What you buy from your utility company is electric current. A meter measures the amount of electricity that flows into your house and, at the end of each month, you pay your utility company for each kilowatt-hour used. In this situation, the flow of electricity is one-way, only flowing from the grid to your house.
Going Two Ways
In a Net Energy Metering electricity flows two-ways. A meter measures the power your solar system creates and any of the power that’s not used flows back into the grid, literally running the meter backward. This portion of electricity (also referred to as net excess generation or NEG) that flows into the grid is credited towards your account.
The Net Difference
If, at the end of the month, more electricity was generated by your solar photovoltaic system than was used, you will only be charged the minimum fee (the amount that electric companies in Hawaii charges for being hooked up to their grid). In fact, credits for energy not used can accumulate over time for those times when you have a higher consumption due to family visiting for the holidays, etc. If you have used more than what your solar energy system generated, then you will be charged the “net” difference between what was generated and what was used, minus the credits you've earned throughout the year.