Solar System Sizing PrinciplesSizing

a system means determining how much energy is required and how many solar modules are needed to generate it.
A solar system must provide enough energy to replace that being consumed daily by the loads (lights, appliances, equipment), plus some to compensate for energy used by the system itself.
To do your system sizing, here are the two main things you will need to know:
1. How much electricity will you be using ?
You calculate this for every load and then add the results together. Electrical use is figured in terms of 'Watt-hours'. This is the appliance's power rating (Watts) multiplied by the average amount of time (hours) it operates daily.
In addition to the electricity used by appliances, the system itself also consumes some power. For example: you do not get back all the energy you put into a battery; an inverter uses some energy to convert electricity from DC to AC; and voltage is lost as electricity travels through wires. A factor to allow for these is including in the sizing instructions.
2. How much electricity will be produced by a solar module ?
You estimate this by multiplying a module's power rating (Watts) by the 'Area Factor' from the map. This gives you the typical  Watt-hours produced per day by a solar module at an average location in the area. Please keep in mind that every solar installation is unique and is affected by many factors, such as:

Local weather patterns: these can vary greatly even within a small geographic area.
Installation: even brief shadowing affects a module's daily output and modules that are not oriented properly toward the sun will produce less power.
Seasonal changes: this estimate is based on a yearly average. The energy produced in winter will be less than average and in summer it will be more than average.
The number of modules needed will vary based on these factors. You may also desire back-up battery capacity and may want to anticipate future needs by having additional solar power available now.

Energy Conservation

As you review your energy needs, remember the importance of using energy-efficient equipment and appliances. For instance, using a 20W fluorescent light for three hours a day, rather than a 75W incandescent light, will give you the same amount of light and save you over 60,000Wh of energy during the course of a single year. This is generating capacity you do not have to buy or power you can use for other purposes.
Energy accumulation
Solar systems produce energy whenever there is sunlight. A 50W module may produce 1,500Wh of energy in a week. If you are powering a cabin that you only visit on weekends, just a couple of modules may give you 3,000Wh of power for your visit. This may be plenty of energy to meet your needs - and all from just a 100W system.

Batteries

Batteries are a major component in solar systems. A number of different types and capacities are available. The best battery for your system depends on many factors and often requires analysis and advice from a solar energy professional. Many small to medium sized systems can use photovoltaic or marine grade batteries. These are designed to be deep-cycled (discharge-recharge) many times and are generally maintenance free. They are available in capacities of about 120 Amp-hours.
Batteries must be able to store enough energy for daily operations. A reserve should be considered to have additional capacity to operate the loads during anticipated periods of cloudy, sunless weather. This reserve capacity is referred to as system 'autonomy' and is rated in days. The amount of autonomy needed varies. For critical loads such as telecommunications you may want 10 or more days of autonomy, for a residence perhaps five days, and only a day or two for a vacation cabin. Of course, solar design can be easy, just call one of the qualified Solar Electric Specialties Engineers. Solar modules should be installed at the correct 'tilt-angle' to achieve the best year-round performance. Generally, this is an angle equal to the site's latitude plus 20°, with modules facing south in the northern latitudes and north in the southern latitudes.