We can design a residential solar system by ourselves. For this one needs to know what a solar system is made of. The basic components of the system are:

  • Solar Panels
  • Charge controller
  • Inverter
  • Battery
The whole designing of the system is based on the energy consumption of the electrical appliances that need to be powered by the solar panels.
The step by step procedure of designing the system as follows:
Determine the Power consumption demand:

Let us take an example, in a house there are 4 CFL of 18 watts which operates for 4 hours in day ( though it is always better to use LED to save and reduce load, but the fact remains that CFLs are more prevalent and used in todays time – our experience and research shows that residences are shifting to use of LED bulbs and fixtures and the new buildings being made are preferring LED technology).

Two 80 watt fans used 4 hours in day and
1 television of 150 watt operates 2 hours in day.
Now, the total appliances usage in watt-hours :
CFL: 4 no’s x 18 watts x 5 hours = 360 watt-hours/day
Fans: 2 no’s x 80 watts x 4 hours = 640 watt-hours/day
TV: 1 no x 150 watts x 2 hours = 300 watt-hours/day
Adding all the above values,
(360 + 640 + 300) watt-hours/day
1300 watt-hours/day
This is the daily energy requirement that needs to be backed by the solar panels.
Sizing the PV panels:
Energy required from the solar panels = 1300 x 1.25(as some of energy is lost in the form of heat)
                                                               = 1625 watt-hours/day
Now, divide this value by the solar insolation for that area where the system is to be installed to get the peak wattage of the solar panels.
(The average solar insolation New Delhi is 5.5 KWhr per square metre daily but you would typically get about 5 units of solar energy production every day)
=325 watts
Therefore, number of 120 watt panels needed: 3 no.s

Inverter sizing:
The total watt of the appliances is
CFL: 4 no’s x 18 watts = 72 watts
Fan: 2 no’s x 80 watts = 160 watts
TV: 1 no x 150 watts = 150 watts
Adding all the three values, 72 watts + 160 watts + 150 watts we get 382 watts.
For safety purposes the inverter should be at least 25%-30% bigger in size.
Therefore, the rating of the inverter should be 382 x 1.3 = 496.6 watts or greater.

Sizing the batteries:
Calculate the total wattage of the load (as calculated above) = 382 watts
Divide this value by 0.8(battery loss) = 382/0.8 = 477.5 watts
Divide this by 0.8 (conversion loss by inverter while converting current from dc to ac current) = 477.5/0.8
 = 596.87 watts
Divide this by 0.7 for the depth of discharge (for long life don’t fully discharge the batteries, keep 30% charge remaining in the batteries)
= 596.87/0.7 = 850 watts
This is the amount of energy which is required from the batteries to run the appliances.
Now let us suppose that backup required from the batteries is 5 hours.
Multiply 850 watts by 5 hours, we get  watt-hours which are required from the batteries.
Now, divide the result by 12 volts, the nominal battery voltage, to get the capacity of the battery, we get 4250/12 = 354 ampere-hours.
So, the battery should be rated 12 volts 354 Ah for 5 hours back up. Or one can even plan for a 24 V system in which case, the Ah capacity would be half of 354 Ah.

Sizing the solar charge controller:
The rating of the charge controller = total short circuit current of the PV array x 1.3
The 120 watt PV module specification is:
  • Power in watts (Pm) = 100 watts
  • Open circuit voltage (Voc) = 21 volts
  • Short circuit current (Isc) = 6.95 amperes.
  • Voltage at maximum power (Vmp) = 17 volts
  • Current at maximum power (Imp) = 6.18 amperes
We assumed that the solar panels are arranged in the parallel combination. The current will get add up i.e. 6.95 x 4 = 28.4 amperes. Therefore, the rating of the solar charge controller is 28.4 x 1.3 = 36.92 amperes.
So, the solar charge controller should be rated 36.92 amperes at 12 volts or greater. You would typically get a 40 A rating charge controller in the market.

This is the basic designing that one can do oneself and evaluate the cost of the system based on the specification of the components.

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