How Is Initial Power Calculated in a Plutonium-238 Powered Pacemaker?

[erinec]

Homework Statement

Plutonium-238 with a half-life of 90 years, emits and alpha particle of energy 5.10 MeV. 180mg of this radioisotope is used to power a pacemaker. Calculate the initial power in mW, supplied by Plutonium-238, assuming a 100% efficiency for the conversion of decay energy into usable power.

Homework Equations

I tried using:
N = N_o(1/2)^(t/T), where T is the half-life
P = W/t

The Attempt at a Solution

I do not know what "time" to use in order to calculate the power. What does it mean by "initial"?
Help would be very appreciated.

 

[mgb_phys]

Initial means assuming that none of it has decayed yet
Watts are joules/second so you need to work out how many decays in the first second