How many panels you need will be a your energy usage measured in kilowatt-hours (kWh), your solar access, and the configuration of your roof or yard.
Find your energy usage by looking at your electricity bills or logging onto your utility and downloading a .csv file. Choose a range of dates that encompass a recent twelve month period. If you just moved in, or otherwise have no such data available, a rule of thumb is annual 3.5 kWh are needed per square feet of living space. For example, a 1,500 sq. ft. home would require 5,250 kWh annually.
If switching to heat pumps from a gas furnace, an additional 3,000 to 5,000 kWh per year for heating and cooling may be needed. If switching to a heat pump water heater from a gas powered one, add around 1,800 to 2,500 kWh per year. If powering a new EV, add about 300 kWh per 1,000 miles driven annually.
Your solar access will obviously vary by time of day and season. If you live in an area with low levels of sunlight, you may need more solar panels to meet your energy needs. The configuration of your roof or yard may limit how many panels you can fit. More efficient solar panels will produce more energy, so you may need fewer of them.
When sending your Quote Request to solar installers on EnergyScope you can specify an estimate of annual kWh needed, allowing for an apples-to-apples price comparison. The installers will perform an analysis using remote tools like Aurora or OpenSolar. They will lay down panels on the available area, and the software will account for solar access, equipment efficiency, roof tllt, etc. You can do this yourself for most locations, but not all, using Google's Project Sunroof.