COMMERCIAL PROJECT PLANNER · USER-ENTERED QUOTES · TRANSPARENT ASSUMPTIONS
Solar System Size Cost & Payback Calculator
Choose the DC system size yourself and combine local solar/energy references with a real installer quote.
Cost, production and simple payback model
Load available local references, then replace every value with your actual tariff, installer quote and project assumptions.
Local references have not been loaded.
This is a planning estimate. Financing, taxes, degradation, export compensation, maintenance, inverter/battery replacement and site-specific construction can materially change the economics.
Use one calculator instead of relying on a price page for every system size
System size is only one dimension of a solar project. The same 8 kW DC rating can have very different installed cost, annual production and financial value depending on roof/site work, inverter architecture, electrical upgrades, location, self-consumption and tariff structure.
When to use the fixed-size shortcuts
WattCostLab keeps fixed 3–20 kW shortcut pages for quick scenario sharing, but the master calculator is the preferred indexable page because it lets a user compare sizes without creating a search page for every numeric variation.
Compare scope, assumptions and exclusions—not just the total price.
Use WattCostLab’s quote checklist before signing a contract. It helps you compare like-for-like project scope without claiming that a referral marketplace is active.
Frequently asked questions
Should I choose system size from roof area or electricity use?
Both can constrain the final design. Annual electricity use provides an energy target, while usable roof/site area, shading and electrical/interconnection limits can cap the feasible system.
Why is installed cost per watt user-entered?
Because WattCostLab does not currently have a verified, comparable contractor-price dataset covering every location and project scope.
Can I load local solar production?
Yes. The tool can request PVGIS specific yield for a reference city, and you can overwrite it when you have a roof-specific estimate.