Excess Watts Calculator for Motorcycle Accessories

Instructions: Input your motorcycle alternator's total peak watts in the text field called "Your Bikes Total Peak Watts." This information can be found in the specifications for your motorcycle. If you have accessories installed on your bke, than input the total watts for all your installed accessories in the text field called "Installed Accessorie Watts." The default value for accessorie watts is 0. Go over the list of power requirements for your motorcycle listed below. The values in this list are a general rule of thumb, and may not exactly match the loads on your motorcycle. If you know the exact values for your bike, than you can change the values. I have made the following assumptions:

  • Some motorcycles burn both the high beam and low beam bulb when the high beam is selected. If you only use the low beam bulb when driving during the day and night, than change the value of the high beam to 0. The wattage for the headlights are for incandescant bulbs, and if your bike uses LED's, than you might want to change the values.
  • Motorcycle alternator peak wattage occurs at higher RPM. At idle the alternator will produce less wattage, so keep this in mind when figuring watts for your bike. This isn't a problem for short periods, because your battery will help with the load requirements, but for longer periods your battery will eventually discharge. Also, operating your alternator at peak output can cause heat related problems, so keep this in mind when adding accessories to your bike.
  • If your motorcycle is air cooled, than change the value for the Radiator Fan field to 0, since your bike doesn't have a fan. If you have an older bike that doesn't use a computer or have a fuel pump, than change these fields to 0.
  • The formula to compute power (watts) is Watts = Voltage x Amps. If your motorcycle service manual gives the peak amps for your alternator, than you can compute watts by multiplying peak amps by 14.4 volts. For example, an alternator that puts out 30 amps will have power (watts) equal to 30 amps x 14.4 volts = 432 watts.
Your Bikes Total Peak Watts: Watts
Installed Accessorie Watts: Watts
Headlight High Beam: Watts
Headlight Low Beam: Watts
Brake Tail Lights: Watts
License Plate Light: Watts
Instruments: Watts
Ignition: Watts
Fuel Pump: Watts
Computer: Watts
Radiator Fan: Watts


(Answer will appear here)

Here is an example table of peak watts for a variety of different motorcycles to give you an example of different alternator output.

Motorcycle Year Fuel Delivery Alternator Peak Output
Kawasaki KLR 650 2009 Carburetor 245 Watts
Harley Davidson Softail Custom 2009 Fuel Injected 489 Watts
Harley Davidson Road Glide 2008 Fuel Injected 650 Watts
Buell Blast 2001 Carburetor 297 Watts
Yamaha FJR1300 2003 Fuel Injected 490 Watts
BMW R1150RT 2003 Fuel Injected 700 Watts
BMW K1200LT 2003 Fuel Injected 840 Watts
Ducati 996 2000 Fuel Injected 520 Watts
Ducati ST2/ST4 2002 Fuel Injected 520 Watts
Harley Davidson Heritage 1998 Carburetor 360 Watts
Harley Davidson Electra Glide 2002 Fuel Injected 585 Watts
Honda Shadow 1100 2002 Caburetor 329 Watts
Honda ST1300 2003 Fuel Injected 740 Watts
Honda Valkyrie 2000 Carburetor 546 Watts
Honda GL1800 2003 Fuel Injected 1100 Watts
Kawasaki Vulcan 1500 2000 Carburetor 377 Watts
Kawasaki Vulcan 1500 2001 Fuel Injected 588 Watts
Kawasaki ZX6R 2001 Carburetor 305 Watts
Suzuki Bandit 1200 1999 Carburetor 405 Watts
Suzuki V-Strom 2002 Fuel Injected 360 Watts

Here is a table of common accessories and their power requirements. The values are a general rule of thumb, and the actual value for the accessory for your bike may be different. Always check with the manufacturer for the accessory.

Accessory Power Requirement
Heated Vest 70 Watts
Heated Jacket 100 Watts
Heated Gloves 30 Watts
Heated Socks 25 Watts
Auxillary Lighting 35-100 Watts Each
Laptop 50 Watts
Cell Phone 3 Watts
Radar Detector 2 Watts
GPS 5 Watts
Portable Music 3 Watts