Back to blog

Plug-In Solar: What It Is, Where It's Legal, and How to Use More of What It Makes

· Last reviewed · Hungry Machines Team

Plug-in solar is a small photovoltaic system you install yourself: one or two panels on a balcony rail, a fence, or a flat roof, a microinverter, and a cord that goes into an ordinary wall outlet. No roof penetrations, no electrician, no combiner panel. The inverter pushes power onto the branch circuit it is plugged into, and the house consumes it before it consumes anything from the grid.

The appeal is obvious for anyone who rents, or whose roof faces the wrong way, or who does not want a five-figure contract. The economics are less obvious, and they hinge on a detail: with plug-in solar, a watt you do not use at the moment it is generated is usually a watt you give away for free.

That is a scheduling problem, and it is the one Hungry Machines Energy already solves.

How it works

The hardware is basic:

  • Panels, usually one or two, with combined capacity anywhere from 200 Wp to about 1,600 Wp
  • A microinverter, which converts DC to grid-synchronized AC and usually shuts down when it stops sensing the grid, so an unplugged cord is never live and the system cannot backfeed a circuit during an outage
  • A cord into a standard outlet

When the panels produce 600 W and your house is drawing 900 W, the grid supplies the remaining 300 W and your meter records that instead of the full 900 W. Nothing is stored and nothing is switched. The solar power simply reaches the nearest load first, because that is how electricity flows.

When the panels produce 600 W and the house is only drawing 200 W, the surplus 400 W flows back through the meter to the grid. What happens next is the whole question.

Where people are using it

Germany is the mature market, where the systems are called Balkonkraftwerke, balcony power plants. A 2024 reform package raised the inverter limit to 800 W, allowed up to 2,000 Wp of panel capacity behind it, and cut the paperwork down to one simplified entry in the national registry with no separate utility notification. A parallel change to German property law, in force since 17 October 2024, added plug-in solar to the list of privileged alterations in the civil code and the condominium act: a tenant can now require a landlord to permit one unless refusal is genuinely reasonable, and an owners association decides by simple majority rather than the three-quarters majority other structural changes need. The registry held about 1.29 million systems in March 2026, up from roughly 480,000 before the reform.

Utah was the first US state to write a rule for them. HB 340, signed in March 2025 and effective that May, defines a portable solar generation device as a movable system producing at most 1,200 W that connects through a standard 120-volt outlet, and exempts it from the interconnection agreement, from utility pre-approval, from related fees, and from any demand to fit extra controls. In exchange the device has to stop energizing the building during an outage and has to meet NEC and UL requirements.

The rest of the US is moving quickly behind it. Eight states have signed plug-in solar laws by mid-2026: Utah, Maine, Virginia, Maryland, Colorado, Connecticut, Vermont, and New Hampshire. New York’s legislature passed a bill awaiting the governor’s signature as of July 2026, and close to thirty states have introduced legislation of some kind. The limits differ, so Utah’s 1,200 W is not a national figure and several states allow more.

The product-safety gap closed as well. For years no North American standard covered a cord-connected inverter that backfeeds a branch circuit, which was the substantive objection inspectors and utilities raised. UL 3700, published in December 2025, is that standard, covering overcurrent protection, touch safety, and ground-fault detection. The first microinverter certified to it shipped in July 2026. Rules still vary by state and utility, so check yours before ordering.

The part that decides whether it pays

Rooftop solar in a net-metered state is slightly different: surplus production can earn a credit, so it may not matter when you use the power. Plug-in solar generally does not earn such credits. The German default for a balcony system is unentgeltliche Abnahme, unpaid acceptance, where the grid operator has to take your surplus and pays nothing for it. A route to the EEG feed-in tariff exists, at 7.78 cents per kWh in 2026, but it requires the metering and registration of a full PV plant, and against 30 to 40 cents per kWh for grid power that credit is worth a fraction of using the power yourself. Utah’s exemption addresses interconnection, not compensation.

So the value of the system is not what it generates. It is what it generates while something is running.

How much that adds up to depends on the day. The same 800 W kit puts out far more under a clear southern sky than under cloud, or angled toward the sun instead of strapped flat against a balcony rail. A single fixed estimate can’t tell those apart. That’s why Hungry Machines forecasts each day from the weather instead of guessing (more on that below).

Now put a typical house underneath it. Between the refrigerator, lights, and everything on standby, base load might be 200-400 W. Midday production exceeds that for hours. Every one of those watts leaves the building and earns nothing.

Half your production going out the door is not a hardware defect. It is the default outcome for a system whose output peaks at noon in a house whose occupants are at work and whose big machines run in the evening.

Which is a scheduling problem

The fix does not involve buying anything. It involves moving the loads you already run into the hours the panels are producing.

A water heater reheating a tank or HVAC cooling the house draws more than an 800 W system makes. An EV charger draws far more. A heat pump on a low compressor stage is in the same range. Any one of them, running at noon instead of at 7pm, converts exported watts you were giving away into watts that offset your full retail rate, delivery charges included.

This is worth more per kilowatt-hour than ordinary time-of-use shifting. Moving a load from an expensive hour to a cheap one saves you the difference between two grid prices. Moving a load into your own solar window saves you the entire price of that kilowatt-hour if exporting pays you nothing, or the difference between that price and your export rate if you have entered one, and it does so on top of whatever your rate structure was already doing.

The catch is that you cannot run this schedule by hand. Solar output moves with the seasons, your rates could move with the day, and the house needs to be the right temperature when someone gets home.

How Hungry Machines helps

Register a plug-in system with its size in kilowatts, 0.8 for a typical German-spec kit, 1.2 for a Utah-legal one, and Hungry Machines pulls a weather forecast for your panel’s tilt and azimuth each night and uses it to discount the price of every time slot for your other machines. Connect a production sensor (the wattage your inverter or plug reports) and the forecast gets sharper over time: after about three days of readings the optimizer has fit your array’s real yield, and after about ten days it has learned which hours a nearby wall or roofline shades. Enter what your utility pays for exported power and the plan starts pricing solar-covered hours at that rate instead of at nothing, and reports the surplus it expects to sell back.

Your EV charger, home battery, HVAC, water heater, and robot are scheduled together against that discounted price curve, in that order: each appliance’s planned draw is subtracted from the forecast before the next one is planned, so the surplus the plan reports is what is genuinely left over, before counting whatever else in the house, the fridge, the lights, anything already running, draws from it too.

Adding a battery

A battery attacks the exact weakness described above. It absorbs midday surplus that no load is there to consume and returns it in the evening, when both your usage and your rates peak. Several plug-in vendors now ship a matched battery for this reason.

Register it as a home battery and it is optimized like any other. Sunny hours price at your export rate, or at nothing if you have not entered one, so charging naturally lands in the middle of the day and discharge lands in the expensive evening window. Whether the battery earns back its price is a separate calculation that considers what you paid and how wide the gap is between your cheapest and most expensive hours.

Frequently asked questions

Do I need net metering or an interconnection agreement for plug-in solar?

It depends on where you live, and it is the question to settle before buying anything. Germany reduced it to one simplified registry entry with no separate utility approval. Utah, in a 2025 law, exempted systems up to 1,200 W from the interconnection application, its fees, and any additional insurance requirement, and seven more states had signed their own versions by mid-2026. Limits and conditions vary, and states without a law still apply the default interconnection process even to an 800 W kit. Ask your utility first.

How much does an 800 W plug-in system actually produce?

It depends on the day, and Hungry Machines forecasts it rather than assuming a fixed shape. Each night it pulls tilted-plane irradiance for your panel’s actual tilt and azimuth from a weather forecast, so a cloudy afternoon shows up as lower expected output, not the same curve every day. Connect a production sensor and the estimate stops being a forecast at all: after a few days of readings it becomes a fit to what your specific panel makes, including any shading from a nearby wall or roofline.

Does Hungry Machines need a sensor on my plug-in solar?

No, but connecting one makes the forecast yours instead of a generic one. Register the system with its size, tilt, and azimuth and Hungry Machines forecasts output from the weather immediately. Add a production sensor (the wattage your inverter or plug reports) and the integration pushes a reading every five minutes; after about three days there is enough data to fit your array’s real yield, and after about ten days a shading profile for the hours a wall or roofline blocks. Skip the sensor and the plan keeps using the weather forecast alone.

Is plug-in solar worth pairing with a battery?

A battery addresses the exact weakness of plug-in solar, which is midday production with nothing running to absorb it. Several plug-in kits now ship with a matched battery for this reason. Register it in Hungry Machines and it is optimized like any other appliance: sunny hours price at your export rate, or at nothing if you have not entered one, so charging tends to land in the middle of the day and discharge into the expensive evening. Whether the battery pays for itself is a separate question that depends on its price and the spread between your cheap and expensive hours.

Next steps

If you already have a plug-in system, the fastest gain available to you is not another panel. It is moving your water heater and your EV charging into the hours the panels are already producing.

To see how the nightly plan is built, read what happens after you connect. To understand how your rate structure interacts with all of this, start with the time-of-use rates guide or the cheapest times to run your appliances. To pick the hardware that connects your home to the optimizer, see how to connect and pricing.

Sources

Rules for plug-in solar are changing quickly in both the US and Germany. Everything below was checked in September 2026, and so was the Hungry Machines model described above: the tilted-plane irradiance forecast, the learned yield and shading, the export rates, and the sequential allocation across your other machines. If you are reading this much later, verify the current position with your own utility before buying.

United States

Germany

General

Join Our Newsletter

Energy-optimization techniques, product updates, and best practices for home automation.