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ZDNET's key takeaways

• Plug-in solar uses microinverters to feed power into a household system.
• So far, in the US, only Utah has passed legislation to allow such systems.
• These systems cost about $2,400 and will take years to pay off.

Ever since handling my first tiny solar panel quite a few decades ago, I've been fascinated by them. The fact that a bit of glass had the ability to harvest power from a big nuclear orb 93 million miles away blew my mind back then, and continues to blow my mind now. Since then, solar panels have grown in size and efficiency and dropped in cost to the point where it's becoming economically feasible for you and me to buy panels and generate our own electricity.

But what do you do with that power?

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Up until now, you've had two options. You could have a standard rooftop solar system, have that professionally connected to your home's electrical system, or you could connect your panels to a portable power station, and use that to power devices in your home. 

The downside to the first option is cost, since installing solar panels on your roof and connecting them to your electrical system is a professional job. The downside to the second option is the limitation of having all the power flow through a power station. 

But what if you could connect your solar panels directly to your home's AC system, and do that yourself?

This is what plug-in solar -- also known as balcony solar or portable solar -- promises.

What plug-in solar isn't, and what it actually is

Let me dispel the biggest myth right off the bat -- plug-in solar isn't an AC plug attached to your solar panels. Household power is AC, whereas solar panels output DC (direct current), so the two aren't compatible. If you tried to plug panels directly into a wall socket, a lot of things would go bang. 

Plug-in solar is a new technology that allows solar panels to be connected to a home's electrical system. The idea is that the solar panels collect power during the day, feed it into the home's system, and that power is used to cover the home's daytime base load, and perhaps a bit more (more on base load in a moment). 

What sits between the solar panels and your outlet is a microinverter. This is a box that converts DC electricity from solar panels into AC that your home appliances understand. It can then feed this AC power into your home's electrical system via a standard plug and outlet (so your outlet becomes an inlet). 

As well as converting DC power to AC, the microinverter has to not only inject that power into your home's power system, but do so safely.

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It's that simple. You don't need to hire a professional; you can do it yourself. It also opens up solar to those living in rented properties or in apartments. In Utah, for example, landlords cannot object to tenants installing plug-in solar systems that plug into an outlet and do not require any permanent modifications to the property. 

What's base load?

Base load is the background load that a house draws during the day when no major appliances are actively in use. It represents the continuous energy draw from essential and standby devices, such as internet and Wi-Fi routers, chargers, refrigerators, and freezers. As a ballpark figure, this is approximately 200 to 400 watts (or 4.8 to 9.6 kWh over 24 hours) for the average US home. 

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How much power you can pull depends on factors like the size of your solar panel setup in watts and how much sun you get in your area (called Peak Sun Hours, or PSH), and not forgetting to remove the losses through inefficiencies, shading, and dust and dirt on the panels (called derate). 

The derate figure can vary, but I've gone for 0.77 (equivalent to a 23% loss), which errs on the side of pessimism.

Energy Collected (kWh/day) = Panel Output (kW) × Peak Sun Hours (PSH) × Derate Factor (0.77)

If you live in Alabama and get an average of 3.5 hours of sun a day, a 1 kW (1,000-watt) solar panel array will collect:

1 x 3.5 x 0.77 = 2.7 kWh

Change to somewhere like Arizona, with about 6.5 hours of sun, and the figure climbs a lot.

1 x 6.5 x 0.77 = 5 kWh

How does it work?

You place your solar panels in the garden or on a balcony (balcony installation is great for people renting or living in apartments), connect them to the microinverter, plug them into an outlet, and that's it, you're harvesting solar power while the sun is shining. As long as you're collecting 200 to 400 watts, you've covered base load. 

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OK, but what about the surplus? 

Well, you have options. The excess can go into the grid (you aren't likely to be paid for this, at least under current domestic electrical tariffs, but that could change), or you can push the surplus into a power station for use later. Adding a power station into the mix is going to boost costs.  

So, what's stopping us from using plug-in solar?

Bottom line: It's primarily government red tape. 

While plug-in solar is available in some countries, like Spain and Germany, where you can walk into a superstore and buy a plug-in solar setup, the US and UK are lagging behind. (Here in the UK, Anker has just unveiled the SolarBank 4 E5000 Pro -- what a mouthful! -- ahead of the changes in the regulations to allow these systems.)

Based on the data available, it seems that Utah is the only US state so far that has legalized the use of plug-in solar units, with 29 other states pushing legislation to make them legal. Utah allows systems up to 1.2 kW without utility approval or fees. As for legislation currently in the pipeline, most states seem to be sticking with that 1.2 kW limit. 

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One exception is Colorado, which wants to push that limit up to 1.92 kW. Compare this to the UK and Europe, which limit the power to 800W. 

Costs and benefits

Cost is also undoubtedly holding back plug-in solar, as these systems are not cheap, especially compared to the cost of electricity (which, on average, is about 17 cents per kWh). 

A 1,200 W plug-in solar kit will currently set you back about $2,400. I anticipate that as more states give plug-in solar the thumbs-up and more players enter the market, prices will come down. 

If you live in Utah and you've dropped $2,400 on a system, how much will this save you? The best-case PSH for Utah is 7 hours, and plugging this into the equation above gives:

1.2 x 7 x 0.77 = 6.5 kWh

That's a lot of power, worth some $0.86 a day (Utah electricity cost $0.1333 per kWh in February, based on government data), or some $315 a year. Assuming everything goes smoothly, you'll recoup your initial investment in 7.5 years. 

If you're just using the plug-in solar system to cover base load during daytime hours, this figure becomes quite dismal. Assuming your base load is a hefty 400 W, your plug-in solar will cover 2.8 kWh during daylight hours, but it's only worth about $0.37 a day, or $135 a year. 

That means it's 17 years before you've paid for the system.

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You either need to be running a lot more during the day than base load -- such as running heavy appliances like AC or heating, or charging EVs -- or pushing the extra power into a power station (which, as I've already mentioned, substantially increases the costs of your setup). 

Is plug-in solar safe?

One concern raised by legislators in some states is that plug-in solar installations may not be safe. Anything can be unsafe (lithium batteries are now a major cause of house fires), so there's a risk that plug-in solar could be abused or damaged to the point of becoming unsafe. All plug-in solar systems have to conform to National Electrical Code (NEC) regulations and require Underwriters Laboratories (UL) certification.

That's about as safe as anything else we use. 

I've had an EcoFlow PowerStream microinverter for over six months, running on an isolated test grid (plug-in solar isn't yet legal here in the UK either, so I've been limited to test systems), and I've had no issues with the hardware.

Do you need plug-in solar?

Right now, plug-in solar is a niche product, which means prices are high and it will take a long time to pay for itself. But as more countries and states open up to plug-in solar, I expect competition to increase and prices to fall. 

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My advice right now is that if you live somewhere sunny and use a lot of power during the day -- maybe you charge an EV or have a Tower of Power, as I do -- you can cover your costs in a few years. But if you live in Alaska, where you get a couple of hours of sun a day, and all your system is going to power is an internet router, it's not worth it. At least for now.

But watch this space, because this is going to evolve a lot over the coming months and years, and prices could change dramatically.