With the current state of technology, local power generation is not able to support local consumers when the grid goes down. The inverters have to sense power at the grid before they can inject power into the grid.
There are two types of grid failure from the point of view of an inverter. The first is a clean break in power: a tree falls on a line and a breaker at the generating source cuts all power to an area for safety reasons until normal delivery can be restored, or Hydro is reorganizing delivery sources and produces a brief interruption as the switch-over takes place. The second type is where the conditions on the grid become unstable which triggers an inverter shut-down. The oven continues to work, and lights may seem to be over-bright or rather yellow and dim indicating voltage outside its normal range. These are unusual events, triggered by perhaps a very hot and humid day when demand for cooling power rises significantly. The inverters detect this low or high voltage and are programmed to shut down until normal conditions return. The result for the producer is a loss of income for the duration of the loss of stable grid power.
Even momentary power interruptions are important. When the power goes down for a few seconds, the inverters must wait for five full minutes after the power comes back to sense the harmonics of the grid supply, harmonize its own alternating current to suit the grid, and then recommence supplying power. In the meantime, a small portion of the contract has been irrevocably lost to the producer. These brief interruptions are surprisingly frequent; I guess Hydro is rebalancing or rerouting power. It is enough to stop the clock on your microwave or reboot your connection to the Internet, but normal resistance applications are largely unaffected.
Example: at 80 cents per kWh at midday in full sun producing 9.5 kWh, the loss is about $0.63 for five minutes out.
Ontario Hydro needs to maintain its lines; from time to time the grid goes down for about three hours during daylight so that poles can be replaced or equipment maintained. When my system was first installed, the state of the forestry affecting lines in my location was poor, with approximately five to ten extended power outages per year due to trees falling on lines during a storm, whether wind, lightning, snow and ice accumulation, traffic accident or simply pole replacement. The grid put significant resources into forestry in the area, and now the situation has improved enormously, with perhaps only one such incident per year. The effort by the grid forestry people reduced my costs. They could help me even more by doing necessary maintenance at night, but no doubt this increases the difficulty of the work and raises safety concerns. I can live with the associated costs.
Example: at 80 cents per kWh at midday in full sun producing 9.5 kWh, three hours outage is worth $22.80 in lost income.
It is clearly important that for a producer, maintenance windows should be timed to have minimal impact on local generation, otherwise losses are incurred.