You need 2 different numbers - the watts when running and the watt-hours per day. The watts when running could come from the data plate, and would be needed to size a big enough inverter. The inverter needs to supply peak power, not average power, and it needs to be big enough to start the compressor as well as keep it going.
The watt-hour per day usage is harder to figure. The best way is to plug the AC cord into a Power Angel or Kill-a-Watt meter, and let it add up the usage. Another way would be to find a scrap hour meter that would record how long the external thermostat supplies power, and multiply the daily hours by the data plate figure. The usage will change based on whether the freezer is in an unheated basement in Alaska, or on a sun porch in Arizona. The good part of this variation is that with solar, is there is more sun available when more cooling is needed.
Once you know your usage, you can figure the required battery and solar capacity. If you are short on cash, get more batteries than solar and top them off with a small genset every now and then.