FUEL GENERATORS AS INTEGRATED
INTO A HOME POWER SYSTEM
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Many of our customers, particularly in the north, utilize a propane (or
other fuel) generator 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 from a PV
system—then 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 turn-key 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 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 and unfortunately there
isn’t a “one size fits all” answer.
There are many factors that should be considered before purchasing a
generator: things like if 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), if you have other
charging sources like solar/wind, how long/often you want the generator to
run and if 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 derating 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) and 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
go with multiple inverters (or separate chargers) to increase your
charging rate. To determine how many hours of generator run time will be
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 inverters’ maximum DC charging rate. The reason for using only half
of your inverter’s maximum charging rate is 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 a dual Outback
VFX3648 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 VFX3648 inverters is 45A each for a total of 90A which is ~21%
of the 420 amp-hour capacity of the battery bank so that 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 inverters maximum DC charging rate 45 (90/2). This gives us an
estimated generator run time of 4.7 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. There, clear as mud right?
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 would
like 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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