Disclaimer:

These recommendations are offered in good faith and are based upon information supplied by various PV customers and several educational websites. It remains the responsibility of the user to verify that all input data and assumptions are valid and to independently confirm that the results are suitable for their intended purposes. Setsolar will not accept any responsibility on behalf of any customer for inaccurate system sizing or wrong equipment assumptions and purchases.

How to size a solar PV system

The basic formulas and information you need to know: W = V x I (Watt = Volt x Amp)
V = W/I and I = W/V

Watts(W)

Definition of watts please click here.

Volt (V)

Definition of volts please click here.

Amp (I)

Definition of Amps please click here.

Please find below an example including all the calculations on sizing a solar PV system.
Example:


If you would like to run the following: 1 x Small TV
5 x Energy Saving lights
1 x Small Fridge

Step 1

Find out what is the wattage of the appliances you wish to run. Most appliances have a rating at the back of the unit.

1 x Small TV (100W)
5 x Energy Saving lights (15W Each)
1 x Small Fridge (200W)

Step 2

Once you have the correct ratings you need to work out how long each appliance will be used for.

Small TV (100W) for 5 hours
5 x Energy Saving lights (15W Each) 8 hours
1 x Small Fridge (200W) 6 hours (which is what an average fridge compressor runs for) 1 x 100W*5= 500Wh
5 x 15W=75W*8 = 600Wh
1 x Small Fridge (200W) *6= 1200Wh

Total W per hour = 375W per hour
Total W per day = 2300kW

Step 3

Now you need to add the losses that you may incur in your system 20% for Battery Losses
10% for Inverter Losses
10% for Distribution Losses 2300Wh/0.8 = 2875Wh 2875Wh/0.9 = 3194Wh 3194Wh/0.9 = 3549Wh
Total usage is 3549Wh

Step 4
PV Module Calculation

When calculating how many panels you need. You need to divide by the amount of available sunlight hours. We normally work on an average of 5 hours of sunlight in South Africa. Solar Panels = Daily usage / 5h of sunlight
= 3549Wh / 5h
= 709Wp

Now you need to add 9% for the tempreature coefficient of PV panels 709Wp/0.91 = 779Wp Once you have your total you need to divide into the solar module size that you wish to use and you will then get how many modules you will need.

Totals watts per hour /Solar module size = 779WpWp / 200W 24V Prime PV Modules
= 3.9 (4) x 200W 24V Prime PV Modules

Step 5
Battery Calculation

For battery sizing you always need to cater for a 50% discharge to prolong the life of your battery. Any further discharging will dramatically reduce your batteries life span. Battery Size = Daily usage x 2 (for the 50% battery discharge) / System Voltage = 3549 x 2 /24V
= 295Ah @ 24V The easiest way to get to this capacity is: 6 x 100Ah 12V Batteries
(3 x connected in parallel and 2 x connected in series) This battery capacity will give you around 1 days storage (autonomy)

Step 6
Inverter sizing

When sizing your inverter you need to use your total Watt per hour. In the case of the example your total watts per hour is 375. So in this case you would need to go for a 500 to 800W Pure Sine Wave inverter. On the load assessment the second last column from the bottom will give you the combined usage of all listed items.

Step 7
Charge Controller Calculation

Charge Controllers = Total PV Wp needed/ System Voltage
= 800 / 24
= 35A You will need a regulator with a rating of no less that 35A @ 24V You may also download our load assessment sheet to help you size your system.

Load Assessment