When selecting a solar electric
back power system for your home or business, it is important to know
approximately how much power you will need to have available to
power emergency loads during a blackout. Unlike an offgrid solar
system which needs to replenish the amount of power consumed
with available sunlight within each day that the system is
operating. A backup power system need only supply power for the
anticipated duration of a blackout, which in most cases is only
a few hours. In fact in most cases, solar panels are not
necessary in such a system because the backup power system's
batteries can be recharged in anticipation of the next power
failure by using utility power once it has returned to normal
operation. The only advantage that solar panels would serve in
such a system would be in the event that a prolonged outage
(More than 24 hours) should occur.
In a typical backup power
system, batteries store the energy that is needed to power the
designated emergency loads for the pre-determine period of time.
Just like a small UPS (Uninterruptible Power Supply) for your
computer can supply power for 5 to 15 minutes allowing you time
to safely shut you computer off, a backup power system supplies
power but in this case for hours or even days, allowing you to
operate your home or business until the power has returned.
In order to select the
appropriate system for your backup power needs, it is important to
match your anticipated power consumption with your back up
system's battery bank capacity. To correctly size a system for your
home or business you must first determine the wattage of each item that you
wish to power during a power failure and also determine how long each item
will run during the power failure.
For example a 60 watt light
bulb that is used for 5 hours will consume 300 watt hours. Watts
multiplied by time is equal to watt hours. A microwave oven that
consumes 800 watts that runs for 15 minutes, consumes 200 watt
hours, 800 watts times .25 hours equals 200 watt hours.
So to correctly size a system, simply make a list of each item
that you intend to run. Next to each item write down it's power
consumption in watts and next to that write down the amount of
time that the item will run during the power failure, then multiply the watts of
the item by the amount of time that it will run and write that
number down in the last column. After you have calculated the
watt hour consumption for each item, simply add each item's watt
hour rating together and you'll have your total consumption.. For example:
Once we have this information,
it's a simple matter to match the number of batteries that you will need in order to store enough power for what you
will consume.
Choosing an inverter for your
backup power system
DC to AC inverters are
available as inverter units only, or may have additional
circuits added that allows them to charge batteries when an
external AC source is fed into the inverter. This type of
configuration is know as an inverter/charger. In addition to
the charger circuit, these units will typically include a
device known as an AC transfer switch.
The advantage to purchasing an
inverter/charger with transfer switch is that it can function
as a highly reliable automatic power backup unit or UPS.
When the utility company is operating
normally the inverter/charger passes the utility company power
through its internal transfer switch to your appliances and
maintains a charge on your battery bank. As soon as the
utility power fails, the inverter automatically stops charging
the battery bank and begins producing its own AC power which
is passed on to your appliances through its internal AC
transfer switch.
When the utility power returns, the
inverter goes back to charging the batteries and again passes
the utility power though the transfer switch to your
appliances. Most inverter/chargers switch from utility power
to inverter power and back again so fast that most of your
appliances will hardly miss a beat.
Sizing the wattage rating of an
inverter for your backup system is a simple matter of
determining the total number of appliances that you would
typically be operating on a concurrent basis, and adding a
buffer of at least 500 watts. In other words if there was a
possibility that you would have your 600 watt microwave, a 200
watts coffee maker and a 200 watt stereo running at the same
time, you would be drawing 1000 watts, then you should choose a
1500 watt inverter. An inverter should never be run at it's
maximum rating for prolonged periods of time, doing so will
shorten the life of the inverter.
Another issue to consider is
the amount of surge current that your appliances draws. Any
appliance that uses a transformer, motor or other magnetic
device draws what is known as surge current at startup. These
devices are otherwise known as inductors. Inductors appose the
flow of electrical current.
When an inductor is first
energized there is a great degree of inertia that must be
overcome for the magnetic field which surrounds the inductor to
reach it's maximum field. Just as it's difficult to initially
push a car by hand that is at rest and gets easier to push as it
gets going. Initially starting an inductor takes a great
deal of current to get it started but backs off on the current
after it gets going.
Devices such as microwave
ovens, refrigerator compressors, fan motors and large
transformer based appliances can draw from 3 to 6 times it's
normal wattage in an initial surge of current. This initial
surge of current typically only lasts milliseconds but it's
enough to shut down an inverter if it's not sized properly. Thus
it's important to choose an inverter that has enough surge
capacity to start such appliances. For example a meager 600 watt
microwave oven will typically require a 2000 watt inverter just
to get it started.
If all of this
information seems a little overwhelming, don't worry our
friendly knowledgeable staff are here to help you every step of
the way.
And finally, be cautious when
purchasing a backup power system for your home or place of
business over
the Internet. Get to know who you're dealing with. Many of the
backup power kits that are available on the Internet are actually
home made configurations. Many websites on the Internet that
would appear to be large reputable companies are actually home
based affairs that operate from an impossible to trace POB
(PO Box). Remember,
you're about to give this individual your personal information
and more importantly your credit card number.
Is his company solvent ? Does
he have liability insurance ? Does he really have the
items that you're about to purchase in stock ? Does he
have any stock ? With the advent of the energy crisis,
dozens of home based dealers with little or no formal training
or experience have cropped up on the Internet.
Even if you don't live nearby, ask the dealer if you can get
directions to his place of business so you can stop by and take
a look at some products. If you can't get directions or a
straight answer from him, then in our opinion, steer clear !
It's important to remember that it takes only minutes to upload
a website to the Internet and only seconds to take it down.
If you
would like to learn more about protecting yourself when shopping
on the Internet
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