To determine your generators size you should tally up all the power requirements for the items on your list, giving you the smallest size for your generator. There are recommendations that the manufacturers adding a 10 to 20 percent buffer to that figure to ensure your generator has sufficient capacity. For instance, if the power requirements add up to 6000 watts, the generator should have a capacity of 6600 to 7200 watts.
Add the start-up wattage requirement. This identifies the peak capacity your generator must have. For example, if you have determined that your sump pump demands an additional 1000 watts during start up, add that to the generator size you calculated.
Start shopping for a generac generator of the appropriate size. If you find that you are exceeding your budget, you can go back through your list and determine if there are any items you can do without during an outage. Remove them from the list and recalculate your requirements.