Do I need to convert units to be compatible with constants? I want to calculate the wavelength of radiation given its energy.
I know I need to use $E=h f$ and $f = c / \lambda$. 
All I'm given is $E = 20 \text{ keV}$, now my true question is:
Do I use $E = 20 \text{ keV}$ to calculate $f$ as it is, or do I need to convert it to $\text{MeV}$ in order to use it? 
I ask this because the planck constant I have is $h = 4.135\times 10^{-21} \text{ MeV s}$ and so if I calculate the frequency with this value and then use it along with $c$ to find my wavelength, will the results not be wrong?
 A: Let me answer your question with a question. How would you use $E$ as it is to calculate $f$?
Suppose you plug $E = 20\text{ keV}$ into the formula and solve it for $f$.
$$\begin{align}
f
&= \frac{E}{h} \\
&= \frac{20\text{ keV}}{4.135\times 10^{-21}\text{ MeV s}} \\
&= \frac{20}{4.135\times 10^{-21}}\times \underbrace{\frac{\text{keV}}{\text{MeV}}}_{\text{did you forget this?}} \times \frac{1}{s}
\end{align}$$
On the other hand, if you convert it to $\text{MeV}$,
$$\begin{align}
f
&= \frac{E}{h} \\
&= \frac{0.02\text{ MeV}}{4.135\times 10^{-21}\text{ MeV s}} \\
&= \frac{0.02}{4.135\times 10^{-21}}\times \frac{\text{MeV}}{\text{MeV}} \times \frac{1}{s}
\end{align}$$
Try them both and see if you find a difference. ;-)
The thing to remember is that the actual value of $E$, the amount of energy, is the same in both cases. $20\text{ keV}$ and $0.02\text{ MeV}$ are just different ways of writing it, just like $\frac{1}{3}$ and $\frac{4}{12}$ are different ways of writing the same value. But it wouldn't make sense for the final answer to depend on how you write your values.
A: You could convert h to 4135 e-21 keV/Hz and use that in your calculations, or 4135 keV / (e21 hz), and use that.  Your calculation is then 20 keV / 4135 keV * 1e21 Hz or 20 keV / 4.135 *1e18 Hz.
