How to Design a Solar PV System

How to Design a Solar PV System

Information and Numbers sourced from

There are many types of solar systems, such as solar water heaters, solar convection fans, and passive solar systems; the main focus of this post is the Solar Photovoltaic system. This system provides a renewable source of power by converting the energy from photons that collide with the solar cells into usable energy that our appliances use.

The components of the system include the panel, the charge controller, the inverter, batteries, all of your appliances, and any auxiliary power source.

  • The panels convert sunlight to energy, although due to constraints of current technology and the laws of thermodynamics, the average efficiency of panels in the best conditions are about 15%-25%.
  • The charge controller will protect your batteries by not allowing them to overcharge which will ruin your battery.
  • The inverter converts the power from the panels from DC to AC, which is what all of our appliances, computers, phones, TVs use.
  • The battery stores the power from the panels; it is important to get a deep cycle battery that can be recharged and discharged over and over. Car batteries are the wrong kind of battery, especially if powering a house.

To get the most efficient system in terms of cost and power, one needs to first figure out how much power is used during a day.

  1. Determine your power consumption demands by doing two things. First calculate the watt-hours used by each appliance per day. Most appliances will list the watt-hours somewhere, and you just multiply that by the hours it is used. Next, all the watt-hours from all your appliances are added up, and this becomes your total watt-hours for your house. Finally you should take this number and multiply it by 1.3. This will be the amount of power your panels need to gather each day as a minimum. It is also good to overshoot this number to be sure you can gather as much as possible in the event of bad weather.
  2. Sizing up the PV system is the next part. This involves the determination of your areas Insolation, or hours of good sunlight, and the rated efficiency and watt output of your panels. In Durham, as well as most of North Carolina, we have an Insolation of about 5. This means that we take our watt-hours we determined we needed and divide it by the Insolation to determine the amount of watt-hours needed by each panel. This number is then divided by the rated output of the panels you have available, which will give you the number of panels needed. It is important to note that this number is the minimum and to prolong the life of your systems and batteries, as well as achieving better performance, extra panels should be installed.
  3. The inverter should be chosen based on the total wattage of your house. It should be rated to handle 25-30 percent more wattage than all your appliances can use at once. This will make sure that you don’t overload and bust your inverter, which can be expensive. In a grid-tied system, the inverter should be rated to the output of the PV panels.
  4. The batteries should be sized next. Take the total watt-hours per day used by the appliances and divides by 0.85 to account for battery loss. Divide this answer by 0.6 for depth of discharge, as the battery should never be discharged below 60% of its capacity. Divide this number the voltage of the battery and then multiply this number by the days of autonomy, or days you need to operate without input from the panels, and this will give you the Amp-hours of your system, or the batter capacity needed.
  5. The charge controller should match the voltage of both the panels and batteries, and should be able to handle a shirt circuit of the system.

Some basic calculations would like this:

  1. Determine Appliance Use = (18 W * 4 hours) +(60 W * 2 Hours) + (75 W *24 *0.5 hours) = 1092 Wh/day
  2. Total PV panels energy needed = 1092 *1.3 = 1419.6 Wh/day
  3. Total Wp of PV Panel capacity needed = 1419.6 / 5 = 283.92 Wp
  4. Number of panels Needed = 283.92 / 110 = 2.58 panels or 3 panels (4 panels for wiring simplicity)
  5. Inverter Sizing = 18 + 60 + 75 = 153 W * 1.25 = 191 W atleast
  6. Battery Capacity = ([(18 W * 4 Hrs) = (60 W * 2 Hrs) + (75 W * 24 Hrs * 0.5)] * 3 Days of Autonomy) / ( 0.85 * 0.6 * 12) = 535.29 Ah — So Battery should be rated 12 V 600 Ah for 3 days of autonomy

This is just the basics and a knowledge of basic electronics and the terms will come in handy.

Solar Panels: Cheap, Free, or DIY?

I would first like to give credit to and for the following information on how to obtain cheap/free solar panels and guides on how to make your own solar panels!

If you live in the Triangle Area, then it is almost certain that you have seen solar panels at either a construction site or on top of street signs. Most often, these are used for low power LED lights so that new traffic signs are more noticeable and so that everyone knows that there is a lane shift or some other issue ahead. For the city and state to continue using solar panels, however, they need to be efficient and effective. So when the relatively fragile solar panels catches a rock on its front face or if they simply age past their prime efficiency, the state is required to swap out the panels.

This means that otherwise decent panels are hauled off, because it is easier to replace than it is to repair. These panels are the panels we will be using.

*CAUTION* It is often times illegal to stop on the side of the highway to look at these panels, so practice safety and caution.

Step one is to obtain all the contact information you can; names and phone numbers will become extremely important. This allows you to look up the companies who supply the DOT with solar panels.

Step two is to actually visit these places, as you will have a much greater chance of walking out with solar panels if you have a face to face meeting, instead of a phone conversation. You need to find out where the panels currently are and if they sent them to another company. Always offer money for the panels, as they will appreciate it, and chances are they may be more willing to give them away for free, especially since they can no longer use them. Even if you end up paying for panels, you will end up saving several hundreds of dollars, maybe thousands.

Step three is to test them with a multimeter. This way you can determine if they are worth using or repairing.

Step four is determining where to place and/or mount the panels. More guides can be found on, however, to keep it simple, they have to generally be facing true south and at a certain angle (45 degrees seems to be best for most situations); your panels don’t have to fit the angles or be facing true south, but they are more accurate they closer you get to these positions.

If you are a DIY person, solar cells can be purchased, and with the good backing and patience, you can make your own panels, which cuts out the middle man. A more in depth guide can be found at

With all this information, I hope y’all are more empowered to begin moving away from Duke Energy and in to a more autonomous, energy independent life!



Cheap/Free Solar Panels:

DIY Solar: