# Electricity and city meters

1. Aug 28, 2006

### John Galt

I live in the USA and I was wondering, if I plug a gasoline generator capable of generating power into the wall outlet in my home and generate more current than I'm using to power my house, will the excess current being generated force the meter to spin the opposite way in proportion to the to the excess current being generated by the generator?

What is the maximum amount of current and voltage that could flow from the house into the grid past the meter without damaging any system components between the generator and the grid?

Just scratching my noggin at this one after some reading.

2. Aug 28, 2006

### Staff: Mentor

You aren't allowed to do it without permission, but if you choose to generate power and sell it to the power company, the power company is generally required by law to buy it from you.

But since the current is a/c, I'd be surprised if the meter would spin backwards.

3. Aug 29, 2006

### Danger

Russ, this isn't an area of expertise for me by any means, but I do know that the meter does reverse itself when you're feeding power into the grid rather than tapping it. I've read several different accounts of the situation, and all agreed. Usually, the amount generated just offsets part of the regular power bill, but someone with a good generator capacity actually ends up with a negative amount due.

4. Aug 29, 2006

### John Galt

If you use AC in your house to power appliances, then why wouldn't AC be able to flow past the meter? Is there a rectifier between the grid and the meter and an inverter between the meter and household load?

5. Aug 29, 2006

### Staff: Mentor

Ya know, I seem to remember hearing that before. I don't understand why that would be, though.

6. Aug 29, 2006

### Staff: Mentor

I'm not following - all of the power we're talking about here is AC, and your electric meter is not in your house circuit, it uses induction to (essentially) passively measure the current.

7. Aug 29, 2006

### NoTime

The meter is a watt meter.
Not a current meter.

8. Aug 29, 2006

### Manchot

Well, the amplitude of the voltage wave is fixed, so if you know the current, you know the power.

On a side note, suppose that you have a generator capable of generating a sine wave in phase with the power grid and with the same amplitude. How would you go about connecting it to the grid? Could you just plug it in?

9. Aug 29, 2006

### NoTime

No, you don't. You have to know the phase as well.

It needs a slightly greater amplitued.

10. Aug 29, 2006

### Staff: Mentor

Yes, wattmeters have voltage taps. It wasn't relevant to what I was talking about.

11. Aug 29, 2006

### NoTime

Well, you said you would be surprised if the meter would spin backwards.
So why would you be?
It has to when reactance is involved.

12. Aug 30, 2006

there is not "opposite direction" to ac... its a sine wave, it changes its direction every 110 times in one second(here we are given 55 hrz).
though you could make a different phase, though that wont helf you anyway with that watt meter...

Last edited: Aug 30, 2006
13. Aug 30, 2006

### Farsight

This is an interesting thread.

It's got me thinking about negative energy, which reminds me of money - my debit is my bank's credit. And if my electricity bill is negative, that means I used negative energy. But there was no negative energy, just positive energy going from B to A instead of A to B. Only the energy seems as real as the money in my bank, which I can't touch or smell or see, regardless of whether I'm in debit or credit. Sure I can touch cash, but that's not really money, it's just a bit of paper that says its money. Money makes the world go round, but does it actually exist in its own right? And where does that leave energy? Hmmmn.

Sorry, I was rambling there. This wikipedia write-up of electricity meters seems pretty good:

http://en.wikipedia.org/wiki/Electricity_meter

14. Aug 30, 2006

### NoTime

While that is more or less true there remains a distinct difference between power source and power load.
When you have reactance what is the source and what is the load switch places. For a perfect reactance no power is consumed, but in the real world it is. Large users are charged for reactive loads.
In any event the old meters would run backwards, don't know it that's still true.

15. Aug 30, 2006

### Danger

Power companies are shifting to new meters? Maybe ones that won't run backwards so they won't have to pay for the electricity that all of these young whippersnappers are backfeeding to them? :uhh:

16. Aug 30, 2006

### Staff: Mentor

How does a power meter know if the power factor of the load is lead or lag except to measure it? Would they assume it is always lag and flip the meter if it is backwards? Since the primary purpose of a meter is to measure real power (and real power is what makes it spin), why would reactive power change its direction? What happens if you have a capacitive load (rare, but it happens)?

Googling turns up mixed results. Some sites say that most existing meters will spin backwards, others say they need a special meter. Ie:
http://www.lipower.org/cei/solar.metering.html

Last edited: Aug 30, 2006
17. Aug 30, 2006

### John Galt

In California, solar power companies install mostly photovoltaic cells in many locations. A big marketing incentive for consumers to buy is that if you aren't using power and your cells are generating it, the meter spins backwards and the power company may even owe you  per kwh at market rate if you come out generating more power than you are accepting from the grid.

I do know that if this is to be done the power companies need to know the size and location of the installation so they can account for the extra electricity flowing into the grid as part of their load management operations. I know that licensed professionals are the only ones who can do this type of installation but I don't know if extra gear is added besides a line splice. I just thought somebody may know a definitive answer, but we all do live in different places with different legal codes and electric grid structure.

18. Aug 31, 2006

### NoTime

It's built just like a bench watt meter without the return spring. Two coils. One passes any current the other is acrost the two lines for voltage. If there is real power a force is generated in the disk that moves it.
If there is a reactance (lead or lag) there is always some resistance in the circuit that consumes real power. When the phase rotates the energy stored in the reactance feeds back onto the grid (also consuming real power) with the phasing such that the disk will move in the opposit direction.
Kind of a two step forward one step back kind of thing.
In the case of a bench meter the needle vibrates.
Oh! They probably don't have meters with pointers any more.

19. Sep 1, 2006

### denni89627

I don't understand how you could simply feed power back into the grid. I understand AC as a sine wave but wouldn't your feed be impossible to phase correctly with the incoming feed? I would imagine fluctuations would turn your generator into something that sounds like a jet engine and then something really bad (or really cool if it's not yours) would happen.

20. Sep 1, 2006

### denni89627

I just thought of something else, but awesome. I was talking to an electrician yesterday that told me something pretty interesting. The garment district in Manhattan still has many buildings fed off of the old DC grid set up at the turn of the century. They should be reading this thread!