There are three basic types of solar electric systems:
- A Grid-tied system operates only when the utility grid is working.
- A Stand-alone off-grid is not connected to the grid at all.
- A Battery-based grid-tied system will allow the system to provide electricity to the house during a grid outage.
If your home or building is on-grid already, a grid-tied system is the least expensive to install and the simplest to operate and maintain. There is no need for owner maintenance or technical ability, and as the system feeds renewable electricity into the grid through your meter. The vast majority of installs in Oregon are grid-tied. Off-grid solar is popular option for sites that are not close to transmission lines or for people who want to be independent of their utility. Increasing, homeowners and businesses are installing grid-tied solar systems in conjunction with batteries. This helps to ensure the house or building will have some power if the grid is down.
Connected to the grid: Meters, Monitoring, & Net Metering
Most electric meters measure electricity moving both into and out of your home or business. Generally, we are taking electricity from the electric grid for our needs. The meter runs “forward” as it counts up the kilowatt hours we have consumed. But if you generate electricity with a grid-tied PV system and you make more electricity than you need, net metering allows this excess electricity to run the other way through the meter and back into the electric grid.
For example, on a sunny summer day a PV system might produce more electricity than needed at the time. Conversely, in the evening when everyone is home, electricity needs may exceed the output of the system.
If you put 10 kilowatt-hours (kWh) of excess electricity into the electric grid during the day, net metering allows you to take 10 kWh of electricity out of the grid later and pay nothing extra for this electricity. In effect, you are allowed to “bank” these 10 kWh and use them later to offset your need to buy this electricity.
Your inverter probably has a display showing cumulative generation, but if you change inverters, you would lose the count to that point. So the best way to monitor the output of your solar electric system is a dedicated meter at the inverter.
If your site is remote and grid electricity is not available, then an off-grid system is the only choice. An off-grid system will be a good choice for you, and save you money over extending the power line, if the following are all true:
- Your residence is over 1/4 mile from existing power lines.
- You are willing to accept that you may not have unlimited power on-demand.
- You are willing to manage your own power system.
If some of these are not true, the cost of owning and operating a stand-alone off-grid system will increase, and may be more than the cost of extending the power line.
Oregon Department of Energy has an Oregon Solar Electric Guide which contains help information on related to sizing and planning off-grid PV systems to meet your electricity needs.
Battery based grid-tied solar
Depending on grid reliability and the length of power outages in a particular service area, some people opt for the simplicity of a grid-tied system with a portable generator for emergency back-up power. Others may opt for the reliable backup of a battery-based on-grid system, which will pick up loads seamlessly during a utility power outage. The price of a battery-based on-grid system is a higher installation cost, and takes more owner time in maintaining the batteries. Modern battery-based on-grid systems are nearly as efficient as grid-tied.