I would love having this many computers crunching but not the electric bill. Thats going to sting a bit.

If you don't mind me asking...what system specs will these have. I saw there were a few choices. Also, do you think you will run any other projects too considering these have the GeForce 9400M graphics?

Something to consider...
Using this kind of power would raise a red flag at the power company. This is how most "grow houses" are found. The power company reports strange and excessive power usage to the authorities.

You might want to start with 50 computers first to get things rolling. Once your comfortable with it, add another 50 until they are all online. Gives you a chance to sort out any potential problems. Might want to have your local police chief take a look at what your doing so when they do get the call, they will already know why. Ever thought of getting servers with 8 cores +? Won't have as many units to control. I currently run two 8 core computers. Easier to manage. Good luck on your project.

I don't know were you living, and what kind of project you will start (maybe a research or a live lab to test applications), but along side all considerations posted here about it, I think that except if you really need test/use it over Mac minis plataform for any distinct reason, I think that you could at least try get some high performance systems / workstations / servers available. It could be less expensive at end.

Some systems available right now, could provide a lot of cores at reasonable prices, and extreme manageability of facilities and features, which probably will be an advantage in comparison with all planned set of 100 Mac minis, including if you consider the power consumption, total power of computing, scalability, and other variables involved.

Other interesting consideration to think about is that, considering all set of 100 Mac minis power consumption, and the power cost during all the time planned for this project (and you mentioned $20k / year...), when put under opportunity costs/return on investments perspectives, could allow you have a great opportunity to invest in some kind of alternative power systems for your house to do it.

And if you have area enough in your roof / backyard, could be a mixed systems using solar, wind, and more sources, which could cover all power consumption requested at least for your project in some day periods, and you ever could install grid tied systems, to avoid shortages or dependencies, with clear benefits on the long term for your property/wallet.

There are a lot of solar / wind system providers, which could help you on that evaluation/project, including some that works with kinds of photovoltaic tiles, concentrate photovoltaics, small/medium size wind generators, power/heat generators (to burn oil / wood / pellets / brickets / gas / biogas, to generate heat and power for your house at sametime), and so on related with efficiency/generation.

The best way is evaluate it under a complete project perspective looking for became your house more efficient in power usage aspects. And it will probably bring valorization for your property, and additionally could use incentives/benefits from power supplier company / city / state.

Just to think about.