Comment by Animats | Hacker News Reader

Comment by Animats | Hacker News Reader

There are several variations on this idea, and the web sites are confusing.

First, there are ones that are just a big UPS. You plug them into the wall, and plug loads into them. They may try to level your power usage. No problem with that. Good for refrigerators. That seems to be what "Pila" is selling. This class of device seems to come with deceptive ads which do not show cords from loads plugged into the box.

Then there are ones that purport to pump power back into their wall outlet. That has safety issues, starting with its male power plug being an output. It can't provide power when there's no input power, because it would try to drive the the whole grid during an outage. It's mostly for taking advantage of variable power pricing. This seems to be what "Regen" is selling.

These are apparently available in Germany, but must be installed on their own dedicated circuit, so you don't have a load and a source on the same side of the breaker. Otherwise, you could potentially have an overload without a breaker trip. Regen is trying to get them into the UK, but old UK ring main wiring is a bad match to this. Gotta have a breaker between load and source.

The trouble with these things is that, plugged into the wrong outlet, they create a gap in overload protection that won't show up until something goes wrong.

That’s largely correct but misses the point that the battery+pv system is legally only allowed to feed in 800W [1]. On our circuits you can draw ~3.5kW without tripping the breaker. This brings you to 4.3kW max load, which - as far as I understand - is still what all the circuitry must be rated for anyways.

The downside is that we have three phase power supply anywhere halfway recent. And that only gives you a single phase. In _most_ cases that’s fine, because you get a net-sum of all three phases at the meter, but technically you are only drawing/supplementing one out of three phases. In practice this likely doesn’t matter since different house put theirs on different phases and it all washes out.

[1] the DC side can have up to 2kW of panels. But that’s not visible to the grid

Home electrical load safety is based on distribution being a tree, with a breaker for each branch. The branch breaker will trip if there's an overload on that branch. Outlet strips with their own circuit breakers are sub-branches. This is all worked out in the US under the National Electrical Code, so that you can't overload a branch circuit without tripping a breaker. The UK uses BS 7671, which has the same goals but different topology. Residential users don't have to do load calculations or make measurements.

This thing breaks the simplified rules that make home electrical safety work. The obvious way to get into trouble with this is to plug in two of them on a circuit with heavy loads. Now you have a big source and a big load without a breaker between them. Germany gets this, and requires that a licensed electrician add up the loads, check the topology, and do the install.

One way to make this work safely would be to require a special symmetrical connector, with both sides protected against touch. That has to be on its own branch circuit, with a breaker between it and any loads. That becomes a legit place to plug in source devices.

This is essentially the same as having a dryer or A/C outlet installed.

You are aware, that the normal Balkonkraftwerk is plugged in with a standard Schuko plug? This _only_ works because there is no licensed electrician involved. The personal cost would add at least 50% of the total expenditure and it is made safe by reducing the safety margin by the up to 800W that the Balkonkraftwerk supplies on that circuit

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