Is “either” only used with two options?

First of all, it's common usage, that is, in native English speech, to refer to multiple choices using either, but it's not entirely grammatical (from a prescriptivist's point of view). The definition of either is:

Each of two. [from 9th c.]  
One or the other of two. [from 14th c.]  

Note that I say it's common in informal speech and usage, but probably not for formal contexts.

An entire page of Wikipedia was devoted to this word, and the definition given was:

Either/or means "one or the other." Its usage, versus the simple or structure, is often for emphatic purposes, sometimes intending to emphasize that only one option is possible, or to emphasize that there are only two options.

Thus, it depends on what the context is. If in speech, then either would be appropriate. If however, in a formal context (business letter, etc.), then using either for multiple choices would be inappropriate.

Either is used where one is required and there are multiple options, usually two. It can be used for more than two but it is most often is used for only 2 options.

In the sentence you're asking about you could say:

It can provide either 100, 150, or 400 amps of power

But it will sound better if you say

It can provide 100, 150, or 400 amps of power

because saying either makes the options sound limited even though there are multiple options.

... while either coordinations are characteristically binary, multiple ones like either Kim, Pat or Alex are also possible. 1

Although commonly stigmatized, (a multiple correlative like either) can add clarity to constructions whose complexity might otherwise cause confusion. For this reason, such constructions are sometimes used even in careful written English, eg in the rubric of an examination paper:
Candidates are required to answer EITHER Question 1 OR Question 2 OR Question 3 and 4. 2

You can use "either" for more than two options, but what do you want to say? The first example (with "either") implies that you would need a different power source to provide each one of the outputs. "Either" emphasizes exclusive possibilities. The second example implies that it (i.e. the same power source) can provide all three options - 100, 150 and 400 amps. In context, though, either example could be construed to include both meanings.