How big should a solar system be?

kW, kWp and kWh

• kW (kilowatt) measures power: how fast energy is being used or produced right now.
• kWp (kilowatt-peak) is the rated capacity of the solar panels under standard test conditions. It is how system size is usually quoted.
• kWh (kilowatt-hour) measures energy over time: what your bill charges for and what a battery stores.

What decides the size

• Your electricity use, from your bills.
• Your goal: offset part of the bill, most of it, or also charge batteries.
• Roof area, shape and strength.
• Orientation and shading, which affect how much the panels produce.
• Utility connection rules and the capacity of your electrical service.
• Budget.

Bigger is not always better

A system sized well above your daytime use may export a lot of power, and how exported power is credited depends on the utility's rules. A properly sized system matches your use, your roof and your goals.

Batteries add a second sizing question

If you want backup, the battery size depends on which appliances you want to run and for how long, which is separate from the size of the solar array.