Off-Grid Solar System Design Step-by-Step.

Steps of Design

  • Defining our loads.
  • Sizing inverter.
  • Sizing panels.
  • Sizing batteries.
  • Sizing a charge controller.
  • Panel connection.

Step 1: Defining our Loads

Device NumberPower/device(w)Number of HoursEnergy (wh)
Lamp118 w472 wh
Fan160 w2120 wh
Refrigerator175 w12900 wh
Total153 w1092 wh/day

Step 2: Sizing Inverter

  • An inverter is used in the system where AC power output is needed.
  • The input rating of the inverter should never be lower than the total wattage of appliances.
  • The inverter must have the same nominal voltage as your battery.
  • For stand-alone systems, the inverter must be large enough to handle the total amount of watts you will be using at one time.
  • For grid-tie systems or grid-connected systems, the input rating of the inverter should be the same as the PV array rating to allow for safe and efficient operation.
  • The inverter power should be greater than the load’s total wattage by 25% or 30%.
  • The inverter’s continuous power = 1.3* total wattage = 1.3153 W = 198.9 W.
  • If the system has motors, compressors, refrigerators, pumps, and washing machines, we need to make sure that the inverter can withstand the starting current.
  • The surge power of these devices is found on the label of them.
  • If you don’t know, you can assume the surge power = 3x – 4x the wattage of these devices.
  • The inverter surge power = Lamp + Fan + 4 Refrigerators = 378 W.
  • We need an inverter with a continuous power of 198.9 W and a surge power of 378 W.

Step 2: Sizing Inverter

Step 3: Sizing Panels

  • The total energy required by the loads is 1092 Wh/day.
  • The total energy needed from panels = total energy * 1.3 (a safety factor to accumulate for all the losses in the PV system in addition to the panels not operating at the optimum conditions) = 1092 Wh * 1.3 = 1419.6 Wh.
  • The power of panels = π‘‡π‘œπ‘‘π‘Žπ‘™ π‘’π‘›π‘’π‘Ÿπ‘”π‘¦ 𝑛𝑒𝑒𝑑𝑒𝑑 π‘π‘’π‘Žπ‘˜ / 𝑠𝑒𝑛 β„Žπ‘œπ‘’π‘Ÿπ‘ 
  • = 1419.6π‘Šβ„Ž2 β„Žπ‘œπ‘’π‘Ÿπ‘  = 709.8W.
  • We will select Sunpower SPR-200-BLK-U.
  • Number of panels = π‘‡π‘œπ‘‘π‘Žπ‘™ π‘π‘œπ‘€π‘’π‘Ÿ / π‘π‘œπ‘€π‘’π‘Ÿ π‘œπ‘“ π‘œπ‘›π‘’ π‘π‘Žπ‘›π‘’π‘™
  • =709.8200=3.549 π‘œπ‘Ÿ π‘Žπ‘π‘π‘Ÿπ‘œπ‘₯π‘–π‘šπ‘Žπ‘‘π‘’π‘™π‘¦ 4 π‘π‘Žπ‘›π‘’π‘™π‘ .
  • The power of panels = 4 * 200 = 800 W.

Step 4: Sizing Batteries

  • Assuming the lowest temperature in the location is -20Β°C. Then the nominal capacity will be 160 Ah instead of 330 Ah.
  • The temperature correction coefficient = 160π΄β„Ž / 330π΄β„Ž=0.48.
  • Ah of batteries = π‘‡π‘œπ‘‘π‘Žπ‘™ π‘’π‘›π‘’π‘Ÿπ‘”π‘¦ π‘›π‘’π‘’π‘‘π‘’π‘‘βˆ—π·π‘Žπ‘¦π‘  π‘œπ‘“ π‘Žπ‘’π‘‘π‘œπ‘›π‘œπ‘šπ‘¦ / π·π‘œπ·βˆ—π‘†π‘¦π‘ π‘‘π‘’π‘š π‘£π‘œπ‘™π‘‘π‘Žπ‘”π‘’βˆ—π‘‡π‘’π‘šπ‘π‘’π‘Ÿπ‘Žπ‘‘π‘’π‘Ÿπ‘’ π‘π‘œπ‘Ÿπ‘Ÿπ‘’π‘π‘‘π‘–π‘œπ‘› π‘π‘œπ‘’π‘“π‘“π‘–π‘π‘–π‘’π‘›π‘‘
  • = 1419.6π‘Šβ„Žβˆ—2 / 0.8βˆ—12π‘‰βˆ—0.48
  • = 616.15 Ah.
  • Number of series batteries = π‘†π‘¦π‘ π‘‘π‘’π‘š π‘£π‘œπ‘™π‘‘π‘Žπ‘”π‘’ / π‘π‘Žπ‘‘π‘‘π‘’π‘Ÿπ‘¦ π‘£π‘œπ‘™π‘‘π‘Žπ‘”π‘’ = 12/12 = 1 π‘π‘Žπ‘‘π‘‘π‘’π‘Ÿπ‘¦ 𝑖𝑛 π‘’π‘Žπ‘β„Ž π‘ π‘’π‘Ÿπ‘–π‘’π‘  π‘ π‘‘π‘Ÿπ‘–π‘›π‘”.
  • Number of parallel strings = π΄β„Ž π‘Ÿπ‘’π‘žπ‘’π‘–π‘Ÿπ‘’π‘‘ / π΄β„Ž π‘œπ‘“ π‘œπ‘›π‘’ π‘π‘Žπ‘‘π‘‘π‘’π‘Ÿπ‘¦=616.15 / 330=1.86 π‘œπ‘Ÿ π‘Žπ‘π‘π‘Ÿπ‘œπ‘₯π‘–π‘šπ‘Žπ‘‘π‘’π‘™π‘¦ 2 π‘π‘Žπ‘Ÿπ‘Žπ‘™π‘™π‘’π‘™ π‘ π‘‘π‘Ÿπ‘–π‘›π‘”π‘ .
  • Total batteries = 2*1 = 2 batteries.

Step 5: Sizing Charge Controller

  • The power of panels = 800 W and the system voltage is 12 V.

Important Note:

  • The series panel connection is dependent on the MPPT range.
  • We try to make the panel voltage at the middle value of this range.
  • If the MPPT range is not available, we will take half of the open circuit voltage value.

Step 6: Panel Connection

  • Assuming selection of the open circuit voltage at the middle of the maximum PV open-circuit voltage = 150/2=75 𝑉.
  • Number of panels in series = 𝐷𝑒𝑠𝑖𝑔𝑛 π‘œπ‘π‘’π‘› π‘π‘–π‘Ÿπ‘π‘’π‘–π‘‘ π‘£π‘œπ‘™π‘‘π‘Žπ‘”π‘’ / π‘ƒπ‘Žπ‘›π‘’π‘™ π‘œπ‘π‘’π‘› π‘π‘–π‘Ÿπ‘π‘’π‘–π‘‘ π‘£π‘œπ‘™π‘‘π‘Žπ‘”π‘’
  • =75 𝑉 / 47.8 𝑉=1.57 π‘œπ‘Ÿ π‘Žπ‘π‘π‘Ÿπ‘œπ‘₯π‘–π‘šπ‘Žπ‘‘π‘’π‘™π‘¦ 2 π‘π‘Žπ‘›π‘’π‘™π‘  𝑖𝑛 π‘ π‘’π‘Ÿπ‘–π‘’π‘ .
  • Panels in parallel = π‘‡π‘œπ‘‘π‘Žπ‘™ π‘›π‘’π‘šπ‘π‘’π‘Ÿ π‘œπ‘“ π‘π‘Žπ‘›π‘’π‘™π‘  / π‘›π‘’π‘šπ‘π‘’π‘Ÿ π‘œπ‘“ π‘π‘Žπ‘›π‘’π‘™π‘  𝑖𝑛 π‘ π‘’π‘Ÿπ‘–π‘’π‘ =4/2=2 π‘π‘Žπ‘Ÿπ‘Žπ‘™π‘™π‘’π‘™ π‘ π‘‘π‘Ÿπ‘–π‘›π‘”π‘ .
  • The open circuit voltage at lowest temperature = number of series panels * open circuit voltage of one panel at lowest temperature.
  • = 2(47.8+(0.065*(25+20))=101.45V < 150 V.
  • The input current of the charge controller = short-circuit current of the PV panel * number of parallel strings * safety factor (1.25 or 1.3).
  • = 1.2525.4 = 13.5 A < 50 A

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