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Prosine 2.5/3.0 Installation & Operation Guide 71
Section 7: Batteries
HOURS. This can then be easily converted to an estimate of the battery amp-hours that the appliance
requires:
BATTERY AMP-HOURS USED = AC WATT-HOURS / 10 (for a 12-volt battery), or
BATTERY AMP-HOURS USED = AC WATT-HOURS / 20 (for a 24-volt battery)
For example, a 100W light bulb that is used for 4 hours will use 400 watt-hours (Wh) and the inverter
will consume approximately 40Ah from a 12V battery, or 20Ah from a 24V battery.
Another useful rule of thumb is that the current drawn from the battery can be estimated from the AC
output watts by using these same factors (10 for 12V systems, 20 for 24V systems). For example,
when running an 800W microwave oven, the inverter will draw approximately 800 divided by 10 =
80A from a 12V battery.
Motors are normally marked with their running current rather than their starting current. Starting
current may be three to six times running current. The manufacturer’s literature may provide more
accurate information than the motor nameplate. If large motors will be started, you may need increase
the battery size to allow for the high start-up demand.
Battery Bank Sizing Example & Worksheet
The following battery sizing example illustrates a typical calculation, assuming an opportunity to
charge the batteries every three days:
Table 3: Battery Sizing Example
This example illustrates how quickly your battery needs can escalate. To reduce the required battery
bank size, you can either conserve energy by eliminating or reducing the use of some loads, or re-
charge more frequently.
Appliance (A)
Power
Consumption
(B)
Operating Time per
day
Daily watt-
hours needed
for this
appliance
( = A x B )
TV & VCR 200 W 2 hours 400 Wh
Small microwave oven 800 W 15 mins = 1/4 hour 200 Wh
3 lamps, 60W each 180 W 4 hours 720 Wh
Coffee maker 600 W 15 mins = 1/4 hour 150 Wh
Hair dryer 1500W 6 mins = 1/10 hour 150 Wh
Total Daily Watt hours of AC load: 1620 Wh
x Number of Days between charges: 3
= Total Watt hours of AC load between charges: 4860 Wh
Battery Ah used between charges
(divide by 10 for 12V system, divide by 20 for 24V system)
486 Ah
Recommended Battery Bank Size in Ah (multiply by 2) 972 Ah