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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 last long!
  • 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 short time.

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.

The lesson 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.

Happy cruising!

Related sites:
www.Battery-Load-Calculator.info Determine your optimum house battery bank size.
www.Marine-Alternators.info How to select the appropriate Marine Alternator
www.Marine-Desalinators.info How to get water from the sea!
www.Marine-Generators.info AC power onboard your yacht is no longer a luxury!