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8
Sizing the Battery Bank
To determine the minimum battery bank ampere-hour rating that
you will need to operate appliances from the inverter, and any DC
appliances powered by the battery bank, follow these steps:
1. List the maximum wattage that the inverter has to provide (as
above).
2. Estimate the number of hours the appliances will be in use
between battery recharges. This will differ depending on
appliances. As an example, a typical home-use coffeemaker
draws 500 watts during its brew time of 5 minutes; it maintains
the temperature of the pot at about 100 watts. Typical use of a
microwave oven is only for a few minutes. Some longer
operating time appliances are lamps, TVs, computers and
refrigerator / freezers.
3. Determine the total watt-hours of energy needed. Then multiply
the average power consumption in watts by the number of
hours of run time. For example: 900 watts for 10 hours = 9,000
watt hours. Using the 900 watts (or 90 Amps) for 10 hours
example as above, then 90 amps is needed for 10 hours. This
provides us with the basic amp-hours (AH) of battery that is
required. Ten hours at 90 amps equals 900 Amp Hours (AH).
This answer is just a beginning because there are other
conditions that determine actual run time. These include:
• AC appliance load and time in use (basic Amp Hour)
• Cable gauge and length (cable losses)
• Charge level of the batteries (between use, chargers have
to be able to fully charge the batteries)
• Temperature of the batteries (colder batteries provide fewer
amps)
• Age and condition of the batteries (older batteries lose
capacity/amp hours)
• Compliance with turning off unnecessary AC and DC loads.
If there is any doubt about sizing the battery bank, it is safe to
over estimate the amp hour requirements of the battery bank.