Good sizing is what separates a project that delivers real savings from a system that disappoints the customer and damages the installer's reputation. An undersized system does not offset the bill; an oversized one wastes money and exports energy that, with Brazil's new "Fio B" grid charge, is credited less and less. This guide walks through sizing a grid-tied (on-grid) photovoltaic system, from the consumption survey to the choice of inverter.
1. Survey the real consumption on the electricity bill
Everything starts with the bill. Take the history of the last 12 months and calculate the monthly average in kWh — this removes seasonal distortions (air conditioning in summer, for example). Also identify three pieces of information that change the design:
- Connection type (single-, two- or three-phase), which defines the minimum availability charge: 30, 50 or 100 kWh that the customer pays regardless of generation.
- Tariff group (B or A). Group A systems require separate treatment of contracted demand.
- Tariff (R$/kWh) including taxes, the basis for the whole payback calculation.
The consumption to be offset is the monthly average minus the availability charge, because that portion is always billed.
2. Find the site's peak sun hours and calculate the kWp
Peak sun hours (PSH) are the site's average daily irradiation converted into equivalent hours at 1,000 W/m². In Brazil they range from about 4.2 (South, winter) to more than 5.8 kWh/m²·day (Northeast and Centre-West). Use sources such as the Brazilian Solar Energy Atlas or the Global Solar Atlas. With the PSH in hand, apply:
kWp = daily consumption to offset (kWh) ÷ (PSH × PR)
The PR (performance ratio) is the system's overall efficiency — losses from temperature, wiring, soiling, inverter efficiency and mismatch. Use something between 0.75 and 0.80 for well-executed Brazilian projects. Example: a customer with 600 kWh/month to offset (20 kWh/day), PSH 5.0 and PR 0.78 → kWp = 20 ÷ (5.0 × 0.78) ≈ 5.13 kWp.
3. Define the modules and their number
With the kWp defined, choose the module power (most are now between 550 W and 620 W, N-type and half-cell). The number of modules is:
Number of modules = (kWp × 1,000) ÷ module power (W)
In the example: 5,130 ÷ 575 ≈ 9 modules (5.18 kWp actual). Always round taking into account the available roof area, the orientation (ideally north in the southern hemisphere) and the tilt. Each 2.3 m² module needs about 2.5 m² including spacing.
4. Choose the inverter and the DC/AC ratio
The inverter is sized by the ratio between the modules' DC power and the inverter's rated AC power — the DC/AC ratio, known in Brazil as FDI, or oversizing. In Brazil a ratio between 1.1 and 1.35 is common: slight oversizing makes better use of low-irradiance hours and increases annual generation, with little clipping. For a 5.18 kWp array, a 4 to 5 kW inverter is usually ideal. Also check the MPPT voltage range, the number of strings and current compatibility.
Choosing between a string inverter and microinverters and calculating oversizing safely are covered in dedicated guides.
Worked sizing example (B1 residential)
| Parameter | Value |
|---|---|
| Average monthly consumption | 650 kWh |
| Availability charge (two-phase) | 50 kWh |
| Consumption to offset | 600 kWh/month (20 kWh/day) |
| Site PSH | 5.0 kWh/m²·day |
| Performance ratio (PR) | 0.78 |
| Required power | ≈ 5.13 kWp |
| Modules (575 W) | 9 units (5.18 kWp) |
| Suggested inverter | 4–5 kW (DC/AC ≈ 1.2) |