The Battery Bank Sizing Guide from Xantrex is a very
useful tool to determine the size of your inverter and battery bank.
However, some additional information is always helpful. Here is what I
have learned from personal experience:
- Your inverter should be sized based on the
total simultaneous load to be applied (including motor start loads)
- Your battery bank should be sized based on your
total daily amp-hour demands (inverter and DC loads)
- Your battery bank should never be discharged by
more than 50% of it's rated amp-hour capacity, or your batteries won't
- Your battery bank will be TOO SMALL!
My reason for this harsh assessment is based on
the fact that static (unloaded) battery voltage is entirely different
from dynamic (loaded) voltage. This is due to internal resistance of
batteries, and the surface are of the plates. The voltage supplied by
any battery is reduced as the current draw increases. Additionally, the
amp-hour capacity of any battery drops as the current load increases.
There is one more important
factor for battery bank sizing:
- Your battery bank must be sized based on the maximum expected current draw, and
depends on the battery type. The maximum charge/discharge rate for
various deep-cycle battery types is:
- Traditional lead-acid batteries: 20-25% of amp-hour capacity
- Gel cell batteries: 30-35% of amp-hour capacity
- AGM (absorbed glass mat) batteries: 35-40% of amp-hour capacity (check your cable sizes!)
As you can see, the battery type can make a big
difference in battery bank sizing based on maximum current load. Try
out our Marine Battery
Load Calculator to help determine your ideal battery bank size.
A fully charged starting battery loaded at half
its rated CCA capacity (load test current) will only put out about 9.7
VDC at 80°F. The voltage of a deep cycle battery will drop even
more under similar load conditions. Keep in mind that most inverters
will trip off to protect the batteries when their voltage drops to 10.5
V or less.
Xantrex provides a fine example of using a
circular saw that uses 1500 watts of power. Their example indicates
that the saw would only use 2 amp-hours at 12 volts if run for one
minute. (Xantrex's example ignores the inefficiency of the inverter).
The point made by Xantrex is that while the saw uses 1500 watts, the
total run time is short, so it uses very few amp-hours. If your battery
bank is sized based on using this current draw for just a few minutes
at a time, you appear to be in good shape. This may not be the case.
My point is that the
saw uses 1500 watts!
A 2000 watt inverter producing its rated output
will draw about 2222 watts (at 90% efficiency) from the battery bank.
At 12 volts, the current draw is 185 amps. Using the rules at left, you
would need a 1500 amp-hour bank of deep-cycle batteries! A battery bank
with 400 amp-hours capacity doesn't stand a chance of supporting a 2000
watt inverter load without help, but it can be done.
Don't let me scare you away from an inverter just
because the battery bank sizing can be complicated. A modestly sized
battery bank will work great to power an inverter under most
conditions. If you have an occasional need for high-wattage power, you
can meet the current demand by simply running your engine - even if
your alternator is not rated for the total current draw. The current
supplied by your alternator will make your battery bank appear much
larger to the inverter, and be enough to run a substantial load for a
In the circular saw example above, your power
supply (battery bank and alternator) needs to provide 138 amps of 12
volt DC current while the saw is running. This can be accomplished with
a 400 amp-hour battery bank assisted
by a 100 amp alternator.
learned is that a large inverter is a very useful piece of equipment.
Your battery bank does not need to be huge if your highest loads are
only used occasionally. Base your battery bank size on the loads that
will run on a regular basis.