How many batteries

ronpozoromo
ronpozoromo Registered Users Posts: 6
I am building a remote off grid cabin in wv(38 deg, 36'N/81 deg/63'W). When I retire I will live in it full time (about 5 years). I will use no more than 1000 watts/day. I am looking at 6 Kyocera 135 watt panels facing south and will be using a gas generator when needed. I would like to use Trojan 105's and mppt controller. My questions are how many batteries, what system voltage, and what inverter/charger would you recommend?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How many batteries

    Well if you're sure about the 1000 Watt hours per day ...

    That's "12 Volt" territory. 100 Amp hours @ 12 Volts would be 1200 Watt hours, roughly speaking. Along that line two of the T105's in series would provide 112 Amp hours for use @ 12 Volts based on 50% maximum DOD. If you want to keep the discharge low you could parallel another set; four batteries in total.

    Now what do you need to recharge that? Trojan recommends 10% @ 14.8 Volts. That's 22.5 Amps for one set, or about 432 Watt array. Double the batteries = double the current = double the array.

    What could you get from six Kyocera 135's?
    810 Watts @ 77% efficiency = 623 Watts / 14.8 = 42 Amps peak potential current. Frankly that would probably work with 450 Amp hours of battery.
    Also, that 623 Watts over 4 hours good sun is almost 2.5 kW hours which should more than meet your proposed needs even with system losses. If you apply the "Icarus Formula" you get 1246 Watt hours daily on the AC side. If this is to be an all DC system you might do even better.

    That said, with 450 Amp hours of 12 Volt battery you're right at the point where you might consider wiring for 24 Volt. And with 800+ Watts of array an MPPT controller is a good idea. You could run two strings of three panels each - high array Voltage to reduce V-drop.

    There's lots of equipment choices for this sort of set-up, depending on what you pick for system Voltage. Some of the choice would definitely be dependent on the budget.
  • ronpozoromo
    ronpozoromo Registered Users Posts: 6
    Re: How many batteries

    4 of the Trojan 105's is what I thought, but I wanted to be sure. I don't want overkill on the panels, but I wanted to be sure I had enough for cloudy days and use the generator as little as possible. The cabin is fairly primitive and I want to conserve-outhouse, rainwater catchment system with shallow pump & bladder tank, small chest freezer converted to fridge, a few cfl lights, and a laptop (3 g works on an Iphone there, so I don't think I'll need satelite internet.) I'll have no TV-no reception-I'd rather read anyway-my hilltop property has beautiful views and I'd rather be outside when I can. I'll run washing machine, vacumn, etc off my generator. Passive cooling, wood heat, propane range... With 4 of the Trojans though I'm not sure about 12v vs 24v though-whatcha think??
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How many batteries
    If you apply the "Icarus Formula"...
    What is the Icarus Formula? Does it involve the number of feathers per arm and the melting point of wax? :^)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How many batteries

    If you can get away with a 300 watt TSW inverter from MorningStar and around a 500-600 Watts maximum of panels with a Rogue 30 amp MPPT 12/24 volt controller, I would stick with 12 volts.

    If you want to stay with 12 volts, and the larger array (not a bad thing), then your next step up would be 45+ amp charge controllers. There are some PWM which are not expensive--but really only good if the solar panel to charge controller distance is not too far (10' - 20' or so maximum). If your distance is farther, you should look at MPPT type charge controllers (which are not cheap).

    If you need higher power (to run the freezer, you may need to start with a 1,500 watt inverter), then look at 24 volt inverters.

    At 24 volts, you can still use the Rogue MPPT charge controller for your planned array (30 amps at 24 volt battery bank is 2x the power vs 30 amps on a 12 volt battery bank).

    I would focus on finding the "best inverter" for your needs (12 or 24 volt, probably with a "search function and low remote shutdown" to save power). That will be the difficult part (finding/setting up the small chest freezer and running it efficiently from your system).

    If you can setup the inverter to turn on and off with the freezer (and other loads), that will save a bunch of power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How many batteries

    For comparative purposes, I have a 16 cubic foot standard refrigerator, plus computer/satellite/phone set up, plus 1/3 HP water pump, plus 1 HP digester pump, plus lights, plus microwave all running off 700 Watts of panel, an MX60 controller, 320 Amp hours of not-very-good batteries, and a VFX3524 inverter that is on all the time (constant 20 Watt draw). How do I do it? For one thing my 3200 feet of elevation gives me slightly higher than typical panel output. For another, I load-shift like crazy; if the power isn't there (fully charged batteries, midday sun) the pumps and microwave do not get turned on (large pressure tank can store about a day's worth of water). And if I do need the power and the sun doesn't co-operate, Honda EU2000i!

    Note this is not a year-round use; there simply isn't enough Winter sun here to do it and getting there in January would be more adventure than I ever want to undertake. Also I would not recommend this extremely minimalist system for "set and forget" operation; it requires a lot of watching, which I like to do. Even so, I'd like to increase both the array and battery bank as it operates a bit too close to the edge.

    The "Icarus Formula" is named for forum member Icarus, proponent of the simple yet effective calculation: nameplate rating of panels * hours of equivalent good sun / 2 = probably AC Watt hours available. It works, too. :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How many batteries

    And as a point of comparison--Marc's 20 watt running his inverter 24x7 is about the same power consumption that you will see for your chest fridge conversion:
    • 20 watts * 24 hours = 480 WH per day
    • Chest Freezer Conversion ~ 250-500 WH per day
    I believe one person here (Wayne from NS Canada?) has setup his inverter so the Inverter is only on when the conversion fridge is running.

    It is not that difficult to do--especially if you pick the "right" inverter. But it does take a little tinkering and such to make it happen. If you are comfortable with the usually home 120 VAC and 12 VDC car/boat wiring--it should not be a big deal.

    However, it will not be "cheap" to do it "right". A good quality TSW inverter with the search and remote inhibit options are pricey.

    You could do it backwoods style too... Get a 12/24 volt high current relay--and turn it on and off with a fridge thermostat. Connect the 12/24 volt relay output to your inverter DC input.

    Use a second, smaller TSW inverter to power the rest of your cabin 24x7 or connect a bypass to your relay to turn on your bigger TSW inverter when you need other 120 VAC power.

    Power conservation is a game of paying attention to details... Watt draw when inverter is running with no load, can the 1,200-1,500 watt inverter start the compressor (should be able).

    I think one person here changed the motor start capacitor on his conversion and used a MorningStar 300 watt (600 watt surge) 12 volt TSW inverter for his setup.

    And surge current is difficult to measure without paying $$$ for a meter that can do peak capture / inrush measurements for current.

    Poster 2manytoyz has a nice website with some examples of his testing:

    http://www.2manytoyz.com/yamaha2400.html

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ronpozoromo
    ronpozoromo Registered Users Posts: 6
    Re: How many batteries

    I've already figured I'd have to spend a bit to get a good inverter. Would'nt an inverter's sleep mode work with the fridge?
  • bsolar
    bsolar Solar Expert Posts: 103 ✭✭✭
    Re: How many batteries

    i'll jump in then shut up ... if you think slightly larger ( i wont attempt to give exact figures) but i would guess maybe 8 panels and 6 cell bank at your quoted power needs, you wont be worrying about the inverter staying 'on' , i dont .. like Cariboo said though, no matter what you wind up with high loads should be when the sun is cooperating .. and you mentioned 'vacuum' .. check out a makita cordless ;)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How many batteries

    For a simple chest freezer / refrigerator conversion--yes, an inverter with search mode should work fine (you may need a separate thermostat to turn the AC power on/off to chest freezer--many chest freezers thermostat do not go above freezing. Personally, I may also add a 120 VAC muffin fan plugged into the thermostat to stir the air when the compressor turns on if you have uneven temperatures in the chest fridge--cold bottom, warm near top).

    For an energy star fridge with self defrost and Internet Terminal in Door--probably not (auto defrost/energy star refrigerators need 24x7 power to run the defrost timers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ronpozoromo
    ronpozoromo Registered Users Posts: 6
    Re: How many batteries

    I've already planned to use my system wisely. I think the old saying "Make hay while the sun shines" really applies to off grid living. I'm frugal by nature, and I want a small but quality system. The stock market crash has made people in my age group (56) reevaluate their retirement plans. Reasons for living off grid is that my energy costs will be much less, I'll have no mortgage, and I'll be living in a beautiful place far from the insanity of modern society but within an hour of a good size city and 15 minutes of a small town. Of course there is another old saying, "Man plans, God laughs", but ya gotta try