Outback Power Systems FLEXpower ONE One Fully Pre-Wired Single Inverter System Installation Manual - Manual (Page 56)

Specifications
54
900-0095-01-00 Rev A
Discharge Rate
Deep cycle batteries express the amp-hour rating as "at the x-hour rate". This is an average rate of
current flow that would take x number of hours to discharge the batteries. Common amp-hour figures
are at the 6-hour rate, the 20-hour rate, and the 100-hour rate. A battery is classified as having fewer
amp hours if it is discharged at a faster rate, such as the 6-hour rate. There is an inevitable amount of
heat associated with the flow of current through a battery. The higher the amount of current, the
greater the amount of heat generated. The heat is energy which is no longer available to the battery
to power loads. Hence, at a higher discharge rate, the batteries effectively have fewer amp-hours
available. Generally, the 20-hour rate is the most common one.
Estimating Amp Hours
To estimate the battery bank requirements, first calculate the amount of power to be drawn from the
batteries during the 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 from a 12-volt battery system.
8.33 = 100 /12
If the light runs for three hours it will consume (8.33 x 3) or 25 Ah of power.
IMPORTANT:
For these calculations, do not use the AC amp rating of a device. AC amps are
measured on a different scale and will not give correct results.
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.
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 AC amps, multiply by volts for the watts
required (watts = volts x amps).
Things to consider:
 Motors typically require 3 to 6 times their running current when starting. Check the
manufacturer’s data sheets for their starting current requirements. If you will be starting large
motors from the inverter, increase the battery bank size to allow for the higher start-up current.