Once you know an array’s size, the next question is how much energy it will make. This calculator estimates daily, monthly, and annual output for a fixed PV array from the panel rating, count, local sun hours, and a real-world performance ratio.
How it works
The core formula is simple:
daily kWh = array kW x peak sun hours x performance ratiowhere:
- array kW = panel watts × number of panels ÷ 1000,
- peak sun hours = equivalent hours of full 1000 W/m² sunlight per day,
- performance ratio = the fraction of ideal output actually delivered after inverter, wiring, temperature, and soiling losses (typically 0.75–0.85).
Monthly and annual figures scale the daily result by 30.4 and 365 days.
Worked example
An 18-panel array of 400 W modules at 5.0 peak sun hours and a 0.8 performance ratio:
- Array size:
18 × 400 / 1000 = 7.2 kW DC. - Daily:
7.2 × 5.0 × 0.8 = 28.8 kWh. - Monthly:
28.8 × 30.4 ≈ 876 kWh. - Annual:
28.8 × 365 ≈ 10,512 kWh.
That comfortably offsets a home using about 900 kWh per month at the continental US average.
Understanding peak sun hours for your location
Peak sun hours is not total daylight — it is the equivalent number of hours per day at full 1,000 W/m² irradiance that delivers the same energy as the real, variable day. It is location-specific and seasonal:
| Location type | Annual-average daily peak sun hours (approximate) |
|---|---|
| Very sunny (desert southwest US, North Africa) | 5.5–7.0 |
| Sunny (Mediterranean, southern US) | 4.5–5.5 |
| Moderate (mid-US, central Europe) | 3.5–4.5 |
| Low (northern Europe, Pacific Northwest) | 2.5–3.5 |
Using a monthly peak sun hours figure instead of the annual average lets you see seasonal variation — how much less the array produces in December compared to June. For a net-metering analysis this breakdown often matters more than the annual total, because it shows whether the system overproduces in summer and underproduces in winter.
The performance ratio in detail
The performance ratio captures everything that reduces real output below the theoretical maximum:
- Inverter efficiency: String inverters typically run 96–98%; microinverters vary.
- Module temperature: Panels lose roughly 0.3–0.5% efficiency per degree Celsius above 25°C (the test condition). A roof-mounted panel on a hot day may sit at 60–70°C, losing 10–20% of nameplate output from heat alone.
- DC wiring resistance: Small but real, especially on long string runs.
- Soiling: Dust, bird droppings, and pollen accumulate and reduce transmission. Cleaned annually vs. rarely can shift the ratio by a few percent.
- Shading: Even partial shade on one module can disproportionately reduce string output without optimizers or microinverters.
Choosing the performance ratio honestly is the most consequential judgement in this calculator. 0.8 is a reasonable default for a well-designed, unshaded install in a mild climate; adjust down for hot climates, shading, or older equipment.
Notes and tips
- For a yearly estimate, use the location’s annual-average peak sun hours; for a single month, plug in that month’s value.
- Panel output degrades slowly over time — roughly 0.3–0.5% per year — so multiply by a degradation factor for long-term financial models. Over 25 years a panel rated 400 W at installation may deliver closer to 360 W.
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