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When researching about photovoltaics feed-in to grid options in my area, I came across various terms: net metering, feed-in limit, zero export / zero feed-in, solar smoothing.
Can somebody explain the various alternatives of how feed-in is realized, and what devices are required to keep within the rules of each?
The options you have for feed-in differ a lot by country due to regulation:
Export to the grid is rewarded. Utility companies may offer net-metering or to buy the electricity produced locally, based on an export meter reading. Net metering means that your electricity meter runs forward (when you consume from the grid) or backward (when you produce more than you consume, and export the rest to the grid). So you are only charged for your "net" consumption.
Export to the grid is rewarded, but limited. Same as above, but with a regulatory limit on the allowable export that can be either a percentage of current production, or an absolute limit in kW, or a requirement to smooth out short-term fluctuations ("solar smoothing", which usually needs a small battery in the system).
Export to the grid is allowed but not paid. The utility company will accept feed-in to its grid but not pay for it. In this case, a normal grid-tie inverter can be installed. However, the utility company may also have limits for acceptable export to the grid, in which case the inverter needs to support export limiting.
Export results in higher electricity bills. This happens in cases where the utility company's electricity meter can only count in one direction, and does so both for incoming and outgoing electricity. In this case, or when unsure if the installed meter is of this type, an inverter with a zero-export solution will help (see below).
Feed-in is forbidden, without enforcement. Feed-in is forbidden in several countries / areas (incl. Spain, Hawaii, …), but usually this is only a regulation only. To comply with it, an inverter with a zero-export solution will work. There are also multiple variants how strict the regulation is. Some require a minimal consumption, to prevent short-term minimal export to the grid that otherwise can be a result of regulation overshoot in a zero-export solution (source, chp. 4.3.12).
Feed-in is forbidden and not tolerated. In this case, feed-in is absolutely not tolerated, even for a second. Some prepaid electricity meters in South-Africa will fall into this category and disconnect a household from the grid when they detect exporting to the grid.
This follows an overview by Victron Energy. Most of these rules have a technical justification about keeping the grid electricity supply easy enough to manage dependably and safe for workers, but the wide differences between countries also show that a lot of political choices are involved as well (supporting solar energy vs. protecting utility company business models).
