The Xantrex site has good FAQs, App Notes, and Load Worksheets to help size systems,,,, and their inverter manuals answer a lot of questions.
Clamp-on ammeters can measure instantaneous AC current and the Kill-A -Watt - as already suggested - can measure usage over time.
Portable generators and inverters produce "modified sine wave" power - so the accuracy of the Kill-A-Watt device may be affected by this but it will be a good approximation.
If the new home will be "off-grid", I strongly recommend the following:
- install two inverters so you can have 120/240 volt power. This will give you the option of running 240 volt well pumps, etc, and a crude back-up solution if one inverter fails.
- size the system well above what your calculations indicate your needs will be. Many installers quote bare minimum systems to get the business. It's much easier to add capacity now than to retro-fit later. A friend of mine has two 3600 watt Xantrex inverters.
- wire in the connection (permanent or twist-lock + cable ) for a back-up generator so that it will be ready when needed.