“How many solar panels do I need to eliminate my estimated $175 electric bill?”
“How many solar panels do I need to power my X Square Feet home?”
Over the years, these have been the most commonly asked questions by homeowners during the initial stages of switching to solar. However, these are not the right questions to be asked. Square footage, average bills and generalizations are not factors that will give you the answer you are looking for.
What do you buy from your utility company?
Electricity is measured by kilowatt hours (kWh). By adding up the last 12 months of your kWh consumption, you will have a concrete picture of your usage pattern. Tools like Green Button Data will allow you to get this amount of kW for every 15 minutes. You can also access your consumption through your utility’s online portal, paper bills or by calling the customer support line. We do NOT recommend you use averages or the amount paid in your last bill. This will yield unwanted results.
If you foresee a future increase, like adding an electrical vehicle or increasing AC usage, or a decrease in usage, like a family members moving out or replacing an old AC with a more efficient one, this will influence the amount of kWh you will need to generate from your solar system.
Let’s calculate now the system size of solar panels you need to produce this electricity.
These are some factors that will influence the system size:
- Azimuth: it is the direction your roof it is facing. In the Northern hemisphere, a south-facing roof will be the best location to install solar panels, followed by west, east and north.
- Roof tilt: it is considered optimal between 18 to 22 degrees
- Shade: This is a crucial factor that will be even more relevant when deciding if you will be installing a Single inverter, optimizers or microinverters.
- Sunlight: How many hours of sunlight your roof receives in a day. In California we average a 6.5 hours of sun daily.
Tools like PvWatts can help you with this calculations.
i.e.: If we want to produce 13,600 kWh in a Northern hemisphere home, with a 20 degrees roof tilt, no shade and with a 180 degrees azimuth (south facing roof) we would need a 7.2 kW system size.
Using standard 300 or 360 watts solar panels the number of panels will vary depending on the wattage of the panel.
80% of the DC energy gets converted to AC
If you lack space in the best roof azimuth to install 24 panels, then you could increase the wattage of solar panels which will reduce the total number of panels needed.