Why Surplus Gets Thrown Away
A rooftop solar system with a net metering or net billing agreement can push excess production onto the grid and get credited for it. Plug-in balcony systems generally cannot. They're legal precisely because they don't export — the enabling laws carve out a device category exempt from interconnection rules on the condition that the system uses zero-export controls and anti-islanding.
The practical consequence: if your panels are making 400W at 1pm and your apartment is only drawing 150W, that extra 250W isn't banked anywhere. It's simply not produced. The inverter throttles back.
This is why your always-on load matters more than your panel size. Buying a bigger kit doesn't help if your home can't absorb the output in real time. A 400W kit in a home with a 400W baseline can outperform an 800W kit in a home with a 150W baseline, despite costing half as much.
How to Find Your Always-On Load
Three ways, in order of accuracy:
- Your utility's hourly usage data. Most utilities with smart meters let you download interval data from your online account. Look at 2–4am on a weekday — that's close to pure baseline. This is free and exact.
- A whole-home energy monitor ($150–300) clamps onto your main panel and reads continuously. Overkill just for this decision, but useful if you're optimizing generally.
- Estimate from appliances. Fridge 60–150W averaged, Wi-Fi router and modem 10–20W, cable/streaming box 15–30W, standby electronics across a home 30–80W, plus anything genuinely always running. Add a bit for a second fridge or freezer in the garage.
If you work from home during daylight hours, your effective baseline is considerably higher than the overnight figure — computers, monitors, lights and a heat pump or window AC cycling all count. That materially improves balcony solar economics, and it's the profile these systems suit best.
What This Calculator Assumes
Production is spread across the daylight window on a smooth curve peaking at midday, scaled so the daily total matches your peak-sun-hours input with a 15% system derate for wiring, inverter efficiency, heat and soiling. Consumption is treated as a flat baseline across the day.
That flat-baseline assumption is deliberately conservative in one direction and optimistic in another. Real homes have spikes — a kettle, a microwave, an AC compressor kicking in — that would soak up surplus the model discards. But real homes also have days when nobody's in and the fridge is the only thing running. On balance the estimate lands in a realistic range, but treat it as a planning figure rather than a guarantee.
Also excluded: panel degradation of roughly 0.5% per year, any inverter replacement over the system's life, and electricity rate increases, which would shorten payback if rates keep climbing at their recent pace.
Legality Before Economics
Balcony solar only became legal in parts of the US recently. Standard interconnection rules were written for 5–25 kW rooftop systems and made no exception for a 400W panel on a railing, which effectively made plug-in solar impractical nationwide through 2024.
Utah's HB 340, signed March 2025, was the first US law to create a separate exempt device category, modelled on Germany's roughly 800W Balkonkraftwerk framework. As of August 2026, eight states have signed legislation — Utah, Maine, Virginia, Colorado, Maryland, Connecticut, Vermont and New Hampshire — with roughly 22 more having introduced bills in the past year.
Two things worth checking separately from state law:
- Your lease or HOA. State legalization doesn't automatically override a rule about attaching anything to a railing. Utah's law is unusual in explicitly protecting tenants; most aren't.
- UL 3700 certification. This safety standard for plug-in solar was only published in December 2025, with certification testing opening in January 2026. Many kits on sale carry component-level certification (the microinverter is UL 1741 listed) without whole-system certification. If your state's law requires certified equipment, verify what's actually certified.
For a full breakdown of state rules and whether the economics work for renters specifically, see our balcony solar guide. If you can't install anything at all, community solar is the alternative worth comparing.