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DIY PV System --
Deciding On What Type of System to Install
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System page...
Deciding What Type of System to
Install
Grid-Tie vs Off-Grid vs Grid-Tie
with Batteries
There are basically three types of PV
systems in common use:
Off-Grid Systems:
Off-Grid systems are independent
of the utility power grid. They generate electricity, store it, and
make it available for use without any connection to the power grid.
Off-Grid systems use PV panels
connected to a Charge Controller to charge a set of batteries. The
stored energy from the batteries is usually converted to regular 120 volt, AC power
by an inverter. But, some small systems just use the DC power directly
from the batteries.
Grid-Tied Systems:
Grid-Tied systems use PV panels
to generate DC power. The DC power goes to a grid interactive (grid-tied) inverter which converts the PV panel DC power to 240 volt AC power
that is compatible with the power grid. The power from the PV panels
goes (via the grid tie inverter) to supply the household power needs.
If the PV system is generating more power than the house can use, the excess
is sent out over the grid to supply others. If the house needs more
power than the PV system can supply, then the extra is drawn from the grid
as usual.
Grid tied systems only work when
the grid is up. If the grid power goes out, the grid tie inverter is
required to shut down immediately.
Grid-Tied with Batteries:
A grid tied with batteries system is
kind of a mix of the two system. It basically operates like a grid
tied system when the grid is up, but it also charges a set of batteries.
If the grid goes down, the inverter disconnects from the grid (to protect
line workers), but it continues to supply power to the house from the
batteries and inverter -- essentially behaving like an off-grid system when
the grid is down.
Here is a run down on the pros and
cons of each system:
Grid-Tie:
Pro:
- Lowest initial cost (because
there is no need for batteries and charge controller)
- Lowest ongoing maintenance cost
(no batteries to maintain and replace)
- Simplest to install
- Most efficient (because there
are losses associated with charging batteries)
- You can start small and add
(with some limitations)
Con:
- No power when the grid is down.
- Access to the utility power grid is
required.
Off-Grid:
Pro:
- Provides power independently of
the utility grid -- you still have power when the grid is down.
- Don't have to deal with the
utility company
- Can save significantly on
initial cost if a long grid extension is needed to get to your house
- Really encourages conservation
and efficiency in the use of electricity -- off-grid people typically get
along on far less power than on grid and do it without any significant life
style changes.
Con:
- Higher initial cost (needs a
set of batteries and charge controller)
- Higher ongoing time and cost (the cost
of batteries over time is significant -- some say its about as much as
buying grid power)
- A good generator will likely be
necessary from time to time
- The system must be large enough
to supply your full power needs during the lowest sun part of the year
(although a generator can be used to supplement during the worst times).
- You are the power company, and
responsible for safe and reliable operation, and maintenance.
Grid-Tie with Battery Backup:
Pro:
- Provides backup power when the
utility grid goes down.
- Provides some of the advantages
of both systems.
Con:
- Still has a battery system
(usually smaller) that is expensive to buy and maintain.
While I would like to have tried an
grid-tie system with batteries, our conclusion was that it was a lot of extra
bother and inefficiency just to have power during the very few times our grid
power is down. Even though we live in a somewhat rural area, our power has
probably been off only at total of about 10 hours over the last 10 years.
We have a small generator that supplies enough power to run the fridge and a few
small things. It seems like most of the time when we have a power failure,
by the time I get the generator out and gassed up, the power has come back on.
While I would not mind the maintenance
involved in keeping a set of batteries healthy, the fact that the battery set
pretty well wipes out any savings the system would produce and caused it to
operate less efficiently made tipped the scales toward the grid-tie.
So, this write-up concentrates on our
grid-tie system, but here are some examples of the other systems from the PV
section:
Some
off grid systems...
Some grid-tie with battery systems...
Some
other grid-tie systems...
A side note:
It is a fact that people who live
off-grid generally use a lot less power than people who live on grid. I
guess when each KWH you use has a large cost initial system cost, and when the
consequences of using too much power is that you run out of power it makes you
very conscious of power consumption. Power becomes a precious resource
that you learn to use with care. The interesting thing is that this
typically does not mean any significant changes in lifestyle -- it just means
paying attention to things that use electricity, improving efficiency, and
eliminating waste. If we could somehow get everyone to live as though they
were off-grid, I am convinced it would cut residential power consumption to less
than a third of what it is now. There must be some good way to do this?
Any ideas?...
Gary November 20, 2009
