Is there a minimum amount of electricity that can be fed back into the grid?

Question

This is not fully related to electrical engineering, but I have been reading lately about Feed-in-Tariffs and solar energy production and I cannot find any information on the amount of energy that can be fed back into the grid.

My question is, can every amount of energy be fed into the grid for a remuneration according to the Feed-in-Tariff or should the electricity be first accumulated (using an energy storage system) to, lets say a kWh, or is any quantity possible to be fed back?

I have looked at the Feed-in-Tariff method and normally the payment is for a kWh, if the price per kWh is 8 cents, would I be paid 0.8 cents for 100 Wh or would I first have to produce a minimum of 1 kWh?

Answer 1

As others have already noted, the question has both regulatory and technical aspects. This answer considers the technical aspects primarily, but includes some regulatory background information. Because regulations change and vary by locale, any future readers of this answer should look for the most current local regulatory information rather than relying solely on information here.

So with that said, let's look at the heart of the question:

My question is, can every amount of energy be fed into the grid for a remuneration according to the Feed-in-Tariff or should the electricity be first accumulated (using an energy storage system) to, lets say a kWh, or is any quantity possible to be fed back?

There are a couple of ways to interpret this, so to give a complete answer, I'll rephrase individual aspects of the question and answer each one separately.

Can every amount of energy be fed into the grid?

The answer to this is generally "no" from both a regulatory and technical viewpoint.

From the regulatory aspect, many Feed-In Tariff (FIT) schemes impose a cap on the size of a photovoltaic (PV) system that is eligible. For instance, the Los Angeles Department of Power & Water has a commercial PV size range of 30 kW to 3 MW for their current FIT

From a technical aspect, distribution systems, which is generally the term used to describe the part of the electric grid from the substation to individual homes or businesses, have physical limits as to the current they can carry. A typical distribution transformer, like the one that feeds my house, also has an associated fuse and the fuse represents a hard limit on power. The one on "my" transformer is rated 10A at 14.4kV, which is the distribution voltage in this area.

Should the electric energy be stored in integral kWh amounts?

Batteries are a very useful adjunct to PV systems. Obviously, PV systems only generate during the day when the sun is shining, so having a way to store some of that energy for later use is extremely useful to be able to continue to use energy when the sun is not shining. However, from a regulatory viewpoint, I don't know of any currently available FIT that requires the use of a storage systems. I think what might actually be meant by this question is the next interpretation.

What is the smallest increment that can be measured by a net meter?

The use of a net meter is often used in association with a FIT. In the old days, energy only went one way -- from a utility's generation plant toward the end users of that energy. Meters designed for that scenario measure kWh in one direction only. With a two-way flow of power in the scenario in which sometimes the energy goes from the grid into your house and sometimes energy goes to the grid from your own local generation, what is often used is a net meter. As suggested in the name it registers the net energy (supplied minus delivered) which can then be used to calculate the bill (or payment!) based on the current tariff, which might be a FIT. Meters for that purpose are generally very accurate -- a typical residential meter in North America is an ANSI standard class 0.2 meter which basically means that its measurement is within 0.2% of the true value. These meters also have a starting load requirement from the same standard which is 0.1A for the typical 200A circuit, which means that a current as low as 100mA (at 240V, that's 24W or 0.024kW) is enough to be measured accurately. For that reason, there are essentially no technical or regulatory limits on the smallest amount of energy that can be fed back into the grid.

I hope that that thoroughly answers your questions. If not, feel free to leave a comment and I'll try to address any aspects not yet covered.

Answer 2

The question has regulatory and technical aspects.
Regulatory and company practice aspects are outside this forums scope and my knowledge.

Technically: If a meter is set to allow feed in then it will usually sum energy supplied to the grid within certain accuracy and resolution limits.

I'd expect that 100 Watts would register and 10 Watts may not.

If there was a lower limit then storing the energy and sending it in bursts would overcome this BUT there would be no economic sense in doing this.
If say 10 Watts did not register then in a month you'd accumulate
10W x 24 h/d x 30d = 7.2 units. Depending on what you are paid for energy that will probably be worth a dollar or two.
If you already had a battery then you would not need to make decisions of this sort, and you would never justify addition of a battery system for such small amounts.

If you only fed in 100 Wh during a billing period then you'd be lucky to see that registered - and would be unlikely to care.
However, 100 Wh/day =~ 3 kWh/month should be able to be accounted for.

Answer 3

My question is, can every amount of energy be fed into the grid for a remuneration according to the Feed-in-Tariff

Yes. Few systems have storage.

... or should the electricity be first accumulated (using an energy storage system) to, lets say a kW or is any quantity possible to be fed back?

What you are trying to say is, "should the energy (kWh) be accumulated and then fed back in at a pre-determined power level (kW) in a kind of 'burst mode'.

The second scenario is unlikely at the moment as the grid generally likes steady state. What is likely to happen in future is that the "smart grid" would like what you are offering so that they could have you build up reserve energy and call it in on demand. One significant source of short term storage would be electric vehicle batteries when left on charge. On receipt of a signal from the grid they could be switched into supply mode to help ride through a short-term peak demand.

Answer 4

My question is, can every amount of energy be fed into the grid for a remuneration according to the Feed-in-Tariff or should the electricity be first accumulated (using an energy storage system) to, lets say a kWh, or is any quantity possible to be fed back?

I have looked at the Feed-in-Tariff method and normally the payment is for a kWh, if the price per kWh is 8 cents, would I be paid 0.8 cents for 100 Wh or would I first have to produce a minimum of 1 kWh?

A kWh is not a lot. Ask yourself the reverse question: is there a minimum amount I would be billed for? No, even if your load was a single phone charger it would very slowly tick upwards.

I have a 4kW system, and rather than a single net meter I have two (essentially identical) meters, one in one out. Both are capable of measuring individual watt-hours; the meter flashes a light for every watt-hour that goes past. So on a bright day the export meter flashes slightly faster than once a second. On dim days many seconds can go by between movements, but they are still counted.

Your meter will almost certainly show two decimal places, corresponding to 10Wh. However, at billing/feed-in-tariff time, you only send in the whole kWh and are paid for those. Fractional kWh you can carry over.

(The UK system at the time I subscribed pays for every kWh that comes off the panels, regardless of whether I use it or export it. Your local currently available scheme may vary.)

If you want the technical details, have a look at an example meter and the IEC standard for accuracy.