In this guide
Short answer: for most apartments, 400 to 800 W is the sweet spot. Pick 400 W if your home uses little power during the day or you only have room for one panel. Pick 800 W if someone is home during the day or you have a fridge, freezer and home office running. Go to 1,200 W only if you have the space, a dedicated circuit, high daytime use and a state law that allows it.
Why size matters more than with rooftop solar
With rooftop solar and net metering, extra power earns a credit. With plug-in solar it usually doesn't: most state laws exclude these kits from net metering. Every watt your home isn't using at that moment flows to the grid for free. So the question isn't "how much can I produce?" but "how much can I use while the sun shines?"
Step 1: find your daytime base load
Your base load is what your home draws when nobody is doing anything: fridge (cycling, averaging 30 to 80 W), internet router (10 to 20 W), TV and streaming boxes on standby, chargers, a freezer, an aquarium. Typical totals are 100 to 300 W. On top of that come daytime loads: a laptop and monitor (50 to 150 W), a window AC unit (500 to 1,200 W), a dishwasher or washing machine.
How to measure it: open your utility's app or smart meter portal and look at hourly use between 10 a.m. and 3 p.m. on a weekday. Or switch nothing on and read the meter for an hour.
Step 2: compare with what each size really produces
A kit rarely produces its label rating. A vertical panel on a south-facing railing peaks at maybe 60 to 75% of its rating on a clear day, and much less in the morning, evening and winter.
| Kit size (AC) | Typical midday output, vertical | Best for | Self-use you can expect |
|---|---|---|---|
| 400 W | 200 to 300 W | Small apartments, empty during the day, one panel of space | 90 to 100% |
| 800 W | 400 to 600 W | Most households, especially with someone home or a home office | 70 to 90% |
| 1,200 W | 600 to 900 W | Larger homes with daytime AC, pool pumps or electric heating | 50 to 75% |
Step 3: check the limits that apply to you
- Your state's cap. 1,200 W in most states with a law, 1,920 W in Colorado, 420 W for self-install in Maine. See the state map.
- Your circuit. At 120 V, 1,200 W is about 10 amps. On a 15-amp circuit that's already shared with a microwave or space heater, that's too much. Read the safety guide before choosing 1,200 W.
- Your space. A modern 400 W panel is about 5.6 by 3.7 feet and weighs around 45 lb. Many balconies fit two at most.
- Your landlord or HOA. Smaller kits are easier to get approved. See Can my landlord stop me?
Example: the same apartment with three sizes
Consider an apartment in New Jersey (25.2¢/kWh), panels vertical on a south railing, light shade, a base load of about 200 W and someone home part of the day:
| Kit | Typical price | Produced per year | Used at home | Saved per year | Payback |
|---|---|---|---|---|---|
| 400 W | ~$500 | ~330 kWh | 95% | ~$80 | ~6 years |
| 800 W | ~$900 | ~655 kWh | 80% | ~$130 | ~7 years |
| 1,200 W | ~$1,400 | ~985 kWh | 70% | ~$175 | ~8 years |
The bigger kit saves more dollars but pays back more slowly, because a growing share of its output is given away. Prices are illustrative; try your own in the calculator.
Panel watts vs inverter watts
Kits are often sold with more panel than inverter, for example 2 × 450 W panels on an 800 W inverter. That's normal: the inverter caps the peak, but you gain output on cloudy days and in winter. The legal cap applies to the inverter's AC output. Keep the ratio reasonable (up to about 1.25 to 1); beyond that, the extra panels mostly add weight and cost.
Our recommendation
- Renting, small space, out all day: 400 W, one panel. Cheap, easy to approve and take with you when you move.
- Most households: 800 W with two panels and a certified microinverter.
- High daytime use and a dedicated outdoor circuit: 1,200 W, or 800 W plus a battery to shift power to the evening.
Next: compare kits on the US market.
Frequently asked questions
Is 400 W of plug-in solar worth it?
Often, yes. A 400 W kit costs less, fits one railing section, and almost all of its output is used by the fridge, router and other devices that run all day, so very little is given away. It saves less in dollars than a bigger kit but usually pays back just as fast.
How much power does my home use during the day?
Most homes have a base load of 100 to 300 W: the fridge, internet router, TV boxes, chargers and devices on standby. Look at your smart meter data or utility app for usage between 10 a.m. and 3 p.m. on a day nobody was home.
Can I plug two 800 W kits into two outlets?
Only if the total stays under your state's cap, which is usually 1,200 W per home or meter, and only if the two outlets are on different circuits. Two kits on one circuit can overload the wiring without tripping the breaker.
Should I buy more panel watts than the inverter rating?
A little more is fine and common: panels rarely reach their label rating, especially when vertical, so 20 to 25 percent more panel than inverter gives more output in mornings, evenings and winter. Far more than that is wasted at midday.
Sources and official texts