Watt hours and battery basics

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
Hello all,

Please forgive my basic questions but I am new to this game. I am expanding my solar capabilities so I have a lot to learn.
Presently, I have a 60 watt, 51 aH "plug-and-play" system from Sun Force. Not much of a learning curve there. I greatly want to expand my system as my place is totally off grid.
In my estimations, my total usage will be about 1.5 kW/day. So, about 45 kW/month. The way I figure, if I had four 100 watt panels and calculate 4 hours of prime sunshine per day that should give me 1.6 kW/day for a total of 4.8 kW/month. Is this correct?
Is it better to buy the same type and wattage of solar panels from the same company? So, all panels are rated at 100 watts or 130 watts or whatever. Or can you actually mix up wattage ratings. Meaning, can I tie my present 15 watt panels to panels with larger wattage panels? Or does it really depend on the amps or each panel?
Battery bank question: I've done my reading but I am still confused about how to figure the appropriate size of battery bank. I tend to think simply. If I draw 5 amps/hr for 8 hrs then that's 40 amp/hrs. Factoring inverter efficiency and not wanting to over discharge the batteries I would figure an 80 amp/hr system.
Is my thinking correct or am I still confused?

Thank you so much,
Patrick
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Watt hours and battery basics

    Okay, here we go! (As Toad would say. :D )

    Off-grid systems are battery based. Do not expect to correlate PV Watts with power consumption/availability. It works like this (basically):

    'X' Watt hours per day / system Voltage = Amp hours per day.
    Battery bank has to be at least twice this size to minimize the Depth of Discharge to the 50% maximum.
    So 1.5 kW hours per day on a 12 Volt system = 125 Amp hours. Times two equals a 250 Amp hour battery bank. (None of this includes system losses or power consumption of the inverter).

    Then you have to recharge that battery bank. That means looking at two factors: peak potential charge current (should be 5%-13% of the battery bank Amp hour rating - it's a good idea to shoot for 10%) and daily Watt hours harvested.

    So for this theoretical battery bank you want to achieve 25 Amp hours at the charging Voltage of 14.2 or 355 Watts. Since panels generally put out about 80% of their "nameplate" rating and charge controllers/wiring uses a bit more it's a good idea to add a derating factor of 77%, hence a 461 Watt array (or thereabout).

    If we check the Watt hour harvest for that array you get 355 Watts * number of hours of "equivalent good sun" which is usually 4, so 1.420 Watt hours per day. That's pretty close for starters. That's when you need to get into specific calculations like expected insolation for your location and losses specific to chosen equipment & wiring.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
    Re: Watt hours and battery basics

    In addition to what Marc says,

    The good rule of thumb for off grid, is name plate rating/2 to account for all cumulative system loses, multiplied by 4 to represent four hours of good sun one can reasonably expect per day, on average over the course of the year.

    400/2=200*4= 800 watt hours.

    As a point of reference, we live off grid with 400 watts of PV, and we generally use ~ 5-800 watt hour per day, using 450 ah of battery.

    Tony
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Watt hours and battery basics

    don't forget that the 4hrs average used for the sun does not mean that on a cloudy week that all of your loads will be fine. that power is delivered to the batteries only when the sun will be out and the time and intensity varies also from month to month due to the seasons in addition to the weather. for those times we recommend another backup source, but if your goal is only to have solar power these items, then you will have to figure for the longest timeperiod you'd be without and plan for the battery capacity needed without going below 50% dod and then the amount of pvs to keep the batteries properly charged. it gets big and expensive quickly.
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  • System2
    System2 Posts: 6,290 admin
    #5
    Re: Watt hours and battery basics

    Thank you all so much for explaining things in plain terms that I can understand. As I think about this I know I will have other questions. Have a great weekend.

    Patrick
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