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= 500kW per hour
5 x 15W=75W*8 = 600kW per hour
1 x Small Fridge (200W) *8= 1600kW per hour

Total KW per hour = 315
Total KW per day = 1820

Step 3

Now you need to add around 30% on your total KW per day to cater for the battery, inverter and wiring inefficiencies

1820 + 30% = 2366

Total usage is 2366Wh

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
= 2366 / 5h
= 473Wp

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

= 473Wp / 140W Prime PV Modules
= 3 x 140W 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

= 2366 x 2 /24V
=197Ah @ 24V

The easiest way to get to this capacity is: 4 x 100Ah 12V Batteries
(2 x connected in parallel and 2 x connected in series)

Step 6

Inverter sizing

When sizing your inverter you need to use your total KW per hour.

In the case of the example your total KW per hour watts per hour is 315. So in this case you would need to go for a 500 to 1000W 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

Regulator/ Charge Controller Calculation

Regulators/Solar Controllers = PV Wp needed/ System Voltage
= 473 / 24
= 20A

You will need a regulator with a rating of no less that 20A @ 24V

You may also download our load assessment sheet to help you size your system. Please note that the load assessment will differ from working it out manually as the assessment takes into account a winter and a battery temperature coefficient.

Load Assessment