Part 2 – Grid Inter-tie and Net Metering

In the last segment I determined the size of the residential (PV) solar electric system I want to install as a minimum of 5.13 kW. The system was sized to produce approximately the same amount of electricity I use on a daily basis and would hopefully eliminate my electric bill. I was going to begin this segment by starting to price a system, but before we do that, I need to go over a few things that effect how this solar electric system will work with the current electric utility infrastructure.

Because your solar electric system will produce more power than you need at some times and less than you need at others, you’ll need a way to store the power that’s generated so you can use it at night and at times when you need more power than the system is generating. Net Energy Metering laws allow owners of solar power generators to use the electricity grid as a battery to store power from their system when they are not using it and to withdraw the power later when they need it.

Grid inter-tie (interconnection) is basically connecting an alternative electricity generator to the power grid. Net Energy Metering is a billing system that works similar to the banking system and allows the inter-tie to work successfully. When your solar electric system is inter-tied/connected to the utility grid, the grid will accept excess electricity generated by your system. If your system generates more electricity than you need at the moment, that extra electricity is deposited into the local utility grid to supply other customers. The deposit is made through your electric meter, turning it backwards, lowering the meter reading. The electric meter keeps track of how much excess electricity is generated by your renewable energy system and sent back into the grid,  and how much grid electricity you consume. If for any reason you need additional power above what is generated by your system or at a time when your system is not generating, it can be withdrawn from the utility grid without cost up to the amount you’ve deposited earlier. Any electricity withdrawn from the grid, above the amount deposited, will be billed at your fixed billing rate. Most utility companies presently, will not pay for the electricity you deposit into the grid above what you consume, instead they will credit your account for the next billing period.

Net metering policies continue to change, a few states now require the utility company to pay the homeowner retail cost for the additional energy. Net Metering is currently offered in more than 35 states, with polices varying from state to state. For a detailed description of each states net metering policies see the DSIRE database for Net Metering. For a detailed description of each states Interconnection Standards see the DSIRE database of Interconnection Standards.

Net metering also helps to respond to today’s stressed power grids by adding additional energy during the peak demand period of the day. The solar powered systems generate electricity during daylight hours when there is a high demand for power and shut down during the lower demand hours of the night. You’ve made your deposit during daylight hours and can withdraw at off-peak periods during the night.

Before we get back to pricing a system I’ll also need to decide whether or not to have a battery backup or to go battery-less. Although both options will allow a grid inter-tie, there is cost and efficiency differences between the two systems. If your not going to be connected to the utility grid you’ll definitely need a bank of batteries and even if you’re connected to the grid, the battery backup system could be handy when the utility grid is down, at which time the the battery backup provides power to appliances and electrical devices. The battery-less system would shut down during a utility power failure and you would not have electricity available to your home until the utility company restored their power. The battery backup system is obviously going to to be more expensive, with the cost of the charge controller, batteries and replacements every 5 to 8 years. Another thing to consider is that the overall efficiency of a battery backup system is less than that of an equally sized battery-less system, which means you won’t get as much energy out of the system as with battery-less. Because of the added cost (about 20% to the cost of the system) and 30% less efficiency with the battery backup system I’ll opt for the battery-less system. I’m not really concerned as much about the possible utility power failures right now.

I didn’t get to pricing a system in this segment like I wanted to, but I felt it was important to cover the concepts of grid inter-tie and net metering before we went any further. I also decided that the solar electric system in this analysis will be connected to the grid and will not have a battery backup for now. In the next segment I’ll start out by introducing the two ratings given solar panel (module) output (DC Watts), determine the roof area (ft2) required to hold the panels and go from there.

Again, this is a learning experience for me, so if you see any errors or omissions, please feel free to share and leave a comment.