What is Wp?
Wp, watt-peak, is the rated DC output of a solar cell or module measured at Standard Test Conditions (STC). STC defines the reference: 1,000 watts per square metre irradiance, 25 degrees Celsius cell temperature, and air-mass 1.5 solar spectrum. Manufacturers flash-test each module at STC during production and bin by Wp class. The Wp rating becomes the module's commercial nameplate.
Wp is the foundational commercial unit in solar. Module pricing is quoted in rupees per Wp. Project sizing aggregates Wp across modules to compute kWp (1,000 Wp) and MWp (1,000,000 Wp). The PM Surya Ghar subsidy structure is denominated in kWp. ALMM module listings reference Wp class. Even consumer-facing descriptions ("a 5 kWp system") trace back to Wp.
Field W output is always lower than rated Wp because field conditions rarely match STC. Cell temperature in Indian summer often exceeds 50 °C, reducing output by the temperature coefficient (about -0.34 percent per °C for PERC). Combined with other system losses, Performance Ratio of 0.78 to 0.82 is typical for Indian rooftop, meaning ~80 percent of theoretical generation reaches the grid.
Why Wp matters
For commercial comparison, Wp is the universal benchmark across manufacturers. Two modules labelled 580 Wp can be compared directly on price-per-Wp basis. Cost-per-Wp is one of the primary selection criteria in solar procurement.
For project sizing, kWp = sum of module Wp values. A 5 kWp residential system is 9 × 580 Wp modules. Annual generation forecasts derive from kWp using yield-per-kWp values.
For subsidy alignment, PM Surya Ghar slabs are in kWp. The Wp-to-kWp aggregation determines subsidy eligibility (1, 2, or 3 kWp slabs).
For policy, ALMM and DCR notifications list approved modules by Wp class.
Benefits of Wp as a unit
- Universal commercial standard. Globally comparable.
- Pricing basis. ₹ per Wp universal.
- Sizing foundation. Aggregation to kWp and MWp.
- Subsidy alignment. PM Surya Ghar uses kWp slabs.
- Tariff alignment. SECI and SERC tariffs reference kWp/MWp.
- Quality differentiation. Power tolerance specification.
Limitations of Wp
Laboratory rating. Field output is lower.
Temperature sensitivity. Real W varies with conditions.
Manufacturer claims need verification. Power tolerance matters.
Wp does not include efficiency. Same Wp from different module sizes differs in efficiency.
Does not include degradation. Year-25 output below year-1.
Wp in Indian module market
| Module class | Typical Wp | Use case |
|---|---|---|
| Residential PERC / TOPCon | 540 to 620 Wp | 1 to 5 kWp rooftop |
| Commercial PERC / TOPCon | 580 to 660 Wp | 10 kWp to 999 kWp |
| Utility-scale PERC / TOPCon / HJT bifacial | 620 to 720+ Wp | MWp scale |
| Half-cut configuration | Standard at most ratings | All segments |
| Cell-level Wp | 6 to 13 Wp per cell | n/a (in modules) |
Quick facts
| Term | Wp (Watt-Peak) |
|---|---|
| Definition | Rated DC output at Standard Test Conditions |
| STC conditions | 1,000 W/m², 25 °C cell, AM 1.5 spectrum |
| 1 kWp | 1,000 Wp |
| 1 MWp | 1,000,000 Wp |
| Indian residential typical | 540 to 600 Wp per module |
| Indian commercial / utility | 580 to 720+ Wp per module |
| Pricing | ₹14 to ₹22 per Wp typical (modules) |
Common mistakes about Wp
- Treating Wp as field W. Field is lower.
- Comparing modules without normalising to Wp. Always compute price per Wp.
- Ignoring power tolerance. +5/0 percent better than ±5 percent.
- Equating Wp with efficiency. Module size matters too.
- Forgetting STC reference. Wp is at STC, not field.
- Quoting nameplate Wp generation. Honest forecasts use yield × kWp.
Key takeaways
- Wp (watt-peak) is the rated DC output of a solar device at STC.
- The commercial unit for solar module rating and pricing.
- 1 kWp = 1,000 Wp; 1 MWp = 1,000,000 Wp.
- Indian residential modules in 2026: 540 to 620 Wp typical.
- Field W output is lower than Wp because field conditions vary.
- STC conditions: 1,000 W/m² irradiance, 25 °C cell temperature, AM 1.5 spectrum.
- Compare modules on price per Wp.
Frequently Asked Questions
What is Wp in solar?
Wp stands for watt-peak. It is the rated DC output of a single solar cell or module under Standard Test Conditions (STC). A 580 Wp module produces 580 watts at STC: 1,000 W/m² irradiance, 25 °C cell temperature, AM 1.5 spectrum. Wp is the commercial unit for module rating, used on every datasheet, invoice, and project sizing.
How is Wp different from W?
W (watts) is generic power, including AC and DC. Wp (watt-peak) specifically refers to the rated DC output of a solar device under STC. Wp is the laboratory benchmark; field W output varies with actual conditions.
How is Wp different from kWp?
1 kWp = 1,000 Wp. Wp is per-module rating; kWp is system rating. A 5 kWp system uses 9 × 580 Wp = 5,220 Wp = 5.22 kWp. Both refer to STC rated DC capacity.
Why is Wp the standard unit?
STC provides a uniform global benchmark. Wp lets modules from different manufacturers be compared on the same basis. Pricing (₹ per Wp), sizing (Wp per project), and capacity reporting all use Wp.
What is typical Wp per module?
Indian residential modules in 2026: 540 to 600 Wp (mono PERC or TOPCon). Commercial and utility-scale: 580 to 720+ Wp. Cell-level Wp is much smaller: about 6 to 13 Wp per cell.
Is Wp the same as Pmax?
Yes, in module datasheets. Pmax (maximum power) at STC is the Wp rating. Both terms refer to the same number.
How does power tolerance affect Wp?
Manufacturer datasheets specify power tolerance, typically '+5/0' percent meaning actual output equals or exceeds rated Wp. Quality modules guarantee positive tolerance; cheap modules may have wider tolerance.
What is the relationship between Wp and module efficiency?
Module efficiency = (Wp ÷ module area) × 100 percent of input power (1,000 W/m² at STC). A 580 Wp module with 2.55 m² area has efficiency 580 ÷ (2.55 × 1,000) = 22.7 percent.
Does Wp degrade over time?
Yes. Initial degradation (first year, 2 percent typical) plus annual degradation (0.5 to 0.7 percent) reduces effective Wp over module life. Year-25 effective Wp is typically 80 to 85 percent of original rated.
How is Wp measured?
Through flash testing at the factory under STC conditions. Each module is flash-tested and binned by Wp class. Standards: IEC 61215 module qualification, IS 14286 Indian standard.
Does temperature affect actual W output?
Yes. Higher cell temperature reduces output by the temperature coefficient (typically -0.34 percent per °C for PERC). At 50 °C cell temperature, a 580 Wp module produces about 530 W actual.
Is Wp the same internationally?
Yes. STC conditions are international (IEC standard), so Wp ratings are comparable globally. Same 580 Wp module from Indian or international manufacturer rated identically.
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- IEC 61215. Module qualification including STC measurement.
- IS 14286. Indian module standard.
- Module manufacturer datasheets. Wp ratings and power tolerance.
- NREL. STC measurement methodology. nrel.gov
- MNRE ALMM. Approved modules with Wp ratings.
- ITRPV roadmap. Module Wp evolution.
- Fraunhofer ISE. Module performance research.
Written by QuickEstimate Editorial, QuickEstimate Editorial (Surat).
Last updated: 4 June 2026.