Solar PV System Size
Professional PV sizing from consumption, peak sun hours, performance ratio, module wattage and system type — with panel count, roof area, generation and CO₂ savings.
DesignInputs (SI units)
From your utility bill
Leave 0 to auto-calculate as monthly ÷ 30
Used to derive physical module area
Leave 0 to skip the roof feasibility check
CEA India ≈ 0.71, EU ≈ 0.25
Site name — printed on the report
Results
Engineering Formula
- E_daily (kWh) = Monthly consumption / 30
- P_req (kW) = E_daily / (PSH × PR × η_sys)
- P_installed (kW) = P_req × oversize factor (On-grid 1.15 · Hybrid 1.20 · Off-grid 1.30)
- N_panels = ⌈P_installed × 1000 / P_module⌉
- A_module (m²) = P_module / (1000 × η_module)
- A_roof (m²) = N_panels × A_module × 1.15 (walkway / tilt allowance)
- E_annual (kWh) = P_installed × PSH × 365 × PR
- CO₂ saved (kg/yr) = E_annual × grid emission factor
Required DC capacity is the daily energy demand divided by the effective yield of one kilowatt-peak. PSH is the number of equivalent hours at 1 kW/m² STC irradiance, and PR (IEC 61724-1) bundles thermal, soiling, mismatch, wiring, availability and inverter losses. The required capacity is then raised by a system-type oversize factor — on-grid plants carry a modest DC/AC margin, hybrid systems allow for battery round-trip losses and off-grid arrays must recharge the bank while serving the load on average days. Panel count follows from module wattage, and physical roof area from module efficiency (a higher-efficiency module packs the same watts into less area) with a 15% allowance for row gaps, walkways and access.
Step-by-step Calculation
- 1.Daily consumption
E_d = kWh_month / 3030 kWh/day - 2.Effective yield per kWp
Y = PSH × PR × η_sys3.744 kWh/kWp/day - 3.Required capacity
P_req = E_d / Y8.01 kW - 4.Recommended capacity
P × 1.15 (ongrid)9.21 kW - 5.Panel count
N = ⌈P×1000 / Pm⌉17 - 6.Installed DC capacity
N × Pm / 10009.35 kWp - 7.Module area
Pm / (1000 × η_mod)2.62 m² - 8.Roof area
N × A_mod × 1.1551.2 m² - 9.Annual generation
P × PSH × 365 × PR13,310 kWh/yr - 10.CO₂ avoided
E_annual × EF9,450 kg/yr
Expected monthly generation
Engineering Explanation
What is Peak Sun Hours (PSH)?+
PSH is the number of hours per day for which irradiance would have to equal 1 kW/m² (STC) to deliver the same total energy the site actually receives. A location with 5.2 kWh/m²/day of plane-of-array irradiation has 5.2 peak sun hours. It is not the number of daylight hours.
What is Performance Ratio (PR)?+
PR is the ratio of the energy a plant actually delivers to the energy it would deliver at nameplate STC efficiency under the same irradiation (IEC 61724-1). It captures thermal derating, soiling, mismatch, DC and AC cable losses, inverter efficiency and downtime. Typical values are 0.75–0.82 for rooftop and 0.80–0.85 for well-kept ground-mount plants.
Why does module wattage affect panel quantity?+
The array's kilowatt-peak target is fixed by your energy demand. Panel count is that target divided by the wattage of one module, rounded up — so 10 kWp needs 19 × 545 Wp modules but 25 × 400 Wp modules. Higher-wattage modules mean fewer mounting points, fewer connectors and less labour, but heavier individual panels.
How is roof area calculated?+
Module area comes from nameplate power and module efficiency: a 550 Wp module at 21% efficiency occupies 550 / (1000 × 0.21) ≈ 2.62 m². Multiplying by panel count gives the pure module footprint, and a 1.15 factor adds inter-row spacing, walkways and edge setbacks. Tilted ground-mount arrays need considerably more land — use the inter-row spacing calculator for that.
What factors affect system size?+
Energy consumption and its seasonal shape, site irradiation (PSH), shading, roof tilt and azimuth, module efficiency and temperature coefficient, cable and inverter losses, system type (off-grid arrays must also recharge a battery bank), available roof area, and any sanctioned-load or net-metering cap imposed by the utility.
Engineering Notes
- •Higher PR means better system performance — a well-maintained plant sits between 0.75 and 0.85.
- •Keep the DC/AC ratio between 1.1 and 1.3 unless project requirements differ.
- •Leave a maintenance gap between module rows for cleaning and inspection access.
- •Actual generation depends on irradiation, cell temperature, cable losses and inverter efficiency.
Design Assumptions
- • Consumption profile is broadly matched by generation (net metering or self-consumption).
- • Module area derived from nameplate wattage and module efficiency at STC.
- • 15% roof allowance for inter-row spacing, walkways and edge setbacks.
- • Monthly split uses a typical low-latitude irradiation distribution.
- • PR already includes temperature, soiling, mismatch, cable and inverter losses.
Engineering Tips
- ◆Keep the DC/AC ratio between 1.1 and 1.3 unless project requirements differ.
- ◆Confirm PSH for the exact site from NASA POWER, Meteonorm or Solargis — not a regional average.
- ◆Model bankable yield in PVsyst or SAM before financial close on utility-scale projects.
- ◆For off-grid, size the array on the worst irradiation month, not the annual average.
Warnings
- ▲Do not assume PR > 0.85 without measured plant data.
- ▲Rooftop structural capacity must be certified before installation.
- ▲Roof area shown is the physical module footprint plus allowance — verify against shading and obstructions.