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Solar Electric Grid-Tied Systems Introduction

The most commonly installed solar system is a solar electric system interconnected with the local power grid, often called a grid-tied solar system. This type solar system has become the most popular mainly due to a 30% federal tax credit and solar rebates available from most utilities in Colorado and many other states (See our incentives page).  These systems are automatic and maintenance free.

One downside to most grid interconnected solar systems is that during a grid blackout most grid-tied solar systems will not produce power even if the sun is shining.  Although batteries can be used in a grid-tied system, the inverters and batteries generally add many thousands of dollars to the cost of the system. 

The vast majority of people simply store the electricity produced by their solar system in the grid, although people with an unreliable utility provider, or who want to be self-sufficient, may want to invest extra money in a battery based grid-tied solar electric system.

System Sizing

A grid tied solar electric system that is sized to produce most to all of a customer's electric usage (dependent upon installation space and budget of course) is fairly common.  The easiest way to size a system that is appropriate for your needs is to contact your utility and request some electric usage history information. 

To be as accurate as possible, the information you will need is the average kWh (kilo-Watt hours) used in the past year.  It is important to include all seasons because a solar system will produce different amounts of energy at different times of year and likewise most customers use different amounts of energy at different times of year depending on the house loads. 

Our pre-designed systems on our price list include average production numbers (in KWH perday), so once you have your average usage it is easy to match a system size to meet your needs.  Please contact us for the latest Rocky Mountain Solar & Wind, Inc. Pre-designed System Price List at info@rockymtnsolar.com

True Net Metering

Some people may ask how a grid connected solar system is able to power their whole house without batteries if the system is only producing energy during the day when the sun is shining.  The key to making it work is net metering.  With a grid connected solar system, the electrical grid becomes your storage for excess energy.  When the solar array produces energy, it back feeds that energy through a circuit breaker into your electric service panel.  Some of that energy goes to power the loads in the house that are operating at that time.  The rest of the energy feeds back into the grid, causing your electrical billing meter to spin backwards, and essentially is banked for later use.  This allows the customer to effectively offset all the electrical energy they use no matter what time of day it is and without having to invest money and time into a battery system. 

If the system is sized to offset near 100% of the customer's usage, the total usage in a year and the total system production in a year should Net out to around zero.  In a given billing period, if the customer had more system production than electrical usage, then the extra energy amount is carried over to the following month.  The production to usage ratio is re-evaluated once per year by the utility.  At the end of the year they check to see if the customer produced more energy total in the year than they used.  At that point the utility zeros out any existing credits on the account.  If there is more production than usage at the end of the year, the utility will usually write a check for the difference at wholesale rates.  It is designed this way to allow customers to easily offset the energy they use, but also not to create extremely small energy producers/sellers.  In other words, designing a solar system larger than your needs does not exactly create a money making machine.  

Optimal Site Conditions

There are a variety of installation options available for a grid-tied solar electric system.  The most common is to install the system flush mounted (the same angle as the roof) onto a roof face.  However it is also possible to tilt the modules in a variety of directions, or to install the system on the ground (either on top of a pole, or using a ground rack).  It is recommended that you have a solar professional make a visit to the proposed installation site and fully evaluate the placement options and design for your specific situation. South is the ideal direction for all types of solar collectors to be facing.  This is because as the sun rises in the East and sets in the West; the solar gain is greatest at mid day, or around solar noon, when the sun is South of your location.  During standard time 9AM - 3PM are considered peak hours (during daylight savings time from 10:00AM - 4:00PM) because these times are when the sun's radiation is the most intense and the angle of incidence to the modules is the least.

Angle or pitch of the solar modules is also important for optimalproduction; a good rule is to install the modules at the angle of your latitude for highest year round production (in the Colorado Springs area that is about 39 degrees).  Usually this only becomes a factor if the proposed installation area is flat.  If installing is onto a pitched roof the typical roof angles (18 degrees - 45 degrees) are usually close enough. Shading is another factor that can play a major role in system production.  Ideally a system would have zero shading all day long at the installation site.  It is more common to have a small percentage of shading usually from trees.  To make a solar system viable, usually the peak hours (9AM - 3PM) must be free of shading.  If the shading at your site is significant, it could reduce not only your production but also the rebate money you receive back from the utility (see our incentives page for rebate eligibility, or contact your local utility). 

It is best to have a solar professional use a tool such as the Solar Pathfinder to evaluate the shading at the proposed installation site.  This tool is able to identify shading at the site for all times of the year, and the sun does not have to be shining to get accurate results.

Roof Types

It is recommended to install a solar system over fairly new and/or long warranty roofing.  This is because it can be expensive to disassemble a solar system so that a re-roofing project can be done and then to re-install the solar system.

It is best to have an existing roof warranty of at least 20 years.  If your roof is near the end of its warranty, it would be a good idea to investigate a re-roofing project before installing the solar system.  Be sure to talk with your solar professional during this process because often times during a re-roofing project various vents can be relocated to better suit the solar array design.  In some areas if you have multiple layers of shingles on your roof, the extra weight from a solar system may have to be evaluated by an engineer.  

A solar system can essentially be installed on top of any kind of roofing material.  However there are certain kinds of roofing that can make an installation more difficult and thus more expensive.  In general, the kinds of roofing that lay flat to the sheeting (composite shingles, standing seam metal, etc.) are the easiest to install over and seal from leakage.  Raised roofing types (concrete, ceramic, or metal tiles) require more labor and materials in order to install a solar system properly.

Author: Lotus, President