Sizing a Solar Array

A Solar Array is just a fancy term for your solar panels, which themselves are combinations of solar cells, combined into a single system.  Today we discuss “sizing a solar array.” The first step is determining what your needs are.  It is always good to eliminate or reduce some parts of your usage before determining the size of your solar array.  For example at our house we got rid of our drip coffee maker and began buying roasted beans, hand grinding them, heating water on a propane stove, and using a french press.  The amount of electricity we need to make our morning coffee is now zero.  Switching to LED lighting, putting appliances on surge protectors that we turn off when not using, and drying our clothes on the line rather than in an electric dryer were some other adjustments we made before going off grid.

If you are currently on grid, you have a very easy meter to determine your usage in your monthly electric bill.  Depending on your climate, you may see higher usage during certain times of the year.  For example in TN, we saw summer months where we were cooling with electricity as our major power need.  One hard rule we have living off grid is that we don’t use electricity to generate heat.  No electric furnaces, space heaters, microwaves, hot plates, crock pots, or electric water heaters for us.  So my suggestion would be to reduce your consumption as much as possible, then review your bill to see how much electricity you are using.  In the US, the average utility customer uses 901 kWh per month of electricity. Your bill will be denoted in kWh which means kilowatt-hour,  Or 1000 watts for 60 minutes.  For the sake of this example, lets assume that you have dropped your consumption to 450 kWh per month(half the average household).  So on average you are using 15 kWh per day.  Once we have this information we need to look at an insolation map, which will show you the average daily hours of sun you receive in your location. Here is a map of the us provided by the NREL.

Armed with the insolation data you have for your  location, back to sizing a solar array.  You need to divide  your daily use by the number of hours you get.  The NREL posts maps for each month so you could break this down month by month and use the worst case scenario if you like.  Where I live in TN we average 4.5 sun hours per day.  This means over the course of an average day, we get the equivalent of 4.5 hours of full sun.  So using our above number of 15 kWh per day and 4.5 average hours of sun, the total array size that I need is 3333.33 watts.  You will notice I went ahead and made the conversion from kWh to watts.  Now you can figure how many panels you need in your array to meet your needs.  You may decide you want to replace 50% of your needs, or you may go all out and decide to take out all 3333 1/3 watts.  Lets take the latter and assume we are going to get a great deal on 10 or more 310 watt panels.  We divide the wattage of the panels into the watts needed, so 3333.33/310, which gives us 10.75 panels.  My suggestion is to increase this number by 25% to account for losses at different points in the system(which we will discuss later).  Doing this gives us 13.44 Panels, which we will round up to 14.  If you followed along, you now now exactly what your usage is, how much you want to replace with solar, and how many panels you need at a specified wattage to meet your needs.  We hope this post on “Sizing a Solar Array” was helpful.  Shot us an email on our Contact Page if you have any questions we would be happy to help!

How do Solar Panels work?

Solar, or Photovoltaic, Panels work by converting light energy from the sun, or photons, into electricity through the “Photovotaic effect.”  Interestingly, a French physicist built the first ever photovoltaic cell at 19 years old, in 1839.  The first practical photovoltaic cell was demonstrated by Bell Laboratories in 1954 and early on they were primarily used in space.

A Solar Panel is a collection of solar cells that are made up of semiconductors.  Semiconductors absorb the photons–through a process I wont describe in detail–and cause electrons to be excited and break free to be converted into usable DC electricity.  Solar cells are arrangements of silicon “sandwiches” where one layer is activated with boron and the other activated with phosphorus.  This helps the charge to be created and directs the free electrons to the wiring that leads to your charge controller.  

The key to remember here is that its the light energy that is converted to usable electricity, so when siting a group of solar panels, also known as an array, you should do an insolation analysis, which is a fancy phrase for looking at whether or not you are going to have full sun hitting the panels or if there will be shadows cast from structures or vegetation.  A part of this analysis will also be determining how much full sun you get in your area and using that information to determine how many panels you need to charge your battery bank.

This post is about “how do solar panels work” but now that you have a basic understanding, How do solar panels workhere are a few things to remember that can reduce to the amount of energy your panels can produce:  dust/pollen/leaves, temperature, and wire resistance.  If you let your panels get dirty or accumulate leaves or snow, it should be quite obvious that the light cannot reach the covered up cells, which will logically result in less energy production.  When your panels heat up, they also become less efficient.  Remember above when I discussed the electron becoming excited and breaking free?  Well the hotter the cell gets, the less potential there is between the different electrons because the ones that are at rest are already a little “excited” because they are hot.  As a matter of fact when sunlight shines in through your window and heats the air in your house, this is specifically due to excited particles, and its no different with a solar panel.  We will discuss wire sizing in more detail in another post, but remember that current is measured in amps, and wire resistance simply means that smaller wires cannot handle high amps.  This is why we use ideas such as stepping up voltage the reduce the number of amps required to move the same amount of energy.   Amps x Volts always equals Watts so increasing one number always decreases the other.