Grunff wrote:

Now that does seem like a good idea - I could integrate both V and I
using opamps, then multiply to get total power - any problem with that
thinking?

In general yes there is because the product of the means is not the same
as the mean of the product. You need Int[v*i]dt and _not_
Int[v]dt * Int[i]dt.

However in your case v is constant (IIRC) so they are the same, more or
less - depending on how good your stabilised supply is and whether
voltage drop in the wiring is an issue.

Drift is always a potential problem with analogue integrators though;
I'd go with your original data logging idea (provided you can sample
fast enough) and then number crunch in the digital domain.
--
Andy