Thinking About Balcony Solar

by David St. Martin · October 3, 2026

The rooftop calculator on this site answers one question: is a roof full of panels a better use of your money than investing it? Balcony solar looked like a smaller version of the same question. It isn't. A plug-in kit sits wherever your balcony lets it, plugs into an ordinary outlet, and usually can't sell anything back to the grid. Each of those changes the math, and each one sent me back to rebuild something I thought I already had.

This is how the balcony calculator came together, and what I learned along the way.

My data only knew one angle

The rooftop calculator estimates production from real runs of NLR's PVWatts model in 14 markets. Every one of those runs assumed a panel tilted 20 degrees, which is about what a typical roof gives you. That's fine for roofs. A balcony panel might lie nearly flat on the floor, lean back against the railing, or stand straight up. 20 degrees tells you almost nothing about a panel at 90.

My first attempt filled the gap with physics: a standard sky model that works out how much sun lands on a tilted surface, scaled to the measured 20-degree numbers. It got the textbook landmarks right. A vertical south panel came out at roughly two-thirds of a well-tilted one, and vertical east or west at about half.

It still missed the part that matters most for balconies. An upright panel lives on low winter sun, and it sees as much ground as sky, so cold clear air and snow on the ground both help it. A sky model that only knows latitude can't see either one. So I went back for more data: 896 PVWatts runs, covering all 14 markets at eight tilts and eight compass directions.

The measured numbers surprised me:

• A vertical south panel makes 0.52 of what a 20-degree roof panel makes in Miami, but 0.72 in Boston.
• Denver reads 0.71 and Marin County reads 0.61, even though they're only 1.6 degrees of latitude apart.
• Upright and facing north, a panel makes about a fifth of what it would facing south.
• East and west walls come out near half, but they aren't equal. Coastal California does a little better facing west, because the morning marine layer costs the east side.

Going by latitude alone would have been off by as much as 8%. Every market now reads its own measured curve, and the calculator only estimates between measured angles, which costs 1 to 2% at the angles balconies use.

The angle isn't really a choice

On a roof, the angle is whatever the roof is. On a balcony, you'd think you could pick. Mostly you can't, because tilt eats floor space.

The panel in the kit I use as the default is 5.5 by 2.9 feet. Lean it back to 30 degrees, long side along the railing, and it takes 2.5 feet of floor. Turn it the other way and lean it back further, and on a shallow balcony it simply won't fit. Stand it straight up and it takes almost no floor at all, but it makes less power.

So I built an angle tool instead of a single tilt box. A side view draws the panel to scale against the depth of your balcony, and a compass shows which way it faces. If the angle you picked doesn't fit, the calculator tells you the steepest angle that does, and what that costs you in kilowatt-hours.

The trade is real. In Marin, four panels facing south make about 1,750 kWh a year at 30 degrees, and about 1,090 standing straight up.

Smaller is often better

This was the part I least expected.

Most plug-in kits can't sell power back to the grid. Some aren't allowed to, and some, like the default kit here, ship with a gateway that stops them from trying. Whatever your home isn't using at the moment the sun is shining is simply lost. So the question isn't how much a kit makes. It's how much of that you actually use.

Here's the same panel and price per panel in Marin, facing south at 30 degrees, for a home using 4,000 kWh a year:

Going from two panels to four doubles what the kit makes, but only adds $83 a year, because most of the extra power arrives at noon when nobody's using it. 730 kWh a year goes nowhere.

Put the same four panels on a home using 8,000 kWh a year and the picture flips: 92% gets used and the payback drops to 4 years. The right size depends on how much power your home draws during the day, not on how much sun you get.

Where the inverter caps you

Every kit has an inverter, the box that turns the panels' power into the kind your outlet uses. It also sets a hard ceiling on how much the kit can push at once. In Germany, where balcony kits are everywhere, that ceiling is 800 watts by law. In the US it depends on the product and, increasingly, on your state.

When the panels can make more than the inverter can pass, the extra is clipped off at midday. A little of that costs almost nothing. Two 450-watt panels on an 800-watt inverter lose essentially no energy over a year, because the panels only exceed 800 watts for a short stretch on the best days. Four 450-watt panels on the same 800-watt inverter are another story: 22% of what they make never gets through.

The calculator shows clipping whenever it's more than 2% of the year, and tells you plainly when a bigger panel buys you nothing.

One inverter per panel, or one for the whole kit

Kits come in two shapes.

Some put a small inverter on every panel. The default kit here works that way: four 300-watt panels, each with its own 280-watt inverter built in. Enphase built its rooftop business on the same idea. Shade on one panel doesn't drag down the others, and the kit's ceiling grows with each panel you add. Four of them give this kit 1,120 watts.

Others run two or four panels into one inverter, often capped at 800 watts. That's cheaper, and it leaves room to put a battery between the panels and the inverter. The catch is the single ceiling: add panels and you mostly add clipping.

Neither is better in general. Per-panel inverters suit balconies with patchy shade. A single inverter suits a small kit that might get a battery later.

Why I didn't add a battery

A battery looks like the obvious fix for all that lost noon power. Store it, use it in the evening. I spent some time on it and decided to leave it out for now, because a battery on a plug-in kit is harder than it looks.

It has to fit the kit. If the inverter is built into each panel, there's nowhere for a battery to sit between the panels and the inverter. It has to be a separate unit that plugs into its own outlet, with its own charger and its own inverter.

It has to know what your home is using. On a kit that can't export, a battery should only release as much as your home is drawing at that moment. That takes a meter on your main electrical panel. Without one, it pushes out power your home doesn't need, and that power is lost.

It changes the legal picture. The solar and the battery both push power into the same household circuit, and the limits on plug-in systems count them together. A setup that's fine with panels alone can go over the line once a battery joins it. Anything that feeds power back through an outlet also needs the right safety listings, UL 1741 for the inverter and UL 9540 for the battery. Rules for plug-in solar are still being written, and they differ from state to state.

It has a hard time outdoors. Most of these batteries can't charge below freezing, so a battery on an unheated balcony loses its winter charging in a cold place.

And the numbers don't help much. The battery only earns money on power you'd otherwise lose. In most of the cases above, picking the right number of panels does the same job for free.

What the calculator does and doesn't do

It uses the same money engine as the rooftop calculator, on top of the measured production data. It counts what you'd actually use, not what the kit makes. It treats anything sent to the grid as worth nothing.

It doesn't tell you whether a plug-in kit is legal where you live. That's changing quickly, and it's the first thing to check with your utility before you buy.

If you're thinking about a kit, try the calculator with your own balcony. Measure the depth first. It decides more than you'd think.