How to Calculate Size and Number of Solar Panels If you are confused regarding how to calculate size and number of solar panels required to provide a reasonable amount of power for your home, you are certainly not alone. It is completely understandable how anyone can be confused when trying to determine the how to calculate the size and number of solar panels needed for a reasonable level of power. I am an engineer (not electrical) by education. I have been in the commercial construction industry for over 15 years. I have surfed and reviewed all of the same web pages looking for a simple formula, equation, or calculator. And I still get confused!!! That’s because most the formulas online are cumbersome and confusing, especially for someone without an engineering back-ground. And the calculators I have stumbled a crossed do an “OK” job, but they all end up tying the energy output to some ridiculous estimate for an installed system, usually upwards of $20,000 – $30,000. And I challenge anyone to find a decent solar calculator that does not try to push a sale or a ludicrous estimate when used. These calculators also need to make assumptions for weather conditions, costs of installation and accessories (i.e. roof mounting, etc.) and must make additional assumptions on the efficiency, cost, and type of panels selected, so there accuracy can vary greatly. Therefore, I have done a lot of research and will attempt to simplify these calculations in order to assist those who are considering supplementing their energy consumption with solar power (and, believe me, you’ll be glad you did). *Note – We are going to interchange a lot of terms power, energy, and electricity to simplify things (much to the disapproval of my college Physics Teacher, Mr. Willard.) 1. To calculate your energy consumption for individual appliances or groups of electrical devices: First, you will need to know the individual power consumption of those devices. The power consumption of appliances is given in watts (for example, 100W incandescent light bulb). To calculate the energy the device or appliance will use, just multiply the power consumption by the time (in hours) of use. For our example, let’s use the 100W incandescent light bulb (assuming that you have neglected to replace it with the much more energy efficient compact fluorescent variety) that is left on for 2 hours. This example is calculated as follows: 100 Watts x 2 hours = 200 Watt-hours, or 200Wh. In order to calculate the total load from a group of devices, you would need to add them all up in this fashion, considering power usage, duration of use, and number of devices. 2. To calculate energy consumption for your entire home (suggested): If you are looking to supplement as much of your energy consumption as possible or even all of it, you need to figure out your average daily energy consumption. Instead of adding up all of the devices in your home, pull out your actual monthly electric bill. Either on the back of your paper bill or on through your on-line account, find your month electrical usage. Based on my actual bill, it is as follows: Period Service Days Billed Usage (KWH) Amount Nov 29, 2012 – Jan 2, 2013 Residential Electric 34 666 $ 105.24 Oct 29, 2012 – Nov 29, 2012 Residential Electric 31 476 $ 76.26 Sep 28, 2012 – Oct 29, 2012 Residential Electric 31 432 $ 69.86 Aug 30, 2012 – Sep 28, 2012 Residential Electric 29 584 $ 92.13 Aug 1, 2012 – Aug 30, 2012 Residential Electric 29 704 $ 110.90 Jul 2, 2012 – Aug 1, 2012 Residential Electric 30 1296 $ 203.24 Jun 4, 2012 – Jul 2, 2012 Residential Electric 28 914 $ 145.61 May 2, 2012 – Jun 4, 2012 Residential Electric 33 581 $ 78.63 Apr 3, 2012 – May 2, 2012 Residential Electric 29 342 $ 58.05 Mar 2, 2012 – Apr 3, 2012 Residential Electric 32 426 $ 69.57 Feb 1, 2012 – Mar 2, 2012 Residential Electric 30 453 $ 73.39 Jan 3, 2012 – Feb 1, 2012 Residential Electric 29 481 $ 78.54 Totals 365 Days 7355 KWH $ 1,161.42 To calculate your average daily electrical usage, just divide the total Kilowatt-hours used by the total days in the year, or 365, so: 7355 KWh / 365 Days in a year = 20.15 KWh/Day And now you know how much electricity, on average, you consume in one day of the year. Use this number to target your solar panel needs and to implement energy saving strategies around your home. 3. Find out how many usable hours of sunshine are available (insolance) in your geographical region In order to estimate solar energy potential, you will need to find a solar insolation map, just like the one in this link en.wikipedia.org/wiki/File:NREL_USA_PV_map_lo-res_2008.jpg. In plain English, this number (with the units of kWh/m^2) simply tells you how many usable hours of sunlight your region gets per day. Your next step is to take the daily kilowatt-hours desired and divide it by the average usable hours of sunlight from the insolation chart. This is basically saying how do we get the desired days worth of electricity from only “x” hours of usable sunlight (I had to add that just to keep myself straight too!). You will also need to multiply that number by an efficiency factor of around 1.15 to take into account the inefficiencies from wiring, charge controllers, batteries, and inverters. So, for our example, I live in an area of Michigan where the average daily usable hours of sunlight is around 4 hours, based on the insolation map. I want to recover around half of my electricity from my solar panels, so the watts needed from the solar panels need is calculated as: 20.15KWh/Day / 2 = 10.075 KWh/Day So say 10KWh or 10,000 watt hours per day would be half of my average daily electrical usage. 4. Find which and how many solar panels will you need Solar panels come in different types and ratings, and the costs can vary depending on the type selected. As expected, the more efficient panels cost more that the less efficient panels. Fortunately, the power rating of all solar panels is also given in Watts. For our example, let’s use a pretty common solar panel and assume we have purchased one 130 Watt Mono-crystalline Photovoltaic (PV) Solar Panel that is rated for 12V Battery Charging. To calculate the energy it can supply, multiply Watts by the hours used. Note: It is common to allow for natural losses by the system for solar panel ratings, and good rule of thumb is probably around 90 percent. Your system may be slightly more or less efficient depending on the installation and application. However, we have already taken into effect losses from the system with our previous calculations, and our insolation number has already taken into account the amount of actual usable sunlight with rainy days, cloudy skies, etc. Any more and we are compounding losses we have already taken into effect for the balance of the system! So, for our lone 130W solar panel in 4 hours of usable sunshine based on our reading from the insolation chart, the calculation is as follows: 130W x 4 hours x 0.9 = 468 WH So, for this simple example, one 130 Watt Mono-crystalline Photovoltaic (PV) Solar Panel is going to provide a daily average of around 468 watt hours at my home in Michigan. In order to find out how many solar panels I need to supplement half of may average daily energy usage, the calculation is as follows: 10,075 Wh per day desired / 468 Wh produced by each panel per day = 21.5 solar panels So for this example I would need an array of around twenty panels to compensate for half of my electrical consumption, assuming I wouldn’t supplement with wind, or take energy saving measures on my own. I’ll be posting soon on how to get a better estimate of solar costs based on costs per watt of panel as well as costs for some of the other factors, but you can look at a selection of panels under any of the solar categories in the meantime.
Posted on: Thu, 21 Aug 2014 08:01:42 +0000
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