So, I typed up the giant response below assuming you meant 1MWh/year, but after thinking about it, I think you mean 1MW of power generation. Because my house, poorly insulated as it is, uses ~1MWh/month. However, take the information I provided and scale it up or down to whatever your needs are. It's still valid.

First thing you need to figure out is your average photovoltaic insolation, average hours of peak sun usable by photovoltaic cells, is going to be. This varies from location to location. This insolation map is a good start. You can't tell how many panels you need until you know how much sun to expect, right? If you're in Maine you're going to need a lot more panels than if you're in New Mexico.

You want 1MWh per year. Let's break that down. I'm going to show all my units here, even the super obvious ones, just FYI. 1MWh is 1,000,000Wh, of course. Since our insolation chart is done in average hours per day, let's look at how many watts per day that is. 1,000,000Wh/year divided by 365days/year = 2739.72Wh/day.

Now, let's say you live in Austin, TX. It's a nice easy location to do the math for, and it's roughly average for the country. Austin, TX has an photovoltaic insolation value of 5-5.5 hours per day. Let's call it 5 hours per day to be conservative with our estimates (and easy with our math). 2739.72Wh/day divided by 5h/day = 547.944W. This means you need 547.944W of solar panels.

However, this is assuming 100% system efficiency. Your system will never get 100% efficiency. Your cables will have voltage drop (loss of power to heat caused by natural electromagnetic induction in the wires), your solar controller will take some power to run, your panels may not be perfectly aimed, and numerous other potential problems. For instance, I have a 1000W system that puts out 620W actual power. Most of this is because it's flat on the top of a school bus. I estimate that if I tilted the panels properly, I'd get closer to 850-900W. You should probably assume a 10-15% loss. More if your wire runs are long. One thing you can do to mitigate this is to buy thicker cables. Copper is expensive (never use copper-clad aluminum in these situations), but heavier gauge cable will help reduce your losses. It is probably worth it just to buy an extra panel or two.