There are plenty of load analysis tools available for free on the internet, so this article seeks to help you use them properly.
Here are some common strategies from weakest to strongest. They are best used in conjunction. And remember to build in a little buffer!
List loads with basic calculations and use Kill-a-watt measurements on cycling loads (fridge, coffee maker, etc) → measure whole house of loads with current transformers → work with homeowner (considering light use changes seasonally, people underestimate TV usage, etc) and remember chargers and phantom loads (24/7)
Once you understand the loads, you can calculate peak power and average daily energy usage after energy conservation changes, eg. efficient lights and fridge + removing electric heaters and phantom loads often reduce energy usage by over 50% !
Keep starting surge requirements in mind for peak power calculations. These apply to everything with a motor as well as microwaves. A safe bet is 3x the operating power.
Once your load analysis is complete, you will have all the info needed to size your inverter and battery bank. All that's left is the solar array and appropriate charge controllers.
First, gather Peak Sun Hour (PSH) data and choose the required solar value based on normal design criteria.
kWh/m² depends on orientation, tilt angle and location
Data sources: NREL Solar Radiation Data Manual, PVWatts, EnergyPlus
Wost Case Design Month: ** Seasonal PSH varies more than annual average! **
Winter-dominated loads: latitude + 15 degrees
Summer: Latitude - 15 degrees
Spring/Fall: At latitude
Critical Design Ratio: Highest Load / PSH Value month (not necessarily lowest PSH or highest load month, check the ratio!)
Lowest ratio = surplus production
Now that you know how many batteries you need for your solar system, or at least the capacity of the battery bank, you need to pick the best ones for your application. There are many batteries out there, please read Battery Technology Selection for more.