8
Battery Sizing
Unfortunately, there are a number of different standards for rating battery energy storage capacity. 12 volt auto-
motive starting batteries are normally rated by cranking amps. This is not a relevant rating for continuous use.
Deep-cycle batteries are rated either by reserve capacity in minutes or by ampere-hour capacity.
Battery reserve capacity is a measure of how long a battery can deliver a certain amount of current - usually 25
amperes. For instance, a battery with a reserve capacity of 180 minutes can deliver 25 amperes for 180 minutes
before it is completely discharged.
Ampere-hour capacity is a measure of how many amperes a battery can deliver for a specified length of time -
usually 20 hours. For example, a typical marine or RV battery rated for 100 ampere-hours can deliver 5 amperes
for 20 hours (5 amperes x 20 hours = 100 amp-hrs).
Actual battery capacity decreases as discharge current increases. A battery rated at 100 ampere-hours, which can
deliver 5 amperes for 20 hours, may deliver 20 amperes for only 4 hours, resulting in an actual capacity of 80
ampere-hours. For this reason, it is difficult to compare rated ampere-hour capacity with battery reserve capacity.
For example a battery with a reserve capacity of 180 minutes has the following calculated ampere-hour capacity:
180 min.÷ 60 = 3 hr., 3 hr. x 25 amps = 75 amp-hrs
However its actual ampere-hour rating will be closer to 100 ampere-hours because it is rated at the discharge cur-
rent required to get 20 hours of operation (about 5 amperes).
To determine the battery capacity you require, follow these steps:
STEP 1 For each piece of equipment you will be operating from the PROwatt 1500, determine how many watts
it consumes. This can normally be found on a label on the product. If only the current draw is given,
multiply the current draw by 115 to get the power consumption in watts.
STEP 2 For each piece of equipment you will be operating from the PROwatt 1500, estimate how many hours it
will operate between battery charging cycles.
STEP 3 Calculate total watt-hours of energy consumption, total hours running time, and average power con-
sumption as in the following example:
Equipment
Power Consumption
Operating Time
Watt - Hours (Power x Operating Time)
TV & VCR
115 watts
3 hours
345
Sewing Machine
150 watts
1 hour
150
Waterpik
90 watts
0.25 hour
22.5
Blender
300 watts
0.25 hour
75
Coffee Maker
750 watts
0.3 hour
225
Coffee Grinder
100 watts
0.l hour
10
Microwave Oven
1500 watts
0.5 hour
750
Totals
5.4 hours
1577.5 watt-hours
