Panel wattage is only one line on the decision sheet
A solar module’s nameplate power tells you its rated output under standardized test conditions. Real roofs are not standardized. Available area, orientation, shade, heat, module dimensions, inverter design and local solar resource all change how many modules make sense and how much electricity the array may deliver over a year.
Instead of asking “What is the highest-watt panel?”, ask “How much annual energy can this roof produce with a layout that fits, stays serviceable and works with the inverter?”
Specifications worth comparing
| Specification | What it tells you | Why it matters |
|---|---|---|
| Rated power (W) | Module output under test conditions | Useful for array sizing, but not a prediction of real hourly output |
| Efficiency (%) | How much module area is needed for a given rated output | More important on space-constrained roofs |
| Dimensions | Physical footprint | Determines layout, setbacks, access and modules-per-roof |
| Temperature coefficient | How power changes as cells get hotter | Useful in hot climates and poorly ventilated mounting conditions |
| Product / performance warranty | Coverage and long-term output commitments | Read exclusions, remedies and who actually supports the warranty locally |
| Mechanical load / certifications | Documented structural and safety testing | Relevant to local wind, snow, mounting and code requirements |
Efficiency versus total roof output
Higher efficiency can fit more rated capacity into a limited area, but it does not automatically make a panel the best value. On a large unobstructed roof, a lower-cost module with slightly lower efficiency may still produce excellent project economics. On a small urban roof, module dimensions and efficiency can be decisive.
Heat, shade and orientation
PV output changes with irradiance and cell temperature. Partial shade can reduce production, and the electrical impact depends on module layout, bypass diodes and inverter architecture. A good proposal should explain the roof layout and production assumptions rather than give only the array’s nameplate kW.
How many panels do you need?
Start with annual or daily energy demand, then estimate the array size using local solar resource and system-loss assumptions. NREL’s PVWatts is a useful reference model because it uses location-specific weather data and makes system assumptions visible. Melnti’s system-size calculator is intentionally simpler and should be used as an early planning estimate, not a final yield model.
Questions to ask before buying
- What exact model will be installed?
- How many modules fit the usable roof after access and setback requirements?
- What annual production assumption is being used?
- What inverter or optimizer architecture is proposed, and why?
- Who handles a warranty claim in your market?
- What mounting system and roof penetrations are included?
Primary sources and further reading
Melnti uses primary or institutional sources where they are available. Always confirm current rules, tariffs and equipment documentation before purchasing.