Solar System Sizing by Usage

Sizing a PV array from monthly consumption

Sizing a residential PV array begins with two numbers: your average monthly bill in kWh and the local peak-sun hours (HSP). The simplified formula is panels = monthly_kWh / (Wp · HSP · 30 · efficiency), where Wp is the panel rating (Watt-peak at STC). The efficiency term lumps together inverter, wiring, soiling and temperature losses, and usually lands between 75% and 85%. In Brazil, HSP runs from 4.5 to 5.5 kWh/m²/day and climbs above 6 in the Northeast. Take a worked example: 400 kWh/month, 600 Wp panels, HSP 5 and 80% efficiency give 400 / (0.6 · 5 · 30 · 0.8) ≈ 5.6, so you'd install 6 panels. The Brazilian net metering rules come from Law 14,300/2022 (GD legal framework), and surplus credits stay valid for 60 months.

Applications

It covers rooftop home systems, commercial and industrial self-consumption, and agrivoltaics (panels mounted over crops). People also use it to rough out a size before asking an integrator for a proposal, or to feed ESG and sustainability reports.

FAQ

Why does HSP vary by region? HSP expresses solar irradiance as hours equivalent to 1,000 W/m², so it reflects both latitude and how cloudy a place tends to be. That's why the Northeast sertão beats coastal Santa Catarina for sun.

What efficiency should I use? A conservative 75-80% covers inverter losses, DC/AC mismatch, dirt on the panels and the heat they run at. Premium microinverter setups can push that to 85%.

Can I oversize beyond my consumption? You can, but the post-2023 GD rules slowly cut the credit you get for surplus pushed back to the grid. Aiming for roughly 100-110% of your consumption usually works best.