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Battery Bank Sizing
975-0012-01-02 Rev A B–5
Back up power systems which use utility power for recharging should use the
estimated number of days of maximum power outage for determining days of
autonomy.
Understanding Amp-hour Requirements
Amp-hours To estimate the battery bank requirements, you must first calculate the amount of
power you will draw from the batteries during your period of autonomy. This
power draw is then translated into amp hours (Ah)—the unit of measure to
express deep-cycle battery capacity.
Amp hours are calculated multiplying the current drawn by the load by the length
of time it will operate.
Watts to amps To calculate amps when the power consumption is expressed in watts, use the
following equation:
A = W/V
where W = watts and V = volts DC
For example:
A 100 watt light bulb will draw approximately 8.33 amps
8.33 = 100 /12
If the light runs for three hours it will consume (8.33 x 3) or 25 Ah of power.
Time and power The length of time a load is operated will affect the power draw. In some cases, an
appliance which draws a large wattage may not consume as many amp hours as a
load drawing fewer watts but running for a longer period of time.
For Example:
A circular saw draws 1500 watts or 125 amps. It takes 5 seconds to complete
a cross cut. Twelve such cuts would take a minute and you would consume 125
A x 0.016* hour = 2 Ah
*1/60 = 0.016
Observation The circular saw, while it draws more power, consumed fewer
amp hours of electricity because it ran for a short period of time.
Calculating Amp Hours
Calculations To determine the amp hours you will consume, you need to list your anticipated
loads and the length of time you will operate each one. Determine the number of
hours per day and the number of days during the week you will use the appliance.
For example, you use the microwave every day, but a breadmaker only once a
week. If you use an appliance for less than an hour, express the time as a decimal
portion of an hour.
Amps to watts All electrical appliances have labels which state their energy consumption. Look
for an amps rating on motors and a watts rating on other appliances.
If the label plate has expressed power consumption in amps, multiply by volts for
the watts required. (watts = volts x amps).