Calculating Excess Electrical Capacity

Written by John Swiatek

So exactly how many "gee-whiz" pieces of electronics will your bike power? Well that depends on a few variables. Basically, your bikes excess electrical capacity is the alternators charging output minus the common operating load. Usually these numbers are shown in "watts".

A "watt" is a unit of measure for electrical power (P). In this case, the charging power is the product of the bikes voltage (V) and peak current (I). So P = V * I. What this mean? Simple... if the bikes alternator has a peak rating of 20 amps @ 14 volts then the peak charging output is (20 * 14) or 280 watts.

A motorcycles electrical system consists of three major parts, the alternator, the regulator-rectifier and the battery. The alternator is responsible for producing the power to keep the battery charged and power all of the electrical loads. The regulator-rectifier converts the alternator output from un-useable AC power to useable 14.4 VDC. The battery is used to both start the bike and buffer the power from the alternator.

To calculate your bikes excess electrical capacity, follow these three steps:

STEP #1 – Look up the "charging output" and type of "fuel delivery" from the manufacturer's specification sheet. Typically smaller displacement bikes will have smaller peak charging output.

Table 1 – Peak Charging Output

The bikes alternator output will change at various engine RPM’s. Here is an example of how the engine RPM affects alternator output on a Harley-Davidson Ultra Classic Electra-Glide:

STEP #2 – Calculate the common operating load. This is the total of all the electrical devices that are part of the bike and will be in operation during normal riding. Do NOT include items like turning indicators and horns since they are only used occasionally. Exceeding the peak charging output for short periods of time is not a problem. The battery will source the extra power needed. However if the power is needed for a long time, the battery will go dead. For this calculation, do NOT include the aftermarket accessories you will be adding; only include the stock items on the bike.

Table 2 – Common Operating Loads

A common operating load for a standard fuel injected bike is about 285 watts.
A common operating load for a standard carbureted bike is about 195 watts.

Some bikes leave the low beam on when the high beam is activated. To conserve power, many bikes automatically turn off the low beam when the high beam is turned on. Many larger bikes have additional lighting and miscellaneous loads like radios make sure you include all of the items that operate continuously while riding. Carbureted bikes require about 85 watts less to operate.

STEP #3 - Subtract the operating load from the charging output; this calculation will approximately predict the excess capacity. Usually the larger displacement bikes will have greater excess capacity, but this is not always true as seen in the following six examples (operating loads are approximate):

Table 3 – Excess Capacity for 2 small, 2 mid, and 2 big bikes

What do you plan to operate?

This handy table shows how much power many common appliances draw. Only a few appliances draw high power. These are heated clothing, laptops and auxiliary lighting. Small electronics like cell phones and radar detectors draw very little. You can usually run as many of the smaller items as you wish with little or no worry. To find the total power required for all of the accessories you plan to use, add the power rating (watts) for each device. Ranges are provided based on make and model.

Table 4 – Common Appliances

What if I don't have enough power?

Sometimes your favorite bike does not have much excess capacity. For example, the data in STEP #3 shows that V-Strom owners may find that powering several pieces of high power heated clothing kills the battery. There are a few things that can be done to conserve a few precious watts:

1. Replace standard lights with low power LED lighting (where possible).
2. Add a circuit that automatically turns the low beam off when the high beam is activated.
3. A dirty fuel filter can cause the fuel pump to use 120 watts, 60 more than normal. A dirty fuel filter is a common cause for a voltage regulator to fail on a fuel-injected bike.

In Conclusion:

Motorcycle manufacturers have been increasing the alternator output in response to the growing number of electrical appliances available. Most bikes can handle a few 40 watt heated garments without any problems.

According to Widder Canada Inc. “Today’s motorcycles of 500cc or larger can usually handle three (heated) garments together without overtaxing the charging system. Three garments would be equivalent to turning on a 100 watt headlight. Most larger bikes would have no problem riding two-up with both rider and passenger each wearing the full set. Another aspect to consider is that the items will not necessarily be on all the time, or if the thermostat is adjusted to less than full capacity, there will be less draw.”

The Gerbing’s web site states: “…the electrical output of the typical motorcycle continued to increase as motorcycle engineers attempted to satisfy the growing demand for electrical accessories. The result is that all but the smallest bikes can now provide the power needed to generate the needed heat.”