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If its for your home, the easiest way would be to look at your electric bill and take the Kw/hrs and divide it by 744(hours in a month with 31 days, adjust depending on month). This will give you your average usage over time. Next throw in a conservative factor such as 120% and you should be in the ball park.

If its for a boat or RV or other appliances, you can take the nameplate data and it should give you amp draw. Multiply your amp draw by your voltage and you'll get the wattage that the item consumes. Add all of the wattages together and again, multiply by 1.2 to get your 120% wiggle room.