I was fortunate to get a commercial electric meter from NSTAR as a professional courtesy. This meter gives me more information than a standard residential meter, and the most interesting is the separate tally of how much energy the grid has supplied to House 5, and how much energy the House 5 PV system has sent to the grid. Here's data for the past two weeks:
- PV generation 270 kWh (19.3 kWh/day)
- PV sent to grid 230 kWh (16.4 kWh/day)
- PV used on site 40 kWh (2.9 kWh/day)
- Grid delivered to the house 59 kWh (4.2 kWh/day)
- Total used on site 99 kWh (7.1 kWh/day - higher than usual because we have a 60W light on 24/7 for our new chicks!)
Grid-tied systems don't have on-site storage, so once the system stops producing, energy used comes from the grid. The grid is the battery. For utilities that are called on to produce their peak power output in the summer, this way of operating is helpful. The PVs produce during the peak demand time period, typically afternoon, lowering the utility peak, and the house's need for power at night helps balance the utility load.
It will be interesting when winter comes and the heat pump load is added to our base load, while the PV system output drops due to lower solar availability. We'll be net importers unless we use the wood stove instead of the heat pump, and we'll be whittling down our net energy credit, which stands at over 800 kWh now.