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Short answer
Rated capacity in kW describes power, while kWh describes energy accumulated over time. A solar-yield estimate combines location and weather information with system assumptions. We use it to compare preliminary scenarios and prepare installer questions, not to guarantee a particular future year or establish final roof suitability.
- Capacity alone does not determine annual generation.
- Regional resource data and a roof-specific shading assessment are different inputs.
- Annual generation does not show how much electricity coincides with a household's demand.
Who this is for: Households, solar installers and teams reviewing a preliminary rooftop solar assessment.
Separate rated power from generated energy
PV system capacity is commonly stated as DC nameplate power under specified test conditions. The inverter has an AC rating. Those ratings and the energy delivered over a month or year are different quantities. A comparison should state which capacity basis is being used.
Orientation, mounting and conversion assumptions affect the result. Inverter clipping is one reason more DC nameplate capacity does not translate into unlimited additional AC output. A useful estimate exposes these inputs instead of presenting capacity as an annual-energy prediction.
Technical references: NLR SAM: PVWatts system design; NLR: PVWatts calculator inputs and cautions.
Seven inputs worth checking in every estimate
Hold the important inputs constant when comparing two estimates, then change one assumption at a time. Otherwise a higher result may simply reflect a larger assumed array or a different weather record.
| Input | Question to ask |
|---|---|
| Location and weather | Which dataset, period and spatial context are represented? |
| Capacity basis | Is the value DC nameplate kW or an AC rating? |
| Roof geometry | Which tilt, azimuth, mounting and usable area were assumed? |
| Shading | Are nearby buildings, trees and roof obstacles represented? |
| Losses | What soiling, availability and other losses were included? |
| Inverter assumptions | What efficiency and DC/AC ratio were used? |
| Time resolution | Is the output monthly/annual, or detailed enough for the question? |
Technical references: NLR: PVWatts calculator inputs and cautions; NLR SAM: PVWatts system design.
Resource maps do not replace a roof inspection
Global Solar Atlas describes resource and photovoltaic modelling for screening and pre-feasibility. Its terrain-shading treatment is not a statement that nearby rooftop obstacles, buildings or trees were modelled for an individual property. A detailed-looking map should not be treated as proof that roof-specific shading is resolved.
We separate regional screening, a preliminary roof assessment and a site-verified engineering design. Usable roof dimensions, obstacles, access, structural and electrical suitability and the actual equipment/layout belong in the installer discussion. These checks answer questions that a regional resource estimate cannot establish.
Technical references: Global Solar Atlas: methodology; NLR: PVWatts calculator inputs and cautions.
Treat weather-based output as a scenario
Typical or representative weather supports a long-term performance estimate. It is not a forecast of the exact output in a particular future year. Default losses also need review when actual shading, soiling or equipment behaviour differs from the assumed system.
For a preliminary comparison, we would show the base assumptions and examine meaningful alternative cases. A result with more decimal places does not make those assumptions more certain. Avoid transferring a model's generic uncertainty statement into a universal performance guarantee for an Indian roof.
Technical references: NLR: interpreting PVWatts results.
Annual generation is not the same as bill savings
An annual energy total does not describe when electricity is generated or when the household uses it. Two households with similar annual use can have different daytime demand. The energy comparison therefore needs a suitable time basis before it can answer a consumption-matching question.
Our recommendation is to take the estimate, its input assumptions and a representative consumption record to an installer. This guide does not calculate tariffs, subsidy eligibility or a financial return. Its purpose is to make the technical estimate easier to question.
Technical references: NLR: PVWatts calculator inputs and cautions; NLR: interpreting PVWatts results.
Common questions
Is kW the same as kWh?
No. kW is a unit of power; kWh is a unit of energy. A system capacity rating does not by itself state how much energy will be generated over a year.
Does a solar resource map include all rooftop shading?
Do not assume so. Check the model. Terrain shading and shade from nearby buildings, vegetation and roof obstacles are different considerations.
Can an initial solar estimate replace the final installation design?
No. It supports preliminary decisions and installer conversations. Site geometry, roof suitability, electrical requirements and the equipment/layout still need site-specific assessment.
Official references
Sources checked 1 November 2026. The linked publishers remain the authority for their specifications and services.
- NLR: PVWatts calculator inputs and cautions
Current official capacity, system input and site-representation information.
- NLR SAM: PVWatts system design
DC/AC ratings, mounting, orientation and inverter clipping.
- NLR: interpreting PVWatts results
Long-term weather-based estimates and limitations of default assumptions.
- Global Solar Atlas: methodology
Resource and PV modelling, screening role and shading treatment; JavaScript-rendered documentation.