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Science & Technology

DIY calculations before setting up your Solar PV OnGrid System (Residential only)

Have you decided to put up a solar system at home. If yes then you should look at this to calculate the estimated savings and costs that are involved and what type of system are you going to use.

Estimate your load requirements

Find out what loads you will be using for your inverter setup. For my case i have selected my TV as an inverter load because it is on most of the time. Avoid heavy loads such as Water Pumps, grinders and other similar appliances. One good choice will be your Fridge if you wattage to spare on your inverter.  I could not include my fridge because an inverter phase wire could not be installed. The 4 sockets are as follows- Water Purifier, Router, CCTV and a free socket to charge my phone.

RequirementsQuantityLoadTotal Load
Fan480320 
LED tubes520100 
Sockets450200 
TV/D2H15050 
Subtotal  670Watts

So based on total load you will need to select an inverter that is a minimum of 800W capacity. if the product is of 1000 VA the take 80% of that value as the load capacity it will be able to handle. Ever purchased a Silver/Gold standard SMPS for your Cabinet? If yes you will understand what i am saying.

Now that you have selected your solar inverter model it is time to select your battery. For loads below 1000W the solar system will be running at 12 V.  If you are planning on a higher capacity you will need to connect two batteries in series and also make sure your panels are is in the same configuration. Now what do we understand from the Ah rating of a battery? It provides the capacity of one. lets say you select a 150 Ah battery. the capacity of the battery is 150*12 =1800 Watts. So on a 600 W load the battery will be able to supply 3 hours of backup.

Wiring

AC wires

Now the inverter and battery has been selected it is time to select wires. Well here is a simple thing to remember , AC current moves through the outer surface of the conductor . Hence the wires are so to increase surface area.

But for DC currents a different wire is available, but you will not find any branded cables for such. Because the loads for a household is low we can get away with the regular ac wires. Now comes the cross section area of the wire what we need to choose.

Resistivity0.0171ohm 1mm2/m 
AreaLengthResistanceMax currentWattage loss
1150.256525160.3125
2.5150.10262564.125
4150.0641252540.07813
6150.042752526.71875

So a 4mm of wire will have a loss of nearly 40 W per hour. that is huge because we are producing only 500 W. So it is advisable to connect 2 sets of connections using 4sqmm wires. 250 W each or if you want less entanglement you can use just a 6sqmm of wire.

Energy Savings Calculator

Google the average monthly irradiance for your area. The unit is kWh/m2/day. My 100 w panel area is .66 m2. so a total of 3.3 sqm area of sunlight i am able to capture. That would mean i should be able to get 3.3  times of the irradaance value at my location. Well it is not so. Let us see some calculations.

MonthDaysPanel Area m2Solar irradiance kWh/m2/dayTotal Irradiance capturedEstimated utilizationAtual units savedUnits saved on month
   kWh/m2/day(100W*5)Pane efficiency 17%kWh / day 
JAN313.34.5515.0152.552552.169667567.2596925
FEB273.35.2117.1932.922812.484388567.0784895
MAR313.35.919.473.30992.81341587.215865
APR303.36.3120.8233.539913.008923590.267705
MAY313.36.1220.1963.433322.91832290.467982
JUN303.34.4314.6192.485232.112445563.373365
JUL313.33.9312.9692.204731.874020558.0946355
AUG313.33.9713.1012.227171.893094558.6859295
SEP303.34.213.862.35622.0027760.0831
OCT313.34.5414.9822.546942.16489967.111869
NOV303.34.4914.8172.518892.141056564.231695
DEC313.34.3914.4872.462792.093371564.8945165
      Total838.7648445

If per unit rate is INR 8, then my savings per year is INR 6700

For Bhubaneswar the average irradiance is 4.8, where as for a city like chennai it is 5.5. For the same system one will get 15% more unit savings. For Chennai the savings can be estimated at INR 7700 / year

ROI

Year0123456789101112
Total-430006700670067006700-53006700670012006700-530067006700
Inverter-5500       -5500    
Wires-3000            
Battery-16000    -12000    -12000  
Panels-16500            
Misc-2000            
Energy Savings 670067006700670067006700670067006700670067006700
13141516171819202122232425
67006700-53001200670067006700-53006700670067006700-5300
   -5500         
             
  -12000    -12000    -12000
             
             
6700670067006700670067006700670067006700670067006700

IF i plot a graph for breakeven analysis i recover the cost in 9 years time.

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