Solar Panel Array Sizing Calculator
Enter daily energy use, site peak sun hours and an itemised loss budget to find the array size, panel count, area and annual yield.
Step-by-step
- Enter valid values to begin.
Method: kWp = daily demand × (1 + margin) / (PSH × derating), where derating is the product of (1 − each loss). The loss percentages are typical planning values, not measurements; a site model such as PVWatts or PVsyst will refine them. PSH is the daily plane-of-array irradiation in kWh/m² (equivalent hours at 1 kW/m²). Off-grid mode lifts demand by the battery round-trip loss on the share of energy that passes through the battery. Annual yield assumes the same PSH every day. Verify the NEC edition and local AHJ requirements before design.
Solar Array Sizing: From Daily kWh to a Panel Count
Core Engineering Principles
Every array sizing job starts with one number, peak sun hours. A PSH of 5 does not mean five hours of daylight. It means the day’s irradiation equals five hours at 1 kW/m², the condition at which a panel earns its nameplate rating. Divide daily energy by PSH and you get the kWp for a perfect world. We never get one, so the second number is the derating factor, or performance ratio, and the way to get it right is to itemise it. Hot cells lose 5–12% in summer, dust takes 2–5%, wiring 1–2%, the inverter about 4%, and mismatch and first-year degradation a point or two each. Multiply them as (1 − loss) terms, never add them; a typical rooftop lands near 0.80–0.85.
Round the panel count up, then look at what you built: twenty-one 400 W modules is 8.4 kWp, not 8.1, and the extra is free margin. Off-grid, every kWh passed through a battery pays the round-trip loss, so the array must be bigger than the load suggests. We add a margin of 10–20% because panels degrade about 0.5% per year and loads creep upward. Size for the worst month you care about, not the annual average, or the battery will tell you about it in January.
kWp = Eday × (1 + margin) / (PSH × DF) • N = ⌈ kWp × 1000 / Wpanel ⌉
Off-grid: Eeff = E × (1 − s + s / ηrt)
NEC & Standard References
NEC Article 690 governs PV systems: maximum voltage with temperature correction in 690.7, circuit sizing at 125% of short-circuit current in 690.8, plus overcurrent protection, disconnects and rapid shutdown. IEC 61215 qualifies the design of crystalline silicon modules and IEC 61730 covers their safety construction. For inverters see IEC 62109 and UL 1741. This tool sizes energy only; string voltage and conductors are separate checks. Verify the adopted NEC edition and the requirements of your local authority having jurisdiction (AHJ).1. DF = 0.92 × 0.97 × 0.98 × 0.96 × 0.98 × 0.99 = 0.815.
2. kWp = 30 × 1.10 / (5.0 × 0.815) = 8.10 kWp.
3. Panels = 8,103 W / 400 W = 20.26, rounded up to 21 panels, which is 8.40 kWp installed.
4. Output = 8.40 × 5.0 × 0.815 = 34.21 kWh/day, or 114% of demand.
5. Area = 21 × 1.95 = 40.9 m² (441 ft²) and annual yield = 34.21 × 365 = 12,487 kWh.
- Use the winter PSH for off-grid. An annual average hides a December that may deliver half as much sun.
- Shade kills strings. One shaded cell pulls a whole series string down.
- Don’t stop at energy. Cold-morning voltage can exceed inverter limits.
- Roof area is not array area. Fire setbacks and vents often remove 30% or more.
- Never assume zero degradation. Use the warranty curve when a contract guarantees yield.