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Many of our customers, particularly in the north,
use propane or
other fuel generators to assist in charging their batteries in low winter
light conditions, or in conditions of little wind. Many who are just
building their homes have the need to run large power tools like welders
or saws—loads that wouldn’t be cost effectively powered by a PV
system. Upon completion they can add solar or wind power to lower or
eliminate their generator’s run times. When a generator is teamed with an
inverter and battery bank, system efficiencies increase and less fuel is
used. These can be designed to be turnkey enough that, when your
batteries reach a certain low voltage set point, the inverter senses this
and turns on the generator to run the AC loads while it’s charging the
batteries.
When the batteries are fully charged, the inverter turns the generator off
and the loads continue to run off the batteries via the inverter. When
the voltage falls, the cycle starts again. This keeps the generator from
running 24/7, or from running inefficiently just to power small loads.
We don’t claim to be generator experts, but the inverter systems that we
sell have to work with them, so we know how to size them and what features
to specify. A lot of people ask us what brand and size generator they
should use with their battery/inverter systems, but unfortunately there
isn’t a one-size-fits-all answer.
There are many factors that should be considered before purchasing a
generator, such as: whether you need 240 VAC power for large loads (shop
tools, well pump, etc.); the voltage/amp-hour capacity of your battery
bank; the maximum charging rate of your inverter(s); other
charging sources like solar/wind; how long/often you want the generator to
run; and whether you want/need the generator to start automatically.
A rough rule of thumb for sizing a generator for your inverter(s) is to
get a generator with a power rating (after de-rating for altitude and fuel
type) that is at least 1.5 to 2 times larger than your inverter power
rating. The reason for this apparent oversizing is not to make more money
for the generator dealer, but because the generator is often charging the
battery bank and running your AC loads at the same time. If the generator
is too small, it won’t be able to do either job very well.
If you want to find out how deeply you will discharge your batteries each
day and how often/long you will have to run your generator to recharge
them, you first need to calculate your average daily energy consumption in
amp-hours or watt-hours. With this information and the amp-hour capacity
of your battery bank, you can also determine how many days of storage your
battery bank will provide between generator charging cycles. Next, look at
the maximum DC charging amperage of your inverter(s); if it is less
than 20% of your battery bank’s amp-hour capacity (at the 20 hour rate)
then you should consider using an inverter with a more powerful charger or
going with multiple inverters (or separate chargers) to increase your
charging rate. To determine the hours of generator run time required to fully charge the battery bank from its deepest depth of
discharge, you should divide your battery bank’s 20 hour amp-hour capacity
by 2 (to account for a 50% depth of discharge which is the lowest
point we recommend you discharge your battery bank) and then divide
that by half of your inverter's maximum DC charging rate. We use
only half of the inverter’s maximum charging rate because inverters don’t charge
at their maximum rate over the entire charging cycle. Reducing the
charging rate to half of its maximum value will give you a more realistic
estimate of your generator run time.
Let’s run through an example so you can better understand what the
heck we are talking about in the paragraph above: Say you have an
Outback Radian 4048 inverter system with eight L-16H 6V 420 amp-hour
batteries and an estimated daily energy usage of 4,000 watt-hours.
The maximum charging rate of the FM80 charge controller is 80A,
which is ~19% of the 420 amp-hour capacity of the battery bank. This
is a good match. The total watt-hour capacity of the battery bank is
48V x 420 amp-hours, which equals 20,160 watt-hours. If we only want
to draw the batteries down to the half-way point (50% DOD), then we
only have 10,080 watt-hours to work with. If we divide 10,080
watt-hours by our daily load of 4,000 watt-hours, we find that the
battery bank will provide 2.5 days of storage at this rate of
discharge. This is a little on the low side since we like to design
systems to have 3-5 days of storage in the battery bank. To
calculate the hours of generator run time required to replenish the
battery bank from the 50% depth of discharge point, we take half of
our battery’s amp-hour capacity 210 (420/2) and then divide that by
half of the chare controllers maximum DC charging rate 80A. This
gives us an estimated generator run time of 5.25 hours, which isn’t
too bad if you only have to do this every 2.5 days. Of course if you
have some large AC loads on while the generator is charging the
battery bank, the inverters might reduce their charging rate to
support the AC load, and this will lengthen the generator run time.
Clear as mud, right? Another consideration about
which generator may be best suited for your project is the elevation
at your site. The higher up you go, the thinner the air is, and it
affects the gas-air mix of your generator. So, a generator needs to
be derated at high elevations, approximately 5% for every 1000 feet
(above 1000 feet above sea level). For example, if you have a
residence at 8000 ft. elevation, you would need to derate your
generator about 35%. A 10KW generator will only be able to give you
about 65% of its rated power at 8000 ft. elevation; therefore you
must keep this in mind to be certain you will still have enough
power for your loads and battery charging at higher elevations. Wind
turbines need to be derated in a similar fashion. If you think you
need a 10KW generator, but you’re at 8000 ft. elevation, you will
really need one sized at about 13 – 14KW.
Whatever brand or size generator you are considering, we recommend that
you purchase a low-speed 1800 rpm generator vs. a 3600 rpm unit because it
will last longer. A remote 2-wire start capability is a plus if you want your inverter system to automatically start your generator if the
battery voltage is low or if a large AC load kicks on. Fuel choice depends
on what is readily available and least expensive at your site. Diesel fueled generators typically last longer than gasoline, NG, or LP fueled
units, but they are much dirtier, and you need to worry about the fuel gunking up if it is stored for a long period of time. A lot of our
customers go with LP fueled generators because they have other heating
loads that require LP, and it makes sense to go with one fuel for all your
needs instead of two or more. We don’t sell generators so we don’t
recommend a particular brand or model over others. The feedback we get
from our customers indicates that Kohler, Onan, Kubota and Honda
generators are very reliable. Generators do require periodic maintenance
and overhaul, so it makes sense to purchase one from a local dealer that
can provide that kind of service. For a basic diagram of a generator
inverter system, see www.oasismontana.com/gen-verter.html
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