# Solar power system sizing, power auditing and design

Now that you have learned a great deal about solar panels from the last lesson, now we will move on to another important subject: Solar panel sizing and design. In this section, I will advice you to take your time. Its quite easy to estimate and understand, you only have to take your time to understand the units involved and the subsequent calculations.

Understanding the solar panel sizing will distinguish you from thousands of solar installers out there. Many of them work on a rough estimation of 2 x 300W solar panel to 1 x 200Ah deep cycle battery. But this system design and calculation will help you understand what you really need and how to achieve the power autonomy you desire.

#### How much solar power do I/my client need?

This is perhaps the most important question in solar power system installation. How will you determine the number of solar panels needed for a particular job?

We have seen several videos on this topic but this one by Michael Justus stands out. Its a 15 minutes video, but I advice you to take your time and learn it over and over again.

Do you need further help on this? You can Whatsapp Wavetra Energy in house solar engineer for further assistance.

Example: A house in Lagos, Nigeria has the following electrical appliance usage:

• 1 x 10 Watt LED bulb used 4 hours per day.
• 1 x 70 Watt fan used for 2 hours per day.
• 1 x 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.

We will power the system with 12 Vdc, 80Wp PV module.

1. Calculate the daily power consumption

 Total appliance use = (10 W x 4 hours) + (70 W x 2 hours) + (75 W x 24 x 0.5 hours) = 1,080 Wh/day Total PV panels energy needed = 1,080 x 1.3 (this is the energy lost in the system) = 1,404 Wh/day.

2. Size the solar panel

 2.1 Total Wp of PV panel capacity needed = 1,404Wh/ 5 hrs (this 5 Hrs is the average period of sunlight in a day in Lagos) = 280.8 Wp 2.2  Number of PV panels needed = 280.8 / 80 = 3.51 modules

Actual requirement = 4 modules
So this system should be powered by at least 4 modules of 80 Wp PV module.

3. Inverter sizing
Total Watt of all load = 10 + 70 + 75 = 155 W
To avoid overloadof inverter, the inverter should be considered 25-30% bigger size.
The inverter size should be at least 195 W or greater.

4. Battery sizing
Total appliances use = (10 W x 4 hours) + (70 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 2 days

Battery capacity = [(10 W x 4 hours) + (70 W x 2 hours) + (75 W x 12 hours)] x 2
(0.85 x 0.6 x 12)

Total Ampere-hours required 352.94 Ah Approx 400Ah
So the battery should be rated 12 V 400 Ah for 2 day autonomy.

5. Solar charge controller sizing
PV module specification
Pm = 80 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40 A at 12 V or greater.