Solar panels come in all shapes and sizes. From small portable units to megawatt scale utility projects, there are myriad solar applications using thousands of different solar panel models. In this article, we'll take a look at the most common sizes. Before we start, we should mention that we'll cover size in both senses of the word: from a power capacity point of view (as in 200-watt panels) as well as the physical dimension of the panels.

#### Cells, Panels, and Arrays

Let's take a moment to discuss the difference between solar cells, panels, and arrays. These terms come up in Sunmetrix as well as other online resources, and it can get a bit confusing. The basic building block of all solar panels is a solar cell. Solar cells are connected in series in a solar panel, and solar panels are connected to form an array. For residential applications, the industry standard is 60 solar cells per panel. A typical solar cell provides 0.5 V therefore each panel provides between 30 and 45 V. You can read a previous article we wrote to learn more about how cells, panels and arrays are related.

#### How do I know how much solar power I need?

Sizing your residential solar electric system is all about determining the right solar fit for your house. Are you interested in reducing your energy bill, neutralizing it, or generating excess energy to sell back to the grid? Each option will entail a different solar panel system size. One of the first steps in system sizing is to get a very good handle on your energy consumption. A quick look at your monthly utility bill will show you your average energy consumption in kWh (utilities may provide you either with daily or monthly average figures). Let's say your monthly consumption is 500 kWh and your house is located in Denver, CO. The question now is which system size will you need to match this level of consumption.

Now that we know our target size of 4 kW, the next step is to determine how many solar panels we need. Most solar panels used for residential rooftop installations are in the 230 - 300 W DC power range. However, certain manufacturers, such as Sanyo and SunPower, offer more efficient solar panels that for the same physical size can provide up to 345 W DC (if you would like to learn more about solar panel efficiency, check out an article we wrote earlier). Note that the DC output values are based on laboratory conditions (also called standard test conditions), when the panels are outdoors and exposed to the elements with ever-changing solar radiation conditions, the output levels may be slightly lower than the advertised amounts.

So how many panels will you need for a 4 kW system? For example, if you use the 250 W monocrystalline panels manufactured by RENOGY, we will need a total of 16 solar panels (4 kW /0.25 kW).

#### What are the dimensions of residential solar panels? (height, width and depth)

A commercial solar panel (left), compared to a residential solar panel (right).

Aside from the power needs used to determine the number of solar panels, you must also consider the physical size dimensions. If the quality and the price are right, you can definitely choose a less efficient solar panel brand, however you will probably need to purchase more units to meet the same system size target. In order to determine the exact number of panels on your roof and their configuration, installers perform detailed calculations. But you can get a pretty good idea if you can combine the information from the system size we discussed above with the physical dimensions of the panels you are interested in.

Most solar panels designed for residential use are 65 inches high and 39 inches wide. The depth of typical solar panels ranges from 1.4 inches to 1.8 inches. All those solar cells, the casing and the glass cover can weigh quite a bit: the weight distribution of solar panels is around 2-4 pounds per square feet (not very light, but easily within the safety limits of most roofs). Let's take a look at an example to better understand how all of these numbers are related to each other. The RENOGY panels we mentioned above have a module dimension of 64.5 inches by 39 inches. The weight is 42 pounds which translates to a pressure of 2.3 pounds per square foot, so most roofs should have no trouble accommodating the panels and the racking system.

Although the total area of the 16 panels for our 4 kW system is around 280 square foot, the actual area needed on your roof will be slightly greater since we need to account for spacing between panels, necessary for mounting. Solar installers normally perform a detailed structural assessment of your roof to make sure that the design is optimized for the available space and the weight of the overall system is distributed evenly across your roof.

#### A quick look at commercial panels

Commercial panels have slightly different power ratings and physical dimensions than residential ones. Commercial panels are manufactured using a larger number of solar cells (up to 96 per panel) and therefore they are taller. Typical dimensions are 77 inches by 39 inches. The installation method used for commercial panels is quite different from that of residential panels. Since commercial projects are much larger, hundreds of panels are usually needed. Most commercial properties have flat rooftops, therefore non-penetrating, ballast mounts are preferred. This technique is less invasive than bolting the racks on the roof surface and it also saves several weeks in installation time.  For residential installations, installers can seldom use ballasts to secure the panels as most residential buildings have slanted roofs.

#### The most diverse class of panels: portable solar

When it comes to portable solar panels, it's really difficult to talk about a typical dimension. These tend to come in a very wide range of sizes. The power capacity of portable panels is generally lower than the capacity of their residential and commercial cousins. The cell efficiencies are also lower. As a result, the amount of power you can generate from portable panels is generally quite a bit lower than for rooftop installations. Portable panels can be used without an inverter to charge most USB and 12 V devices such as cellphones, GPS units, music players, etc. For instance, the Nomad 7 unit by Goal Zero can only generate 7 watts of power. It's dimensions are 6.5 x 9.5 x 1 inches and it weighs only 1 pound. A 7-watt capacity is puny compared to the 250-watt residential panels we discussed above, but if all you need is on-the-go power to charge your mobile devices, it may be all you need. We should note that portable does not always mean small. The 600-watt indoor/outdoor solar power system manufactured by Sainty International is 48 x 40 x 31 inches and it weighs more than 150 pounds. Obviously this unit can do much more than charging portable electronics.

#### There is a solar application for any budget: just get the sizing right!

One of the biggest advantages of solar power is scalability. If all you need is a few watts of portable power, then you should go for a small solar panel with a few solar cells. If you are interested in a residential solar installation, chances are you will be using similar solar cells, but you will need a whole bunch more. Visiting Sunmetrix Discover is a great next step in your quest to determine the best size for your needs and to see how many kWh you can generate at your location.