Estimate how many solar panels fit on your roof or meet your energy needs, the optimal tilt angle, and system cost.
Method
Allow for chimneys, vents, roof edges and access paths — typically 60-80% of total roof area is usable. This simplified rectangular-roof estimate does not model complex roof geometry.
Accounts for inverter, wiring and temperature losses — typically 75-85%. Used for both the roof-area and energy-need estimated production.
| Item | Value |
|---|
On the Panel Count tab, choose "Roof area" if you know your available roof dimensions and want to know how many panels fit in a simple row/column layout, or "Energy need" if you know your daily electricity usage and want to know how many panels would cover it. Enter your panel's length, width and wattage, an optional panel gap, and your peak sun hours and performance ratio (used for both methods). Use the Tilt tab for a recommended tilt angle and orientation guidance, and the Cost tab to estimate cost by price per panel, per watt, or per kW installed.
The number of panels you need depends on which question you're asking. If you're asking "how many panels fit on my roof," this calculator compares a portrait and landscape rectangular row/column layout - accounting for panel spacing - and recommends whichever fits more panels. If you're asking "how many panels will meet my energy needs," it's your daily electricity usage divided by how much energy one panel produces per day, which depends on its wattage, peak sun hours, and a performance ratio accounting for real-world losses. Where both roof and energy data are entered, the results also compare the two.
Simple mode reduces the roof's length and width by the square root of your usable percentage (so total usable area still matches Roof Area × Usable %), then fits panels into rows and columns. Detailed mode instead trims a setback from each edge and deducts an obstruction area you enter. For each orientation, the number of panels along a dimension accounts for the gap between panels so no gap is added beyond the final panel edge:
This remains a simplified rectangular-roof estimate and does not model complex roof geometry, hips, valleys or irregular shapes - an installer's layout plan may fit a different number.
To estimate how many panels are needed to cover a target daily electricity usage, this calculator first works out how much energy a single panel produces per day, then divides your daily usage by that figure.
Peak sun hours is the equivalent number of hours per day at maximum sunlight intensity, not total daylight hours. Annual coverage compares total estimated annual production with annual electricity use - it does not mean the property is self-sufficient at every hour, since production and consumption occur at different times of day and year.
This calculator uses a simple, widely cited rule of thumb for a fixed (non-tracking) panel mount: set the tilt angle roughly equal to your location's latitude for balanced year-round production. To favor summer output, reduce the tilt by about 15 degrees; to favor winter output, increase it by about 15 degrees. For a typical fixed system, panels generally face toward the equator - south in the Northern Hemisphere, north in the Southern Hemisphere - subject to roof orientation and site conditions.
This is a starting point, not a precision figure - your roof's actual pitch, local weather patterns, and production goal all affect the truly optimal angle. A solar installer can refine this using local production modelling.
The Cost tab lets you choose a pricing method: price per panel (a panel material cost estimate only), or an installed price per watt or per kW (an estimated installed system cost, using whichever panel count is available from the Panels Needed tab). It does not automatically assume rebates, incentives or tax credits, and does not calculate payback period or return on investment.
Example 1 — Roof area method. A 10m × 6m roof with 1.7m × 1.0m panels and no gap: portrait fits 5 along the length × 6 along the width = 30 panels; landscape fits 10 along the length × 3 along the width = 30 panels - a tie here, so either layout works. Adding a 20mm gap reduces portrait to 5 × 5 = 25 panels and landscape to 9 × 3 = 27 panels, so landscape becomes the recommended layout - illustrating how spacing can change which orientation fits more.
Example 2 — Energy need method. A household using 20 kWh/day, with 400W panels, 5 peak sun hours, and an 80% performance ratio: each panel produces (400 ÷ 1000) × 5 × 0.8 = 1.6 kWh/day, so 20 ÷ 1.6 = 13 panels needed - a 5.2 kW system producing roughly 20.8 kWh/day (about 7,592 kWh/year) against a 7,300 kWh/year target, or about 104% estimated coverage.
Example 3 — Tilt angle. At latitude 35°: year-round tilt ≈ 35°, summer-favoring tilt ≈ 20°, winter-favoring tilt ≈ 50°.
A common mistake with roof-fit estimates is dividing total usable area by one panel's area without checking that panels actually fit as physical rectangles within the roof's dimensions - this can overstate panel count, which is why this calculator compares a portrait and landscape row/column layout instead. Ignoring the gap needed between panels is another frequent error, since it reduces how many whole panels fit along each dimension. For energy-based sizing, a frequent error is using total daylight hours instead of peak sun hours, or skipping the performance ratio and assuming a panel's rated wattage is what it delivers all day - real systems always produce less than their theoretical maximum. Treating "estimated annual coverage" above 100% as meaning no grid power is needed is also a common misunderstanding, since production and consumption timing don't align hour by hour.
This calculator estimates a simplified rectangular-roof panel layout or energy-based panel count, and suggests a tilt angle from a latitude-based rule of thumb. It does not model roof shading from trees or nearby structures, roof orientation/azimuth, roof pitch beyond the tilt suggestion, or panel layout constraints from a real roof's complex shape - the roof-vs-energy comparison shown is a simplified planning comparison, not a professional solar layout assessment. It also does not provide a financial payback period, return on investment, or incentive/rebate calculation. For a precise, project-specific system design, get a site assessment and proposal from a licensed solar installer.
This depends on your daily electricity usage, panel wattage, peak sun hours and performance ratio. Use the By Energy Need method above with your daily kWh usage for an estimated panel count, system size, and estimated annual energy coverage, or By Roof Area if you want to know how many panels physically fit on your roof.
This depends on your roof's usable length and width, your panel's dimensions in portrait vs landscape orientation, and the spacing between panels. Use the By Roof Area method above with your roof size, panel dimensions and optional panel gap for a simple rectangular row/column layout estimate - it compares both orientations and recommends the one fitting more panels.
A common rule of thumb is to set the tilt angle equal to your latitude for balanced year-round production. Use the Tilt tab above with your latitude, hemisphere and production goal (year-round, summer, or winter) for a recommended angle and orientation guidance.
This depends on how many panels you need, each panel's dimensions, and the spacing between them. Use the calculator above to work out panel count and layout first, then use the panel area and gap shown to estimate total roof space required.
A simple rule of thumb sets tilt equal to latitude for year-round balance, latitude minus about 15° to favor summer production, or latitude plus about 15° to favor winter production. Use the Tilt tab above with your latitude for a recommended figure.
For a typical fixed system in the Northern Hemisphere, panels generally face toward the equator (south); in the Southern Hemisphere, they generally face toward the equator (north) - both subject to roof orientation and site conditions. Select your hemisphere on the Tilt tab above for this guidance alongside your recommended tilt.
System size (in kW) is your panel count multiplied by each panel's wattage, divided by 1000. Use either method on the Panel Count tab above to get a panel count, and the calculator will show the resulting system size automatically.
Performance ratio (typically 75-85%) accounts for real-world energy losses between a panel's rated output and usable electricity — including inverter conversion loss, wiring resistance, panel heat, and dust/dirt. This calculator lets you adjust it, along with peak sun hours, for both the roof-area and energy-need methods.
It compares your estimated annual solar production with your annual electricity target (production ÷ target × 100). It does not mean the property is self-sufficient at every hour, because solar production and electricity consumption occur at different times - coverage above 100% means estimated annual production exceeds annual usage on average, not that grid power is unnecessary.
Where both roof and energy data are entered, this calculator compares the number of panels needed for your energy target against the number estimated to fit on your roof, and shows the difference and resulting system size. This is a simplified comparison based on the figures you enter, not a substitute for a professional solar layout assessment.
No - it estimates panel layout and production from the dimensions, energy need and sun hours you enter, not shading, roof orientation, or complex roof geometry specific to your property. A solar installer's site assessment or a satellite-based tool can account for these factors precisely.
The Cost tab lets you choose a price per panel (panel material cost only), or an installed price per watt or per kW (an all-in installed system cost estimate). It does not automatically assume rebates, incentives or tax credits, or calculate payback period or return on investment, which depend on local electricity rates, incentives and financing that vary significantly by location.