How much generation will I need....

fordraceboyfordraceboy Registered Users Posts: 12
Quick background--

Off-grid home, no power available and a small genset to help charge batteries.

Battery pack will be 12 T105RE batteries. That's roughly 9kWh available based on a 50% DOD. Looking at a small freezer and a fridge/freezer both, and then minimal LED or CFC lights. The appliances draw right at 1kWh each according to the almighty Kill-a-Watt meter. Lighting would be minimal usage, and minimal draw- I haven't ran a test on them for draw and consumption yet.

I figure to be needing a charge on the batteries every 3-4 days to stay above the 50% mark. My question is this-

How much input into the batteries will I need? I start looking at watt output and then battery drawdown and all that stuff and then get lost in my own mind. It's in Oregon, so solar is spotty at best, so I'm wondering if small wind will keep up? I've got decent windspeed on the property (it sits along a ridgeline) but have not measures overall and average speed at hub height. Not sure of the best way to do this, as I don't wanna buy a tower if wind isn't feasable.

Generator is an option of course, but thats not real handy when I'm not around...

Essentially, how many watts do I need to produce? How do I measure windspeed at hub height without a tower?


  • Ralph DayRalph Day Solar Expert Posts: 902 ✭✭✭✭
    Re: How much generation will I need....

    Wind isn't feasible without a tower.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How much generation will I need....

    Welcome to the forum.

    You've got a problem or two to be sure. For example twelve T105 batteries. Configured for what Voltage? If it's 12 Volts that's six parallel strings: a disaster waiting to happen. If it's 24 Volts that's three parallel strings: much better but not without potential failure even if wired perfectly.

    This brings up the matter of peak charge current. Let's it's 24 Volts with three parallel strings. That's 3 * 225 Amp hours or 875 total. Right away another problem: ideally you'd have 87.5 peak Amps of charge current and that would push the limit of an 80 Amp MPPT controller. You'd need about 2727 Watts to drive it, whether solar or wind.

    So the first thing I would do is re-evaluate the load requirement and see if that battery capacity could go down. 3-4 days of autonomy is not desirable, no matter what you may have heard. It is much better to plan on an average 25% DOD daily, and have the generator to recharge on day 3 if the sun still refuses to shine.

    If you could do wind this would be easier. But as has been said and proven repeatedly on this forum small wind is not that easy, cheap, or reliable. Mind you some would argue that a 3kW turbine isn't small, and it isn't; that is what you'd be looking at for wind power alone.

    Height, clear air, consistent speed around 20 mph. Then if you have a good turbine and a good tower and good luck it works.

    The only good news I can give you at this point is that when you're not around the power usage should go way down (especially if you can shut all loads off) and thus it's much easier to maintain a charge in those batteries. I leave mine for half a year and when I come back they're fine.

    Again I think you should revisit the load requirements first.
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