The size, i.e. how many solar panels will be in in your home’s system will be based on 1 of 2 criteria:
1. Your annual electricity consumption in Kilowatt hours (kWh)
With this criteria for system size, your total kWh consumption over the last 12 months is collected from your utility company. That number is then used to size the system with the aim of producing all the electricity your home will consume. Using the most recent 12 months is the most accurate approach because it includes all 4 seasons and the coinciding fluctuations in consumption.
2. The goal for production you’ve set on your own
Some consumers don’t want 100% of their consumption provided by their solar power system. They want some lesser amount, or perhaps greater if they’re anticipating adding to their electrical load in the upcoming months. For example, someone may have plans to buy an electric car in the next 2 months and have calculated it will increase their consumption by around 3,000 kWh/year, so they want a system that will deliver that 3K kWh in addition to their current annual consumption.
Additional information you need to know how many panels:
Wattage of panels – most modern panels range in size from 330-400 watts. The wattage of a panel is included in its model number, for example, Panasonic 330 is a 330 watt panel, LG 375 is a 375 watt panel, etc. Over time, panels have increased in wattage and efficiency, and this trend is expected to continue.
Sun Hours – knowing the productive hours of sun exposure for the area(s) where your panels will be installed is determined by installers. Using a “Sun Eye” or similar instrument, the Sun Hours are used to calculate system production based on panel wattage. The daily Sun Hours used for this calculation is 3.8. 3.8 hours of productive sun a day may sound low, but it is an industry standard based on national averages and how residential solar power systems have to interact with utility companies. Impediments to this maximum of 3.8 typically include roof facets not facing due South, smoky skies from fires, cloud cover, or shading from trees or tall neighboring structures.
Because determination of Sun Hours is not something easily calculable without special equipment, estimates are used below to calculate a system size based on consumption and the maximum average of 3.8 Sun Hours per day.
Solar panels come in a variety of sizes
With the majority being around 6.5’ x 3.25’ or 21 square feet. Most building codes require a 3 feet space around the edge of a group of panels on a roof. If you know the dimensions of the facets of your roof, you can estimate how many panels will fit on any given facet.
It is prudent to consider that if the calculator above states you need 35 panels, and your most directly south-facing roof facet can only fit 20 panels, the additional 15 panels placed on areas of your roof with weaker sun exposure will produce less energy. This means 35 panels is unlikely to meet your production target. When sun exposure goes down, panel count goes up in order to meet a production target.