Once you have chosen to go Solar, the very important question arises. How many solar panels do I need?

The first part of this is to figure out how many Watts of solar you need. This will depend on how much energy you want to generate. If you are shopping for a grid-tied system, you will find the answer in your electric bills. Every month, the electric company has tallied exactly how much energy (kiloWatt hours or kWh) you used so they could bill you for it.

The meaning of that kWh figure and kW is commonly confused. The kWh is merely kW over time. For example: if 10 kW is consumed for 2 hours, that is 20 kWh. An analogy is kW is like a speedometer, indicating how much power is being consumed at one point in time. And kWh is like an odometer, indicating how much power has been consumed over time.

All you have to do is look at the history of kWh usage on your paper electric bills or your online account with the power company. We recommend reviewing at least 12 months of electric history. You will definitely notice a difference in usage from winter to summer and basing your needs on just one month’s bill can be a big mistake.

The next decision is how much of the electric bill do you want to offset. You could eliminate it completely, cut in in half or do something in between. You do want to be careful not to put in a system that will generate more than you use. In most cases, you do not get a payback on the excess power generated. When in doubt, shoot for just under your usage to avoid over-paying for your solar. For example, if you added up all your usage for a year and it was 12,000 kWh and you decided you wanted a system to generate 95% of that do 12,000 x .95 = 11,400 kWh so that is your target output.

All of that applies to a grid-tied system where there will be electric bills for reference. If you are sizing an off-grid system, there is a little more math involved. You will need to list everything you want to power, how many Watts it uses and how many hours per day it will be running. For each item, multiply the Watts times the hours to get Watt hours. Add up the Watt hours of everything on the list. Divide that by 1,000 and you will have the kWh that you need.

Now you know what you need to generate and the next step is to figure out how many Watts of solar will do that. There are many factors that affect solar output including where you live (latitude and local weather patterns), the orientation of the solar panels (the direction they face and tilt angle) and shading.

The best way to deal with all these factors is to use an online calculator. We suggest PV Watts which is a very reliable and impartial tool. You can Google it or go directly to https://pvwatts.nrel.gov/. Follow the prompts which start with entering your Zip Code so your production numbers will be location specific.

It will also ask for a system size in kiloWatts (kW). A good starting point is to take that kWh number you want to reach and divide it by 1,500. In our example above where you wanted to generated 11,400 kWh it would be 11,400/1,500 = 7.6 kW. Once you see the output on that system size, you can adjust the size up or down to get to the goal.

You will also need to enter the tilt and Azimuth of the system. Tilt is just the angle from horizontal. If the system will be installed on a 5:12 pitch roof that will be a 24 degree tilt angle. Azimuth is the number on the compass that corresponds to the direction the solar panels will be facing. If you have a compass handy, that’s great. If not, you can figure this out by downloading a free compass app on your smartphone or you can use the ruler tool on Google Earth (the “heading” is the Azimuth).

A hint here is that solar panels facing South (Azimuth 180) will do the best. Solar panels facing East (Azimuth 90) or West (Azimuth 270) will still do pretty well but if they are facing North (Azimuth 360) they will generate significantly less power.

Shade can be a little trickier, typically you want to go for an unshaded area which requires no adjustment in the PV Watts calculator. If that giant tree in your back yard is a problem, the obvious solution is to remove it. This can be done guilt-free as the solar you will be installing has the carbon footprint equivalent of planting thousands of trees.

Early morning and late evening shade aren’t a big deal, but if there is a lot of shade in the middle of the day from an unremovable object like your neighbor’s two-story house, you may want to have a solar professional do a shade assessment. These can often be done remotely and will be free for a potential customer. The resulting shade percentage can be entered in PV Watts under “System Losses”.

After working with the PV Watts calculator you will know how many watts of solar you will need. Remember in PV Watts the system size is in kW so you will multiply that by 1,000 to get back to watts. Using our example above, if 7.6 kW was the right system size you would do 7.6 x 1000 = 7,600 Watts. The last step in the process is figuring out how many solar panels that will be. The factor here is solar panel efficiency.

The standard 60 cell solar panel will be about 5.5’ x 3.5’ and these range from 260 Watts to 330 Watts. Take the amount of Watts you need and divide it by the Wattage of the panel and that is the number of panels you need. Continuing with our example it would be 7,600/260 = 30.4 or 7,600/320 = 23.75. This means you would need 30 of the 260 Watt panels or 23 of the 320 Watt panels.  This seems to make the 320 Watt panels the obvious choice until you look at the price tag. The price per Watt will be much higher on these panels so how do you know if they are right for you?

The answer almost always comes down to the amount of available space. If you have enough space on your roof to do the job with 260 Watt panels, you will save some money by doing so. If you are struggling to find room to get to the Wattage you need, it might be worth it to go for the more expensive high efficiency modules.