Newbie Shed Project

Filetmignon
Filetmignon Registered Users Posts: 2
Fixing up an old shed in the back yard and want to off-grid solar supply and use as a small workshop. Thinking just a few low-watt leds and a small radio to start. What amp/hr battery would you think for just light, weekend usage? 400w inverter? Thanks all.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Newbie Shed Project

    To have a "successful" off grid solar power installation--You really need an idea / target regarding your power usage.

    We can do it other ways too--For example, if you want to use a 400 Watt inverter to its maximum capacity (400 watts), we can design a battery bank and solar array around the requirement. But if your needs are 20 watts of lighting and a 10 watt cell phone/battery charger--then that would be over kill.

    Since you said 400 Watt inverter--Some really quick numbers that will give you an idea of how it works together (using some very quick rules of thumbs).
    • 400 watt inverter needs about 160 AH @ 12 volt battery bank to power the inverter at full rating.
    A 160 AH battery bank will need around 5% to 13% rate of charge:
    • 160 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 151 Watt array minimum
    • 160 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 301 Watt array nominal
    • 160 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 392 Watt array "cost effective" maximum
    If you have two days of storage (no sun) and 50% maximum discharge, the battery bank will supply:
    • 160 AH * 12 volts * 0.85 inverter eff * 1/2 days storage * 0.50 max discharge = 401 Watt*Hours
    Assuming you get a minimum of 4 hours of full sun per day (on average) for ~9 months of the year, then to supply a minimum of 401 WH per day:
    • 401 WH * 1/0.52 system efficiency * 1/4 hours of sun = 193 Watt Array minimum (based on loads)
    Note, if you draw 400 Watts from your inverter, you will use your daily power allotment in ~1 hour.

    Personally, I would probably suggest a pair of 6 volt @ ~220 AH golf cart batteries and a very nice MorningStar 300 Watt 12 volt TSW inverter.

    And up size the solar array and re-run the rest of the calculations to see if that give you better mix of output power and stored energy (less peak power, but more power for more hours)... So, since I am here, rerunning the calcs on my suggestion:

    A 220 AH @ 12 volt battery bank (2 golf cart batteries) will need around 5% to 13% rate of charge:
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 207 Watt array minimum
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 414 Watt array nominal
    • 220 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 539 Watt array "cost effective" maximum
    If you have two days of storage (no sun) and 50% maximum discharge, the battery bank will supply:
    • 220 AH * 12 volts * 0.85 inverter eff * 1/2 days storage * 0.50 max discharge = 561 Watt*Hours
    Assuming you get a minimum of 4 hours of full sun per day (on average) for ~9 months of the year, then to supply a minimum of 561 WH per day:
    • 561 WH * 1/0.52 system efficiency * 1/4 hours of sun = 270 Watt Array minimum (based on loads)
    If you want the minimum solar array size--You can go with 207 Watt array. If you want to meet your daily "optimum" battery loading, 270 Watt array (based on minimum of 4 hours of sun per day).

    However, if you want extra power on sunny days, then 414-539 Watt array would be very nice--In fact, such an array would supply daily usage (or more) for ~9 months of the year.

    So--It all depends on your needs. You can play with battery size, inverter size, and array size. I suggested the golf cart batteries because they are cheap and rugged/forgiving (just remember to check/add distilled water every month +/-). The Morningstar inverter is a real "tank" of an inverter, and for a small 12 volt inverter, has remote on/off and "search mode" (low power usage until >6 watt load is plugged in--does a little battery saving automation).

    Your questions/comments?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Filetmignon
    Filetmignon Registered Users Posts: 2
    Re: Newbie Shed Project

    Appreciate the response and helpful info, Bill. Thanks. I'm brand new to solar and this is an experiment to take the first steps. Didn't expect to need larger batteries but your calculations make sense, even just to supply a couple hundred watts. Thinking i might skip the inverter for now and start with leds. I could probably get good enough lighting for <100 watts.
    Would also love to hear any pointers/suggestions on battery to led arrangements.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Newbie Shed Project

    System design really not only depends on "peak load" (watts), it also needs to address how many hours a day you want to run the loads.

    For example, a microwave running 1,500 Watts but only 15 minutes a day:

    1,500 Watts * 1/4 hour per day = 375 Watt*Hours

    And a little laptop running 30 watts for 12 hours per day:

    30 Watts * 12 hours = 360 Watt*Hours

    The microwave will drive the minimum size of the battery bank and AC inverter--But both examples above will draw about the same amount of power over a 24 hour period and need about the same size solar array (and also the size of the battery bank to run the loads for 1-2 days with 50% maximum discharge).

    You don't know what will really drive a system design (peak power, or Watt*Hours per day) for the battery bank until you "do the math".

    Similarly, the sizing of the solar array is based on 5%-13%+ rate of charge for the battery bank and Watt*Hours used per day--Either could end up being the driving factor for the array.

    If the shed is relatively near the house--Wait until the ground gets nice and wet. Dig a 18"+ deep trench and put a large diameter conduit (or two) in the ground between the house and the shed. Pull 120/240 AC wire, phone/network cables (different conduit) to the shed. In the end--It will probably be cheaper and more useful vs a dedicated solar power system.

    Don't get me wrong, having solar will be a nice experiment--But if you need a fair amount of power at the shed for projects--A day wiring up (more or less) permanent AC power to the shed will be a worth while project (of course, there are building codes to review and local laws which may throw a monkey wrench in the process regarding AC power and outbuildings--If you care).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset