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
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.
(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
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
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