Expert: George V. Noller - 1/25/2008

Question
I've looked at a number of systems that generate power for home use and they all say that when there is excess, it can flow back into the electrical grid and gain the homeowner revenue.  Here's where I'm confused.  If the voltage on both sides of the meter is the same, how does it flow backward?  Isn't this basic electrical theory?  Does the utility have to do something to sense the availability of this source of power? I know that many utilities share power and I guess this is the same technique?
Thank you!

Answer
Joseph,

I don't blame you for being confused.  The theory of power flow can be very confusing to a layperson.  Let me give you some basic information and see if this helps you to understand.

You made a statement of basic electrical theory and how power flows if the voltage is the same.  In DC theory, that is the concept.  So in your car's electrical system, for example, to charge the battery the voltage regulator raises the voltage compared to the battery and power flows into the battery.

However, the power grid is an AC system.  There are some different theories on how AC systems work.  For starters, there are two types of power flow;  watts and VARs (volt-amperes reactive).  Watts are what is consumed by devices to do work, such as lighting a light bulb or running a garbage disposal.  VARs are what provide the electromagnetic support on an AC system to maintain voltage and allow the watts to flow through that magnetic field (which is changing 50 or 60 times a second depending on where you live).

To move watts through the power system, you have to have voltage to support the movement, but the movement only occurs when there is an angular difference in the sine wave between the sources.  VARs moving through the system occurs when there is a difference of voltage magnitude.

So, to answer your question, real power (watts) will flow from the house to the grid when the power angle of the house is faster than the power angle of the grid.  This will happen when the solar panel is making more power than the house can consume.  And yes, this is the exact same principle used for utilities to share or buy and sell power to each other.

I hope that answers your question.