I do not know how to enter mathematical symbols on this computer so I will do my best.

d= delta(change in)

T=temp C

t=time seconds

J=joules

m=mass grams

S= specific heat of target (aluminum .89J/Cg)

I am attempting to use a small piece of aluminum as a target attached to DMM temperature probe.

J=SxMxdT

Watt=Joules x Seconds

mass of target=1.912

J=(.89J/Cg)x(1.912g)xdT

Watts=Ans/dt

Measuring temperature change over a period of time should yield watts.

The first question that comes to mind is the effect of ambient temperature.