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.

Requirements Quantity Load Total Load
Fan 4 80 320  
LED tubes 5 20 100  
Sockets 4 50 200  
TV/D2H 1 50 50  
Subtotal     670 Watts

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 wiresNow 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.

Resistivity 0.0171 ohm 1mm2/m  
Area Length Resistance Max current Wattage loss
1 15 0.2565 25 160.3125
2.5 15 0.1026 25 64.125
4 15 0.064125 25 40.07813
6 15 0.04275 25 26.71875

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.

Month Days Panel Area m2 Solar irradiance kWh/m2/day Total Irradiance captured Estimated utilization Atual units saved
Units saved on month
      kWh/m2/day (100W*5) Pane efficiency 17% kWh / day  
JAN 31 3.3 4.55 15.015 2.55255 2.1696675 67.2596925
FEB 27 3.3 5.21 17.193 2.92281 2.4843885 67.0784895
MAR 31 3.3 5.9 19.47 3.3099 2.813415 87.215865
APR 30 3.3 6.31 20.823 3.53991 3.0089235 90.267705
MAY 31 3.3 6.12 20.196 3.43332 2.918322 90.467982
JUN 30 3.3 4.43 14.619 2.48523 2.1124455 63.373365
JUL 31 3.3 3.93 12.969 2.20473 1.8740205 58.0946355
AUG 31 3.3 3.97 13.101 2.22717 1.8930945 58.6859295
SEP 30 3.3 4.2 13.86 2.3562 2.00277 60.0831
OCT 31 3.3 4.54 14.982 2.54694 2.164899 67.111869
NOV 30 3.3 4.49 14.817 2.51889 2.1410565 64.231695
DEC 31 3.3 4.39 14.487 2.46279 2.0933715 64.8945165
            Total 838.7648445

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

Year 0 1 2 3 4 5 6 7 8 9 10 11 12                          
Total -43000 6700 6700 6700 6700 -5300 6700 6700 1200 6700 -5300 6700 6700                          
Inverter -5500               -5500                                  
Wires -3000                                                  
Battery -16000         -12000         -12000                              
Panels -16500                                                  
Misc -2000                                                  
Energy Savings   6700 6700 6700 6700 6700 6700 6700 6700 6700 6700 6700 6700                          
13 14 15 16 17 18 19 20 21 22 23 24 25
6700 6700 -5300 1200 6700 6700 6700 -5300 6700 6700 6700 6700 -5300
      -5500                  
                         
    -12000         -12000         -12000
                         
                         
6700 6700 6700 6700 6700 6700 6700 6700 6700 6700 6700 6700 6700

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

Solar Savings

 

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