does it matter when i use most power?

kellskells Registered Users Posts: 1
Hi All,
i live in spain and am of grid and installing a solar panel system. I am told ,that to calculate the number of panels and batteries needed, I need the to predict the total Kwh i need per day. Is it not important when the more 'thirsty' electrical items are used?


  • BB.BB. Super Moderators, Administrators Posts: 30,048 admin
    It depends...

    If you assume that, for example, you want to store enough energy for 2-3 "dark days" (bad weather) and 50% maximum discharge to run your daily loads (refrigerator, computer, office equipment, etc.), then it does not really matter if the energy is used during the day or the night.

    If you assume that your larger loads during the daytime are ones that can avoid using during bad weather (water pumping, vacuum cleaning, running washing machine, shop tools, etc.). then you can have a smaller battery bank (maybe 1 day with 50% max discharge).

    In the end, it is difficult to make for a optimally designed off grid power system... It will generally produce more energy that you need most of the year--Just to provide enough power for winter/poor weather (no sun, no power).

    In a hot climate, if you want to run an air conditioning system during summer, then you loads are somewhat connected to how much sun you will get (hot weather, lots of sun, lots of A/C).

    If you have a major load, such as pumping water, then perhaps you get a water pump optimized to run directly from solar panels, and pump to a cistern/tank when the sun is up (use water tank to "store energy" vs a battery bank). And, if needed, use a small pump to pressurize the water for your home.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,105 ✭✭✭✭
    kells said:
    Hi All,
    i live in spain and am of grid and installing a solar panel system. I am told ,that to calculate the number of panels and batteries needed, I need the to predict the total Kwh i need per day. Is it not important when the more 'thirsty' electrical items are used?

    As Bill has eluded to, it certainly does. Many solar calculators don't take into account seasonal variations. My energy use matches my available energy quite well. I air condition during the summer, and use wood heat during the cloudy winter.

    While I might use 12-15 kwh's of energy during the summer, a calculator might assume I need that during the winter and oversize my system many times over. In the winter, I run around 2 kwh's a day.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
  • jonrjonr Solar Expert Posts: 1,360 ✭✭✭✭
    edited February 2018 #4
    When (as in daily) effects wear on the battery.  Given a choice, use power while the sun shines (which also provides an efficiency gain).  If that's not possible, then the early morning hours  (to minimize the time the batteries are discharged).  Worst is around sunset.

    I am available for custom hardware/firmware development

  • NANOcontrolNANOcontrol Registered Users Posts: 169 ✭✭✭
    I have another take which may or may not be your situation. I use power real time when it is generated and have more panels than would be used in most systems and a MUCH smaller battery. Laundry and dish washing are done mid day. When it isn't sunny, I delay things to another day. It is a lifestyle change, but you aren't investing in batteries which will just go bad.
  • softdownsoftdown Solar Expert Posts: 2,912 ✭✭✭✭
    As others have said, use your panels....not your batteries. Try to use the major energy users before in the morning, after the sun is hitting the panels, to early afternoon. 
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
  • mcgivormcgivor Solar Expert Posts: 3,274 ✭✭✭✭✭
    edited February 2018 #7
    There is also a learning curve I'm sure all/most have experienced over time, the details which could never be accurately accounted for in a calculation. Seasonal variations are the biggest challenge as timing when to use excess power is a moving target, albeit a slow moving one, it takes at least a year to understand how the system will perform, naturally weather is unpredictable so having a backup generator is useful. At first I waited until float was reached before doing laundry, which was usually after 13:00h, later I began after the first hour of absorption ~11:30h  because the battery requiments were well below array capacity, this is one example of learning ones system. In my case being 17°N means there's little difference throughout the year in terms of hours of sunlight, the further north or south of the equator the bigger the difference, so adjustments to schedules wI'll be correspondingly greater. The single most important important load is a refrigerator, which may account for 50% of total useage ( never underestimate the refrigerator ). After charging loads obviously account for most of battery capacity so minimizing is crucial, the addition of a TV for example raised my consumption by ~0.5kWh per day according to monthly logs, over the years I did without. Off grid  living  requires a conservative mindset, unless of course money is no object, then anything is possible.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • mike95490mike95490 Solar Expert Posts: 8,610 ✭✭✭✭✭
    I have a timer set to start pumping water after my batteries have been in absorb for a while.  When they first hit absorb, they are drawing a lot of power, but after an hour, the amps go down, and then gear can run directly off solar and not cycle the batteries.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

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