Sizing a balcony solar kit means balancing the panel watts you want against the space, sunlight and local rules you actually have. For many apartments, one 350-450 W panel suits a small or shaded balcony, and two panels totaling roughly 600-900 Wp suit a standard sunny one. Panel watts are not the same as inverter output, and the rules differ by country, so check yours before you buy.
Quick answer
Most people should start with one panel if space is tight or shade is heavy, and two panels (about 800 Wp) if the balcony gets good sun. Add more only where local limits, mounting space and daytime use justify it. Skip the battery until you can measure a daily surplus.
- Best for small or shaded balconies: one 350-450 W panel
- Best for sunny balconies: two panels, about 800 Wp
- Best for evening use: a 1-3 kWh battery, only after measuring surplus
- Panel rating (Wp) is DC peak power. The microinverter’s AC rating is what feeds your home circuit.
- Two panels of about 400 Wp commonly produce roughly 600-900 kWh a year in Germany. Your result depends on sun, direction and shade.
- Shade at the actual railing position can cut output more than panel choice does.
- Batteries of 1-3 kWh are more realistic than large units for an 800 W system.
- Rules for plugs, inverter limits and registration differ by country and state. Confirm with your utility.
- What is the difference between panel wattage, Wp, and inverter output?
- Should I choose one panel or two panels?
- Sizing table by balcony and use
- How much electricity will a 400 W or 800 W kit generate?
- How do direction and shade change the size you need?
- Worked example: from bill to kit size
- Do I need a battery?
- How many panels can fit safely on my balcony?
- Local rules: Germany, the UK and the US
- What do owners say?
- Frequently Asked Questions
What is the difference between panel wattage, Wp, and inverter output?
Panel wattage (Wp, or watt-peak) is the DC power a panel can make under ideal test conditions. Inverter output is the AC power the microinverter can send into your home circuit. The brief’s sources make the same split: panel wattage is a DC peak rating, while the inverter’s AC limit decides how much can be fed in.
This is where many buyers get confused. An “800 W kit” usually means an 800 W AC microinverter. It may be paired with two 400 Wp panels, or with more panel capacity than that. In Germany, simplified plug-in systems commonly use microinverters up to 800 W AC with up to 2,000 Wp of connected panels, according to the sources behind this guide.
Real panels rarely hit their peak rating. Clouds, heat, angle and dirt all pull output down. So a panel rated 400 Wp may deliver 250-320 W on a good day and far less on a gray one.
Should I choose one panel or two panels?
Choose one panel when space is limited, shade is heavy or your daytime use is low. Choose two when the balcony has good sun and you have the width to mount them securely. A standard module is commonly 400-440 Wp, so an 800 W-class system generally means two panels.
If your balcony is narrow, mostly shaded, or your railing is questionable, then pick one panel.
If your balcony gets several hours of direct sun and you have room for two modules, then pick two panels, about 800 Wp.
If your local rules allow more DC capacity, you have mounting space, and you use the extra power during the day, then consider adding panels. Otherwise, stop at two.
Sizing table by balcony and use
Use this table as a starting point. Annual figures for two panels come from the brief’s German estimates. The one-panel row is roughly half of that. Panel size is typical, not exact, so check the datasheet.
| Setup | Balcony width needed | Approx. annual generation | Likely daytime use | Battery suitable? |
|---|---|---|---|---|
| One 400 W panel | Roughly 1.1-1.7 m of railing, depending on orientation | About 300-450 kWh (half the two-panel range) | Fully used by fridge, router, standby loads | Rarely |
| Two 400 W panels (800 W AC) | Roughly 2.2-3.4 m side by side | About 600-900 kWh in Germany; 650-950 kWh for a typical 800 W system | Used well if someone is home or the base load is high | Maybe, 1-3 kWh |
| Larger array (up to 2,000 Wp where allowed) | Often needs a roof, wall or ground mount, not just a railing | Higher, but the inverter cap clips peaks | Needs real surplus demand | Only with measured surplus |
Panels are commonly around 1.7 m by 1.1 m, so width depends on whether you mount them upright or lengthwise.
How much electricity will a 400 W or 800 W balcony solar kit generate?
In Germany, a typical 800 W system is estimated at roughly 650-950 kWh a year, depending on region, orientation, shading and angle. Two panels of about 400 Wp are commonly estimated at 600-900 kWh. One 400 W panel will make about half of that.
You can redo the maths for your own location. Here is the formula and an example with stated assumptions:
- Formula: panel watts × peak sun hours × system losses × days
- Assumptions: 800 Wp, 3.5 peak sun hours a day (an example; look up your own), 20% system losses (factor 0.8), 365 days
- Result: 0.8 kW × 3.5 × 0.8 = 2.24 kWh a day, or about 818 kWh a year
- Value: at an assumed $0.17 per kWh, that is about $139 a year if you use every kWh yourself
Swap in your own sun hours and electricity price. Power you cannot use at home may earn nothing, depending on your utility.
How do direction and shade change the size you need?
South-facing panels generally produce more than east- or west-facing ones, and tilt and shade can change output a lot. If your balcony faces east or west, expect lower yield and consider the second panel. If it faces north, one panel may not be worth it.
How does balcony direction affect the kit size I need?
A south-facing balcony may meet your goal with one or two panels. An east or west balcony gives a longer, flatter curve: more morning or evening power, less at noon. That can match a household that is home in the morning and evening. Vertical mounting on a railing also changes the angle, so output usually differs from a tilted mount.
How much should shading reduce my expected output?
Measure it at the railing, not from memory. Sources on balcony solar stress that real shade should be assessed at the actual panel position. A guide on checking balcony shade before you buy covers how to do this. As a planning rule, this guide uses a 10-20% cut for light, brief shade and 30% or more when shade hits for several midday hours. These are planning assumptions, not measured figures. Even partial shade on one panel can drag down a string, so keep modules clear of railings, planters and the balcony above.
Worked example: from bill to kit size
Follow these steps with your own numbers. The example household is fictional.
- Daily use: 9 kWh a day (about 3,285 kWh a year).
- Daytime share: 30% is used while the sun is up, so 2.7 kWh. If you work away from home, check a smart meter or app for the real number.
- Raw output of two panels: 800 Wp × 3.5 peak sun hours × 0.8 losses = 2.24 kWh a day.
- Orientation and shade adjustment: an east-facing balcony with some afternoon shade, assumed at 0.75. That gives 1.68 kWh a day.
- Compare: 1.68 kWh is less than the 2.7 kWh daytime use, so almost all of it is used at home. No surplus, so no battery.
Now change one thing. If the household is empty by day and uses only 1.0 kWh then, surplus is 0.68 kWh. A battery that small is not worth the cost. One panel (about 0.84 kWh a day on the same assumptions) would fit better, and a second panel only adds surplus.
Do I need a battery, and what battery size is appropriate?
Most balcony systems do not need one. Batteries around 1-3 kWh are often more realistic for an 800 W system than very large units, because storage should match your usable daily surplus. Size the battery to the smaller of two numbers: your daily surplus and your evening use.
Using the second scenario above, a 0.68 kWh surplus fits inside the smallest battery you can buy. Owners report that batteries add substantial cost and may not pay back when daytime use is already high. For a German view on this, see this article on 800 W balcony solar with battery storage. Log your surplus for a few weeks with a monitoring app before buying storage.
How many solar panels can fit safely on my balcony?
Count what fits within the railing’s rated load, not just what the balcony’s width allows. Most balconies fit one or two panels. Wind adds force, so mounting hardware, railing strength and landlord approval all matter.
Can my balcony railing support the panels and mounting system?
Check the mounting kit’s stated load limits and your railing type. Panels are large sails in wind. Do not hang them on thin or decorative railings. If you are unsure, ask your landlord, building manager or a structural professional. Poorly fitted hardware is a common complaint among owners with non-standard balconies.
Balcony measurement checklist
- Usable railing width, and panel dimensions from the datasheet
- Railing material, thickness and condition
- Cable route to the outlet, without crossing walkways or pinching doors
- Shade periods from morning to evening, measured at the railing
- Outlet location, and whether it is a dedicated, properly grounded circuit
- Wind exposure, especially on upper floors
- Landlord, HOA or building permission
Local rules: Germany, the UK and the US
Plug-in solar rules differ by country, and in the US they vary by state and change quickly. Do not copy another country’s sizing. Always confirm with your utility before you buy or plug anything in.
What inverter size is legal for a plug-in balcony solar system?
In Germany, simplified systems commonly use microinverters up to 800 W AC, with up to 2,000 Wp of panels. The UK differs. Plug-in systems there need a factory-fitted UK plug with a 5 A fuse and inclusion on the ENA Type Test Register, according to the brief’s source. See this explainer on plug-in solar legality in the UK. For the US, the sources here do not name any state’s rule, so ask your utility and landlord what applies to you.
What safety, registration, plug, and grid-connection rules apply where I live?
Rules for registration, metering, plugs and grid connection depend on your location. Follow these safety points everywhere:
- Never use an extension cord for a solar kit.
- Never bypass the microinverter’s grid shut-off. It stops power flowing into a dead grid.
- Use only certified equipment that your utility accepts.
- Hire a licensed electrician if your outlet is old, ungrounded, on a shared circuit, or if you are unsure.
- Ask your utility whether registration or a meter change is required.
What do owners say?
Owners like
- Simple plug-and-play installation compared with a full rooftop solar installation
- Useful reduction in daytime electricity use when panels get direct sun
- Compact size that suits renters and apartment residents
- Battery storage can increase the share of solar energy used after sunset
- Monitoring apps show generation and help spot shading or connection problems
Common complaints
- Actual generation is often much lower than the headline panel rating in cloudy weather or winter
- Railings, vertical mounting and nearby buildings can cause unexpected shading
- Mounting hardware can be hard to fit securely on non-standard balconies
- Cable routing and plug placement are less convenient than product descriptions suggest
- Batteries add substantial cost and may not pay back when daytime use is already high
Summarised from owner reviews and long-term user reports; individual experiences vary.
Frequently Asked Questions
What size balcony solar kit do I need?
Most apartments fit one panel (350-450 W) if space or sun is limited, or two panels (600-900 Wp) on a sunny balcony. Check your local inverter and DC limits first.
Is an 800 W balcony kit really 800 W of panels?
Not always. The 800 W usually refers to the microinverter’s AC output. Panels may total 800 Wp or more, depending on local rules.
How much will two panels save me?
Using 600-900 kWh a year (the German estimate for two 400 Wp panels) and an assumed $0.17 per kWh, savings are about $102-$153 a year, but only for the power you use at home. Your price and sun hours will change this.
Will a battery make my kit pay back faster?
Usually not. Batteries add cost, and owners report they may not pay back when daytime use is already high. Add one only after measuring a real surplus.
Do I need an electrician?
Plug-in kits aim to avoid major wiring, but get a licensed electrician if your outlet or circuit is questionable, or if local rules require an inspection. Never use extension cords, and confirm requirements with your utility.
Sources
