Determining and Understanding the Electrical Power Requirements

for Your Boat
 

In order to determine the ampere draw of your vessel, you need to know what electrical equipment you have and what the 12-volt amp draw is. The following is a listing of typical 12-volt equipment aboard most boats and their average amp draw per hour.
 

12-VOLT ITEM---AMP DRAW
Bilge Pump (500 gph) 2.0
Bilge Pump (1000 gph) 2.9
Bilge Pump (1500 gph) 4.9
Bilge Pump (2000 gph) 8.4
Navigation Lights (3 mile) 1.5
Live Well Pump 7.0
Fresh water Pump 4.0
Refrigerator (12v) 6.0
Ice Maker 6.0
Head (Macerator) 9.0
Anchor Windlass (900Ib) 75
12v. House Lighting. . .. 0.15 per 10watts
Spot Lights (1OOk cp) 8.0
Spreader Lights (3k cp) 3.0
RADAR (24 mile) 5.0
GPS 0.8
LORAN 0.7
VHF Radio - transmit 6.0
VHF Radio - receive 0.5
Depth Sounder (LCD) 1.0
Depth Sounder (Color) 3.0
SSB - Transmit 3.0
INVERTERS*
SSB receive 2.5
Autopilot 5.0
Stereo (50 watt) 0.5
Fan .... . . . .1.0
TROLLING MOTORS (12 VOLT)
24lb Thrust 27
30lb Thrust 30
36lb Thrust 36
42lb Thrust 40
55lb Thrust 55
 

*INVERTERS vary on 12 volt amp draw depending on 115 volt Ale draw. It is generally recommended by the manufacturers that you have 20% of your inverter's voltage capacity in battery amp hour capacity. This should provide 1 hour of full inverter capacity. However, be sure to add additional 12 volt amp draws.
Additionally, the above amp draws are for "on time" while the batteries are being used. Remember, a refrigerator, fresh water pump, head macerator, etc only draws power intermittently.

Reserve Capacity Definition
The time in minutes @ 80 F that a battery can provide 25 amps and maintain a voltage of 1.75 volts per cell (10.5 volts for a 12 volt Battery). Reserve capacity directly relates to the ability of the battery to meet accessory power demands. When vehicle current demand exceeds alternator output, battery reserve capacity is critical.

Calculate the Power Requirements for Your Boat

In order to determine the proper battery or bank of batteries to supply power for all your accessory needs, use the chart on the left add up the amperage draw of all the on-board powered accessories, (it is advised to add at least 20% over this requirement as a 12 volt battery's capacity varies as the battery ages). The chart below will determine the approximate runtime based on the calculated loads.
 

Series VS. Parallel Installations

Batteries can be arranged differently to achieve increased capacity or increased voltage to match your specific requirements.

Parallel Installation; Two batteries connected + to + and - to - in a parallel system that increases capacity and maintains a specific voltage. This configuration doubles the power or amp hour rating of the battery while maintaining the voltage. Thus, two 25-amp hour 12-voit batteries in parallel will give you a 50-amp hour 12-volt system.

Series Installation: A series system increases the voltage and keeps the battery capacity the same. The same two batteries in a series arrangement will increase the voltage to 24 volts and maintain a battery capacity of 25 amp hours. To install batteries in series, one battery's positive post is connected to the second battery's negative post.