.
That inverter-charger has 3 reviews, between Amazon.com and amazon.ca; all 3 report rather immediate failure. Not exactly glowing endorsements... Note that a 2000W inverter won't necessarily run all 15A(1800W) devices; oversizing is your friend.
Think carefully about your power requirements, are you *sure* you won't want to exceed 15A? I currently have a 'workshop' (20' cargo container) that relies on a buried 15A feed from a house ~100M away, plus a 100' 10-gauge extension cord to bridge the last gap. It's a limiting amount of power; there is significant voltage sag if I try to run a spaceheater as a test load; on max the '1800W' heater is getting voltages below 100V and falling as the wires overheat, and pulling perhaps 1450W instead of the 1800 that it should. I can run almost any tool I need to, though I can't buy the most powerful versions, and a welder or high-power compressor are out. However, I don't have enough juice to run a powerful blower or vacuum for dust control while working with my tablesaw or shop saw. A bit annoying. I'd like an 5000W phoenix-victron inverter-charger and a battery bank to suit, but I'm definitely not willing to spend the money just now!
Yeesh, that got long, and a bit discouraging, sorry! Doesn't mean it's not workable, or worth doing, just a lot of things to consider.
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