How Many Solar Panels Do I Need?

The most frequently asked question I receive from people who want to develop their own solar electric system is, "how many panels will I need?"

This is a perfectly reasonable question, but it is one that is extremely difficult to answer without a lot of information and careful thought. This is because the number of panels you need depends on factors such as these:

1. How much electricity to you need, or want to make?

2. How much sunshine do you get at your location?

3. What size, or wattage of panels do you wish to use?

4. What are inefficiencies in your system?

5. How will the solar panels be positioned? Flat? Tilted? Adjustable?

You can get a pretty good estimate of the number of panels you'll  need by going to this  solar panel estimator. You need to gather some information to feed it. What you need and where to find it is detailed below.

How Much Electricity do you Need or Want to Make?

Maybe you want to convert your home from utility power to 100% solar power. Then you can get a good idea of how much electricity you need to make by looking at your electric bill. Kilowatt-hours (KWH) is the unit of electric consumption over a period of time. One KWH  represent the use of 1000 watts of electricity for one hour, or 500 watts for two hours. Whether you wish to replace all of your utility electric consumption with solar, or a fraction of it, you need to get an idea of the number of KWH you hope to make.

Besides looking at your electric bill you can also identify specific loads you want to power with solar. Look at the faceplate for a wattage rating. There is a clever little meter called the Kill-a-watt meter that connects between an appliance and a wall outlet. It gives a reading of volts, amps, watts, and KWH over an elapsed time. The Kill-a-watt meter is very informative about how much electricity different devices consume. 

To compute KWH of a device, multiply the watts times the hours it runs in a unit of time such as a day or a week or month. This gives you watt-hours. Divide watt-hours by 1000 to get KWH. Your electric bill indicates the sum of all the kilowatt-hours consumed for lights, appliances and other electric devices for the month. You could divide by 30 to get a daily figure. The amount of electricity you will want to make will be the sum of the KWH for the electric devices you plan to power with solar energy. For the purposes of the estimator, you need to give a figure that reflects the number of KWH you hope to produce each day.

How Much Sunshine do you get at Your Location?

There's the rainy season when you may not get any sun, then there's the dry season when you may get  day after day of full sun. Inbetween there is huge variation. The sun might go in and out of clouds, mostly sunny one minute, partly sunny the next.

There is also the angle of the sun as the seasons change. A sun that hangs low in the sky has much more atmosphere to travel through before it reaches the earth compared to a sun that is high overhead. In summer, days are longer as one moves away from the equator, so you can expect seasonal changes in the intensity of sun and length of daylight, as well as changing weather conditions. These things affect the amount of electricity a solar panel can make.

It would be daunting to actually try and measure the amount of sunlight that falls on your site. Fortunately, NASA has already done it for us. The term, insolation, refers to the energy in sunlight falling on a flat surface. It takes weather and seasons into account and gives the number of KWH of solar energy falling on one square meter per day, on average.  Go to the NASA Surface Meteorology and Solar Energy site and create a login by giving your email address and a password. You will specify your location anywhere on the earth's surface either by typing in latitude and longitude, or by clicking on a globe and zooming in to your exact spot.  Once there, you can select from a host of interesting datasets, including monthly average insolation values for flat surfaces and also for surfaces tilted towards the equator. For the Solar Panel Estimator, select the Insolation Average, Min and Max, and the Radiation on Equator-pointed Tilted Surface parameters. Note the yearly average figures you find for your location.

What Size or Wattage Panels do you Wish to Use?

Nowadays, solar panels come in a wide variety of sizes and wattages. The only times it matters what size panel to choose is when you must fit within physical space dimensions, or if you are trying to match the electrical characterisitics of two different brands of panels. Choose what is economical and meets your voltage and wattage requirements. High wattage panels are not necessarily superior or inferior to low wattage panels. You do need to pick a size, however, and stick to it for each array you put together. Panels with different electrical characteristics generally do not go together well.  The estimator will want to know what wattage you pick.

What are the Inefficiencies of your System?

Every component has a certain efficiency. That is, the output energy of a device will always be less than the input energy. In our world, there is no such thing as perfect efficiency where output equals input. There is always resistance, heat dissipation, and current loss. Inefficiencies multiply. It doesn't take very many components, each with relatively high efficiency, to seriously degrade the overall system efficiency. Take my system:

Outback MX60 charge controller: 98%

Outback GVFX inverter                90%

Batteries:                                   75%

Multiply .98 x .90 x .75 and you get a lousy 66% efficiency overall, and there is probably even more degredation through transmission loss over the distance between the panels and the other equipment.

The estimator will want to know the efficiency of your system. If you know the ratings from your manufacturer, great. If not, figure it will be somewhere between 50 and 70 percent. In other words, almost half the solar energy you produce is wasted. Sad, isn't it. It should be noted that just as little inefficiencies multiply to big inefficiencies, so also do little improvements multiply to have dramatic effects. Strive for greater efficiency wherever you can.

How will the Solar Panels be Positioned?

The output current from a solar panel is highest when its surface is perpendicular the sun's rays. It falls off markedly as the angle changes. Fixed panels are a compromise. They make for a simpler construction and they can be firmly fastened to withstand brutal winds. The downside is the sun's angle changes constantly, rising in the east, setting in the west, and also reaching higher angles in summer and lower angles in winter.  You will notice on the NASA site that the month-to-month insolation KWH values can change by a factor greater than three times, and one reason is better alignment to the sun. The insolation values have a powerful effect on the number of panels you will need. If you double the insolation value, you cut the number of panels you need in half, and at $500-$1000 each, the frugal solar designer will make as much use of sun angle as he or she can. Try playing with different parameters in the Altenergyweb Solar Panel Estimator such as different insolation values obtained from NASA for your site, and you'll see that seemingly small changes will make a big difference in the number of estimated panels needed, and consequently, the price.

Here is the link to The Solar Panel Estimator.  At some point in your planning, you may also need to estimate how many batteries you will need. I'm working on a tool for that. Click on the next link to get to a preliminary version of my battery calculator.

And oh, yes. Just in case you forgot about the solar panels and other PV information on Altenergyweb, I , here's a few more links.

Build a Grid-Tied Solar Electric System

Solar Panels for Sale (We can ship to Africa, Asia, Central America, Pacific Region, South America, Europe, and North America) 

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