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Green Collar Careers - Grid Tied and Off Grid

The main difference between a standard electrical inverter and a grid-tie inverter is that the latter also ensures that the power supplied will be in phase with grid power. This allows individuals with surplus power (wind, solar, etc) to sell the power back to the utility in the form of net metering or the arrangement your local utility offers.

A grid tie inverter also must disconnect from the grid if the utility grid goes down (blackout). In the event of a blackout, the grid tie inverter will shut down to prevent potentially harming the line workers who are sent to fix the power grid.
 

Alternating Current (AC) alternates in polarity;  that is the same wire will alternate between +120 volts and - 120 volts, 60 times every second.  As most power is generated from an alternator, there is a shape or wave form referred to as sinusoidal or sine wave for short. Synchronizing the inverter AC output with the utility AC requires developing a wave form with the same shape and timing as shown on the right.  This is a must for grid tied systems.

Some earlier and lower cost inverters produce a series of steps that simulate a sine wave but not exactly the same shape and are considered "dirty power".  While these units can be synchronized from a timing standpoint, they don't meet any utility requirements for inverters. Dirty power also shortens the life of electronics with coils; motors and transformers in power supplies like TV's and PC's.

Nearly all utility companies require an inverter that they have previously approved.  To identify inverters approved for the utility your customer is using, search through the Database of State Incentives and Renewable Energy and find the utility service.  There should be a link to their requirements for Grid Tied systems.

Off Grid systems only generate electricity when the sun is shining.  Moreover with a fixed mounted system about 3 hours before or after solar noon solar PV production drops off significantly.  It may be worth considering a tracking system depending on the application and demand cycles.

Off Grid systems are generally augmented with batteries and a back up generator.  Batteries are not the ideal storage solution particularly in colder temperatures;  at 20° F. the capacity of a battery is about half of what it is at 70° F.  Sealed batteries offer low maintenance but they are more costly.  Typically a deep cycle batter will loose half of its capacity in 800-1000 charge/discharge cycles where the discharge cycle never drops below 50% reserve power.  Drop that to 20% reserve and the its about 400 cycles to 50% capacity loss.  Every battery is a little different but that's generally what to expect.

Grid Tie System
Off Grid System