DC non invasive power measurment
The typical way of doing it would be with a clamp-on power meter.

The clamp-on DC power meter works by measuring two things in parallel:


The magnetic flux in a ferromagnetic clamp that encloses the wire with a DC current flowing in it.
The voltage across the load (or source).


Inside the meter, a measure of the flux is converted (scaled) into current, and multiplied with voltage to provide instantaneous power. The instantaneous power is time-averaged to provide average, or RMS, power.

This isn't totally non-invasive, as you need access to the battery leads - you'd clamp the meter around one of the leads, and attach the voltage probes to the battery terminals. But that's the best that you can do, unless the wheelchair has a power/energy logging function that's available to the user.

The meter itself might have a logging function that totalizes the energy, or you might attach it to an external logger. Finally, instead of using a dedicated power meter, you could use a data logger, and an external voltage-output DC current probe. The logger would log the voltage and current on two channels, and you could do the power and energy calculations yourself.