The 650GS has reasonable electrical power from the 400 watt alternator but accessories can load the system to it's limit and beyond. The 400 watt rating is at maximum RPM so at lower RPM the full 400 watts is not available. When considering electrical loads with accessories it is worth making a list and considering carefully what is feasible.
A voltmeter in the dash can be a good addition to monitor electrical performance and know when the systen is being pushed to the limit. The voltmeter should show 13.8 to 14.4v at highway speeds when the battery is being fully charged dependent on the voltage regulator and the battery. Below 13v then the battery is being discharged.
1/ Engine Sensors, Fuel Pump etc are averaged as difficult to measure due intermittant operation.
2/ Maximum current rating for Dash Connectors is 1 amp so maximum power is below 12 watts.
3/ A continuus load of 300 watts seems a safe level for long distance travel at highway speeds.
4/ Yes some of the power estimates are a bit arbitrary but you get the idea.
Heated Clothing can draw high power from an electrical system, a guide to typical maximum power is below.
Too often Mfr's specify current rating without specifying the voltage so power consumption cant be calculated.
You can see from the figures above how much care is needed in selecting clothing.
Swapping to HID lighting can cut headlight power to 35 watts, LED H4 globes are now available and light intensity is improving with up to 2800/3000 lumens. This povides 35-40 watts power savings. See advmonster.com H4 globes.
LED driving lamps are now down to between 10 to 30 watts and becoming better in light output very quickly.
At first glance it seems insignificant but replacing dash lamps, tail and stop lamps with LED globes has a significant saving of power. There is 20 watts of potential savings there.
The big saving is is swapping to HID or LED headlight and driving lights.
BMW show wire sizes on electrical diagrams measured in square mm. Many people look at the wires and consider the wires too thin or under rated but are confused by the high quality thin insulation compared to the rolls of wire available in auto stores which often use a generic mm wire size rating which is a quite different rating.
The current handling of any wire size is varied by the length and whether it is in a larger bundle of wires.
The BMW wire sizes and conservative current ratings are below with equivalent AWG sizes.