Balcony Solar Calculator: output, savings and payback
Two modules on the railing, an 800 W inverter, $800 spent: when has that paid for itself? This calculator works out the annual output from orientation, tilt and shading, derives the actually self-used share from your household base load, and turns that into yearly savings, payback time and lifetime profit.
The electricity price is prefilled with the current national average – enter your own per-kWh rate and the result fits your household.
Result
Pays for itself in
10.6 years
$70.55 saved per year on $800.00 of purchase cost
- Annual output
- 786 kWh
- 893 kWh per kWp
- Of that self-used
- 415 kWh
- self-consumption 53%
- Savings per year
- $70.55
- about $5.88 per month
- Profit after 20 years
- $829.76
- $1,629.76 in savings minus the purchase cost
- Fed in unpaid
- 371 kWh
- no revenue without a feed-in contract
- Share of annual consumption
- 4%
- covered by the system
- Best and weakest month
- October 84 kWh · June 37 kWh
- CO₂ avoided
- 291 kg/year
- Vertical on the railing loses about 30 % against a tilted module. If your bracket allows a 30° tilt, it is clearly worth it.
- All values are estimates based on monthly averages. Weather, exact shading and actual consumption behaviour can shift the result by 10–20 %.
Show calculation
- 1Annual output: 880 W of modules, South Vertical on the railing (90°) (67% of the optimum) = 786 kWh/year
- 2Specific output: 893 kWh/kWp
- 3Oversizing 1.1× versus 800 W: clipped on sunny middays = 30 kWh/year
- 4Self-consumption at a 150 W base load: 415 kWh/year (53% of output)
- 5To the grid, unpaid: 371 kWh/year
- 6Savings: 415 kWh × $0.17 = $70.55 in the first year
- 7Payback: $800.00 ÷ $70.55 = 10.6 years
- 8Over 20 years: $1,629.76 in savings, $829.76 after deducting the purchase cost
How it's calculated
A plug-in balcony solar system only saves money if the power is used inside your own home. Surplus goes to the grid unpaid without a feed-in contract. That's why this calculator doesn't assume a flat self-consumption rate but compares the daily generation curve with your household's base load.
output [kWh/year] = module power [kWp] × irradiation × orientation factor × shading × performance ratio
self-consumption = share of generation that stays below the base load
savings [$/year] = self-used [kWh] × electricity price [$/kWh]payback [years] = purchase cost ÷ annual savingsWhy orientation matters so much
The reference is a module tilted 30–35° facing south. Every other mounting delivers a share of that – vertical mounting on a balcony railing, the most common setup, reaches about 70 %.
| Orientation | flat (10°) | tilted (35°) | vertical (90°) |
|---|---|---|---|
| South | 92 % | 100 % | 70 % |
| Southeast / southwest | 91 % | 95 % | 66 % |
| East / west | 85 % | 81 % | 51 % |
| Northeast / northwest | 80 % | 63 % | 31 % |
| North | 77 % | 55 % | 20 % |
Inverter limit and oversizing
Plug-in systems are capped by their microinverter – 800 W in Germany since 2024, and typically 800–1,200 W where US states have legalized plug-in solar. Modules may be rated higher, which is sensible because the rated power is only reached under lab conditions. Well above the inverter limit, sunny middays get clipped; the calculator shows that share.
Worked example
Typical rental-apartment setup: two modules totalling 880 Wp, an 800 W microinverter, mounted vertically on a south-facing railing with light afternoon shading. Household with a 150 W base load, purchase cost $800.
Inputs
- Module power: 880 Wp
- Inverter power: 800 W
- Orientation: South
- Mounting: Vertical on the railing (90°)
- Shading: Light (briefly at the edge)
- Region: US Midwest
- Household base load: 150 W
- Annual electricity consumption: 10,500 kWh
- Storage (optional): 0 Wh
- Purchase cost: 800 $
- Your electricity price: 0.17 $/kWh
- Feed-in payment: 0 $/kWh
- Annual electricity price increase: 2 %
- Annual module degradation: 0.5 %
- Time horizon: 20 years
- Performance ratio: 80 %
Result
10.9 years
Pays for itself in
- Annual output
- 719 kWh
- Of that self-used
- 399 kWh
- Savings per year
- $67.87
- Profit after 20 years
- $767.79
- Fed in unpaid
- 320 kWh
- Share of annual consumption
- 4%
- Best and weakest month
- February 77 kWh · June 33 kWh
- CO₂ avoided
- 266 kg/year
Calculation
- Annual output: 880 W of modules, South Vertical on the railing (90°) (67% of the optimum) = 719 kWh/year
- Specific output: 817 kWh/kWp
- Oversizing 1.1× versus 800 W: clipped on sunny middays = 16 kWh/year
- Self-consumption at a 150 W base load: 399 kWh/year (56% of output)
- To the grid, unpaid: 320 kWh/year
- Savings: 399 kWh × $0.17 = $67.87 in the first year
- Payback: $800.00 ÷ $67.87 = 10.9 years
- Over 20 years: $1,567.79 in savings, $767.79 after deducting the purchase cost
The variables explained
- Module power (Wp)
- Sum of the rated power of all modules. Common kits: 2 × 440 Wp = 880 Wp. More modules pay off as long as clipping stays small.
- Inverter (W)
- Maximum feed-in power. 800 W is the German limit; check your local rules for plug-in systems.
- Orientation and tilt
- Together they set the share of the optimum (south, 35°). Vertical on a railing means less summer but relatively more winter output – the low winter sun hits the surface almost head-on.
- Shading
- Neighbouring balcony, tree, roof overhang. Partial shading has an outsized effect because the inverter follows the weakest string.
- Base load (W)
- What runs permanently: fridge, router, standby. It determines how much of the generated power is used directly. Typically 80–300 W.
- Storage (Wh)
- Optional plug-in battery. It shifts midday surplus into the night – but never more than the night-time consumption.
- Electricity price ($/kWh)
- Your per-kWh rate from the bill. Prefilled with the national average – your own tariff is more accurate.
- Purchase cost
- Modules, inverter, mounting hardware, cables – plus storage and an electrician if applicable.
Common mistakes
- Counting output instead of self-consumption: you only save what stays in the house – the rest goes to the grid unpaid.
- Taking the rated power at face value: 880 Wp delivers 500–900 kWh a year depending on mounting, not 880 kWh.
- Confusing vertical with tilted mounting: on a railing you lose about 30 % compared with a tilted module.
- Underestimating shading: a tree that only blocks the afternoon can easily cost a quarter of the annual output.
- Adding storage without adding its cost: a battery raises self-consumption and the purchase price – both belong in the same calculation.
- Treating price increases as certain: they shorten the payback on paper but are an assumption about the future.
Assumptions and limits
- The base load is treated as constant. Real households have daytime peaks (cooking, laundry) that add self-consumption – the calculator errs on the conservative side.
- The daily curve is modelled as a half-sine over a monthly generation window, split into sunny and overcast days. For reliable individual figures an hourly simulation is needed (PVGIS, NREL PVWatts).
- Yield data comes from PVGIS (2005–2023 average) for one reference site per region – your location may differ by a few percent. The terrain horizon is included, shading from neighbouring buildings and trees is not; that's what the shading field is for.
- Storage is only charged as far as it can be discharged at night; round-trip efficiency 90 %.
- No maintenance, insurance or removal costs, and no subsidies or tax credits are included.
- This is not tax or legal advice; registration requirements and landlord rules still need checking.
Frequently asked questions
When does a balcony solar system pay for itself?
For an 800 W kit at $800, vertical south mounting and a 150 W base load the payback is roughly six to eight years at US electricity prices. Three figures decide it: purchase price, electricity rate and self-consumption share. In countries with high electricity prices such as Germany (0.39 €/kWh) the same system pays back in four to five years.
How many kWh does an 800 W balcony solar system produce per year?
500–900 kWh in Central Europe, more in sunnier regions. Tilted and facing south, 900–1,000 kWh per kWp is realistic; vertically on a railing about 650–700 kWh per kWp. East or west sits in between, north far below.
How much money does balcony solar save per year?
Savings = self-used kWh × electricity price. With 600 kWh of output, 65 % self-consumption and $0.17/kWh that's about $66 a year; at German prices around €150. Anyone at home during the day or with a higher base load does better.
Why does only self-consumption count?
Plug-in systems are usually operated without a feed-in contract. Power you don't consume at the moment it's generated flows to the grid unpaid – modern meters don't run backwards. That makes your household base load the most important lever.
Is a battery worth it for balcony solar?
It raises self-consumption noticeably – in the example from 65 % to over 80 % – and the savings with it. But if the battery costs $600, it still extends the payback. Do the maths together: enter the storage size and add its cost to the purchase price.
How much module power may I connect to an 800 W inverter?
Oversizing is sensible because the rated power is rarely reached. Above roughly 1.3× the inverter rating, sunny middays get clipped noticeably. Germany allows up to 2,000 Wp of modules behind an 800 W inverter in the simplified procedure; US rules vary by state and utility.
What does a vertically mounted module on a railing deliver?
About 70 % of what the same module would deliver tilted towards the south. In return the winter share is relatively better because the sun is low – and in winter that power is particularly easy to self-consume.
Do I have to register a balcony solar system?
In Germany yes – free registration in the Bundesnetzagentur's Marktstammdatenregister, which takes a few minutes; the separate utility notification was dropped with the Solarpaket I. In the US it depends on the state and utility: plug-in solar is explicitly permitted only in some jurisdictions, so check the interconnection rules first.
Does balcony solar pay off facing north?
Rarely. Mounted vertically facing north only about 30 % of the optimum remains – roughly 250 kWh a year for 880 Wp. That pushes the payback well beyond ten years. An east or west facade is almost always the better choice.
Where does the preset electricity price come from?
From the EIA's average US residential rate (and Eurostat for European countries). That's a national average – your own per-kWh rate is on your bill and gives a far more accurate result. Just overwrite the value.
Sources and background
- EIA – Electric Power Monthly, average residential electricity price
- Eurostat: household electricity prices (nrg_pc_204), consumption band 2,500–4,999 kWh, taxes included
- NREL – PVWatts Calculator (site-specific solar output)
- PVGIS 5.3 (European Commission, JRC): yield data by site, orientation and tilt – the data basis of this calculator
- Bundesnetzagentur: Marktstammdatenregister – registration of plug-in solar devices