Solar Roof Potential Calculator
Thinking about rooftop solar? Enter the location, roof tilt and orientation and get a physics-based estimate of annual sunlight energy per square metre for that roof — before you talk to any installer.
By Aryan Khan, Founder & Editor · Updated October 2026
For example, 40 m² of usable south roof at modern panel efficiency holds about 8 kW of panels — at 4.5 peak sun hours a day that is roughly 36 kWh daily, before shade and weather argue.
How to use Solar Roof Potential Calculator
Area times sunlight, with the assumptions printed on the label:
The Solar Roof Potential Calculator gives an honest estimate of what your roof could generate — from usable area, orientation and sunshine, to capacity in kilowatts and rough annual yield. Enter your roof's facts and read the potential — a screening number for deciding if a quote is worth chasing.
- 1Enter the property location.
- 2Set roof tilt and orientation.
- 3Read the estimated annual energy per square metre.
What people use Solar Roof Potential Calculator for
1. Before calling installers
Walk into quotes knowing your roof's rough ceiling — a 3 kW roof quoted as 9 kW exposes itself. Sun geometry support comes from the Solar Position Calculator.
2. Comparing houses
Two candidate homes, two roofs: orientation and usable area separate the solar winner before the viewing ends.
3. Landlords and housing associations
Portfolio screening: which roofs justify full surveys. This calculator is the sieve, not the survey.
4. Curiosity, quantified
'Could my house power itself?' deserves a number with its assumptions attached, not a shrug — here it is.
Worked examples
Usable area 40 m² × ~0.20 kW per m² (modern panels at 1,000 W/m² test sunlight) ≈ 8 kW capacity. At 4.5 peak sun hours: ~36 kWh/day, ~13,100 kWh/year at the panels — before inverter, shade and weather losses, which real quotes then deduct.
At mid-latitudes a south-facing tilt near latitude angle is ideal; east or west roofs typically yield ~15–20% less, a flat roof a bit less again, and north-facing (northern hemisphere) drops steeply. The estimate's orientation factor carries these rules of thumb openly.
What drives the estimate
The inputs, and how much each matters.
| Input | Why it matters |
|---|---|
| Usable roof area | Sets capacity: ~0.2 kW per m² of modern panels |
| Orientation and tilt | The yield multiplier — south and ~30–40° tilt lead at mid-latitudes |
| Local sunshine | Peak sun hours vary ~2.5 (cloudy north) to 6.5+ (desert Southwest US) |
| Shading | The silent killer — one chimney can tax a whole string |
A screening estimate, not a quote: it excludes your tariff, battery, roof condition and installer pricing. Treat any number without those as the start of the conversation.
How to interpret the result
The most common errors come from using a correct result to answer the wrong geographic question. Keep these points in mind for Solar Roof Potential Calculator:
- For photography, plan around sun altitude, not clock time: golden hour is an altitude band, so its clock time migrates with the seasons even at a fixed place — the calculator's windows handle that for you.
- When comparing day length across a year, remember the curve is your latitude's signature: near-equatorial sites hover at twelve hours while high latitudes swing wildly. Choose expectations by latitude, not by calendar folklore.
Limits & appropriate use
Solar arithmetic's limits are small but real: the official zenith encodes average refraction, not today's atmosphere; coordinates are rounded to your input's precision; and terrain is absent unless you add height yourself — a mountain rises the sun late and the formulas won't know. Time adds the political layer: zones and DST are human law, and western edges of big zones (Spain, western China) make solar noon wander hours from clock noon. Every output here therefore carries its convention and both clocks, because an unanchored sunrise time is an argument, not a fact.
Lunar and seasonal outputs carry their own gentle limits: phase from the synodic approximation is hours-precise, which outruns every planning need but not ephemeris science; day length is geometric daylight, not sunshine, and clouds belong to climate's department. Escalation is rare and well-defined — astronomical almanacs and ephemerides for science, terrain-aware apps for mountain photography, and the tz database maintainers whenever a government invents a new DST rule at 48 hours' notice, as they delight in doing.
- Science-grade ephemerides → astronomical almanacs/JPL horizons.
- Mountain sunrise/sunset → terrain-aware horizon adjustments.
- Legal daylight definitions → jurisdictional statutes (they vary!).
Data & methodology
Solar declination and hour-angle geometry integrated over the year for the given plane tilt and azimuth.
For source details, licences and model limitations, see Data Sources and Methodology.
Key terms worth knowing
- Equation of time
- The sundial-vs-clock correction from orbit eccentricity and tilt.
- Civil twilight
- Sun between 0° and −6°: usable outdoor light without lamps.
Editorial guidance: MapBench explains the method and limitations so the tool can be used as a transparent reference, not as a black-box answer. For legal boundaries, safety-critical routing, surveying, engineering, regulated decisions or official statistics, verify the result with the responsible authority or a qualified professional.
Category: Sun & Moon. Results are intended for everyday analysis and planning unless a page explicitly states otherwise.
Method, assumptions & limits
- This tool’s method
- Solar declination and hour-angle geometry integrated over the year for the given plane tilt and azimuth.
- Before relying on the answer
- Check the date, location and time zone together. A solar or lunar event is tied to all three, and some events do not occur at high latitudes on a given date.
- What the result cannot establish
- A calculated event assumes the model's horizon and atmospheric conventions. Buildings, terrain and local weather can alter what you observe.
Coverage: Worldwide. Search, basemap and external data requests may use third-party services; browser-based does not mean every operation is offline. Read the source and licence notes, calculation methodology and privacy policy. If a result looks inconsistent, report a correction with the inputs and mode used.
Frequently asked questions
Clear answers about the calculation, accuracy and appropriate use.
Is this a quote?
No — it is a solar-geometry estimate of the energy available to the roof plane. Real yield depends on panels, shading and weather; an installer's survey comes next.
What does it account for?
Latitude, season, roof tilt and compass orientation, using standard solar-position maths.
Are sunrise and solar times exact to what I will see?
Calculated events use astronomical geometry. Terrain, buildings, weather and atmospheric conditions can shift what an observer actually sees.
Does daylight saving change the Sun's position?
No. Daylight saving changes the civil clock label, not solar geometry. Timezone rules affect displayed local time.
Why can polar locations show unusual results?
High latitudes can have days when the Sun does not rise or set, so some solar events legitimately do not occur on a selected date.
Can I skip the site survey and order from this number?
Please don't. The estimate screens roofs and sense-checks quotes; ordering needs measured dimensions, a structural once-over and a shading survey. Roofs are expensive to be wrong about.
Which costs and losses stay outside the estimate?
Tariffs and export rates, battery storage, scaffolding, roof repairs and your usage pattern — plus inverter and soiling losses a survey quantifies. The estimate is generation potential, not a bill forecast.
How accurate is the yearly figure?
Order-of-magnitude honest: expect a real system's yield within roughly ±20–25% of the estimate, weather and shade permitting.
What are peak sun hours?
The day's sunlight expressed as hours at full 1,000 W/m² strength — Chicago averages around 4–4.5; Phoenix around 6.5; London around 2.5–3. Your region's figure drives the estimate hard.
East-west split roofs?
Common and fine: two smaller arrays facing east and west give a flatter day of generation at a modest total discount versus pure south.
Does it size a battery?
No — storage sizing follows your usage pattern, not the roof. The estimate stops at generation potential, deliberately.
Old or shaded roof?
Shade and structure dominate everything else; a shaded 60 m² roof loses to a clear 30 m² one. Get the shading surveyed before believing any number, this page's included.
Related tools and resources
The geometry: the Solar Position Calculator; shadows that tax you: the Shadow Length Calculator; peak time: the Solar Noon Calculator.
More free tools
Sources, methodology & editorial trust
- ✓ Browser-first processing where supported
- ✓ External sources are documented
- ✓ No account required for public tools
- ✓ Review data sources
Calculations are documented with their assumptions, limitations and source context rather than presented as unexplained answers.
Reviewed and maintained by Aryan Khan, Founder & Editor of MapBench.
Part of our sun & moon hub
Solar Roof Potential Calculator sits in the sun & moon cluster, alongside 27 other tools that answer neighbouring questions with different methods or datasets.
You may also need
When this answer is only part of the job, these clusters cover the next step.
Read the guide behind this tool
Chasing Golden Hour: A Photographer's Solar Toolkit — Golden hour is an altitude band, not a fixed clock time. How to plan light with solar tools — windows, horizons and moon phase — instead of guessing.