Friday, September 8, 2017

Outlet Inverters Again

A few weeks back, I posted about a solar panel company that advertised that you could just plug the output of the inverter into an outlet, and it would feed not only that circuit,  but through the breakers to all other circuits of the same voltage. I thought it odd that circuit breakers could be unidirectional.   I would expect that feeding 10 amps into the circuit from the inverter plus 10 amps drawn by appliances would pop a 15 amp breaker.   Everything I can find online tells me that breakers are bidirectional.   Total amperage theough the breaker in both directions cannot exceed the breaker capacity.

I keep coming back to this because it makes verification of functionality painless;  just put the panel on the driveway and plug into the outlet by the garage.  If the meter slows down,  it works, and justifies buying a bunch more,  and the expense of installing panels on roof and having electrician connect directly to house wiring instead of wasting an outlet. But even a 500 watt panel at 12 VDC is 41 amps.  A 1000 watt panel is 9 amps,  starting to be a problem.

I had originally thought of the 220 VAC inverter to directly feed the AC compressor and secondarily all the 110 VAC circuits.   But I do not have an unused 220 VAC outlet.   But 1000 watts after conversion to 220 VAC is less than 5 amps,  this is getting more and more sensible.  Test with one of the 110 VAC systems,  then order the 220 VAC system.


Rick C said...

I'll admit to being cynical, but also, in this case, deeply skeptical.

I wouldn't buy such a thing without written proof they'll indemnify you if you wind up islanding the local grid in a power outage and injure an electrical worker.

Jay Kominek said...

breakers are certainly "bidirectional" as you put it, which makes sense since they're being used with AC.

the issue is that you're reasoning about the current draw and its relationship to the breakers incorrectly.

"10 amps from inverter circuit" + "10 amps to appliance circuit" does not mean that any breaker sees 20 amps.

the exact same 10 amps is flowing in the circuit the inverter is attached to, and the circuit the appliance is on. per kirchoff's laws, it's like a loop (that reverses itself at 60Hz). so two different breakers both see 10 amps.

donumvitae said...


A couple of points on this issue:

First, a 15 amp breaker does NOT trip at 15 amps. Due to the way they are designed, they will take over a minute to trip at a 50% overload. They will trip faster for a larger overload.

Second, a breaker most certainly will pass power in either direction. Remember, you have alternating current in your house. It is passing current in both directions all the time. In many breaker panels there is no structural difference between the main breaker (feeds power into the bus bars) and the other breakers (draws power from the bus bars).

Third, if you have 20 amps of load and you are backfeeding 10, then the breaker will see the difference (10), not the sum (30).

Fourth, you cannot simply hook an inverter up to an outlet and have it work. It has to be synchronized very precisely to the line current to work. This is the concept behind so-called 'Grid-Tie' solar systems. They are specifically designed to synchronize with the utility power. As a result, they WILL NOT OPERATE when the utility power is down. This means that you may have more solar power than you need, but if the power goes out, your house will be dark. There are very strict regulations regarding this, due to the aforementioned islanding issue.

Fifth, they say it will feed all other circuits of the same voltage. That is sort of true. Most houses in this country are wired 120V/240V split-phase. This means that the two hot bus bars in your panel are 180 degrees out of phase, so line to line is 240V and line to neutral is 120V (line to ground is also 120V). If you feed a circuit on the 'A' bus bar with this device it will not cross over to circuits on the 'B' bus bar.

In other words, I don't think it is snake oil, but it is certainly useless if the power goes out.


Clayton Cramer said...

Any technical question that I ask gives me lots of useful information. And yes, these are grid-tie systems.

Will said...

Can a grid-tie system be reconfigured to work as a standalone when the grid is down? If not, why bother buying it?

Clayton Cramer said...

I believe my backup generator already has isolation, so with no grid, I would have a standalone system.