How does plug-in solar work?

What is plug-in solar?

Plug-in solar is a small solar kit you set up yourself and connect by plugging it in, the way you would a lamp. People also call it balcony solar, plug-and-play solar or portable solar. They all mean roughly the same thing.

The state laws passed so far give a more careful definition. Utah's describes a movable system with a limited output that connects through a standard 120-volt outlet and has protection to stop it feeding power into the grid during an outage. Others add a safety certification such as UL 3700. What is not in the definition is a roof, a permit or an installer's crew.

How it works, step by step

1. 1
   The panel makes direct current

   Light hitting the panel produces a small electric current. It is direct current, which is not what your home runs on.
2. 2
   The inverter converts it

   A small inverter, often built into the back of the panel, changes it to alternating current and matches it to the timing of your home supply. It will not run without that supply to match.
3. 3
   The cord carries it to an outlet

   A standard plug on the cord goes into an outlet on one of your household circuits.
4. 4
   Your home uses it first

   Anything switched on takes the solar power before it draws from the grid. Your electric meter records less coming in.

Does it send power back to the grid?

It can, if your home is not using everything the panel makes. If you are out all day with only the fridge and the router running, some of it will leave. The kits are small so that most of it gets used inside.

Do not count on being paid for what leaves. Where the laws speak to it, plug-in devices are kept out of net metering. Virginia, New Hampshire and New Jersey all do this, which also means no bill credit for what you send out. If you want a payment for exported power, that is a different, larger kind of solar system.

What happens in a power outage?

It shuts off. This is on purpose. If the grid is down, a device that kept pushing power into the wiring could put electricity onto lines that crews think are dead. So the laws ask for a feature that stops the device energising the building during an outage. Maine's law describes an inverter that shuts off within 0.2 seconds of losing grid power.

What can a plug-in kit power?

Think of it as taking the edge off your usage, not running the house. In good sun, a kit in the usual size range can cover a fridge, a router, a laptop and some lights at once. In poor light it makes much less. It will not run an oven or a dryer, and it is not meant to. Those still come from the utility.

How much you save depends on what is running while the sun is up. Someone home all day gets more out of it than someone who leaves at 7 a.m. and gets back at 6 p.m. To see what that could mean in dollars where you live, use the plug-in solar savings calculator. Our guide to what a plug-in solar kit costs in the US will cover prices once we have real ones, and the methodology page shows how we work out savings.

Plug-in, balcony and plug-and-play solar: the same thing?

Mostly. "Balcony solar" comes from Germany and Europe, where panels are often hung on a railing. In the US the same kit can go on a railing, a wall, a fence or the ground, so many laws say plug-in or portable instead. "Plug-and-play" is a marketing phrase and is also used for some kits that are not the kind these laws cover. If a product says plug-and-play, check what it actually needs to connect.

Safety and the limits

A device is supposed to be tested and certified to a recognised standard, which is why the laws mention UL and similar labs. Check the label on a kit against what your state asks for. The rules also limit the output, usually to 1,200 W, and some ask for a notice to the utility. Outlets in older homes can be a weak link, and a licensed electrician is the right person to ask.

Is it legal where you live?

That depends on your state. Some have a law in force, some have signed one that has not fully started, and some have only a bill. Our plug-in solar laws by state tracker shows each one with the date we checked. To see how different the paths are, compare Utah, which has had a law in force for a while, with the recently signed laws in New Jersey and California. If you would like a note when yours changes, join the alert list. In Britain the rules are national, and we cover them in how plug-in solar panels work in the UK.