Solar glossary

kW versus kWh. A kilowatt (kW) is system size — how much power the array can produce at once. A kilowatt-hour (kWh) is energy over time — what your utility bills you for. A 12 kW system is a size; the kWh it generates per year depends on your location and roof.

Net metering. The arrangement under which your utility credits power you export to the grid. Full-retail net metering credits at the same rate you pay; other structures credit less. The rules are set by your state and utility.

Interconnection and permission to operate (PTO). Interconnection is the utility's approval to connect your system to the grid; PTO is the final green light to switch it on. These are separate from your building permit.

AHJ. Authority having jurisdiction — the city or county body that issues your permit and inspects the work. Dealer fee is the amount added to your system price to buy down a loan's interest rate. Escalator is the annual percentage increase built into many lease and PPA payments. PPA means you pay per kWh produced by a system someone else owns; a lease means a fixed payment for the equipment regardless of output.

How to choose

Check size in kW, savings in kWh

Quotes that only give you dollar savings and never the system size in kW are hard to compare. Ask for both.

Find out your export arrangement by name

Ask your utility whether you would be on net metering, net billing, or an avoided-cost export rate.

Look for the escalator in any TPO contract

It is often a single line deep in the agreement and it compounds over 20 to 25 years.

Distinguish permit from PTO

Passing local inspection does not mean you may turn the system on. Wait for the utility's permission to operate.