Name the module before counting
Nine 450 W modules total 4.05 kWp. Ten 400 W modules total 4.00 kWp but can occupy a different footprint. Read the exact dimensions and mounting information. Separate watts, area and kWp before comparing proposals.
Measure what the home buys
Use annual electricity kWh, then examine daytime demand and future loads. A battery can shift surplus but cannot create energy in winter. Do not divide annual use by a guessed yield and call the result a roof design.
Compare a smaller and a larger option
Ask for incremental installed cost and generation when adding modules or using a second roof face. Additional output may mostly be exported, so value it at the applicable export arrangement. Include access and network constraints. Test both surveyed scenarios.
Make the final design specific
A survey must check shading, roof condition, mounting, cable routes and inverter configuration. Use the roof checklist and quote checker to request missing detail. No bedroom rule or online count replaces these checks.
Sources and review dates
Research-based guidance. Source checks do not imply product testing or verification of an individual installer.
- Energy Saving Trust: Solar panels. Solar suitability, generation, roof and installation considerations; not a local quote dataset. Scope: UK. Checked .
- Trinasolar: Vertex S+ NEG9RC.27. Example 450 W module format 1762 x 1134 x 30 mm; identify exact module revision on a quote. Scope: Product specification. Checked .
Registry sources-2026-11-01. Tariffs have a 30-day internal review interval. Methodology and assumptions.