A solar array's nameplate wattage (Wp) tells you its output under laboratory test conditions, not what it produces over a real day. To estimate actual daily energy, multiply the array's rated wattage by the site's peak sun hours (the equivalent full-intensity exposure time) and by the system's overall efficiency (accounting for inverter conversion, wiring, soiling, and temperature losses): Daily kWh = Wp × Sun Hours × Efficiency / 1000.
| Quantity | Formula |
|---|---|
| Daily generation | Wp×Sun Hours×Efficiency/1000 (kWh) |
| Monthly generation | Daily kWh × ∼30 |
| Yearly generation | Daily kWh × 365 (or sum of monthly figures using each month's own sun hours for accuracy) |
| Typical system efficiency | 75–85% |
For a genuinely accurate yearly estimate, apply this formula separately to each month's own peak-sun-hours figure (which varies seasonally) rather than multiplying a single daily estimate by 365 — the simple ×365 approach used here is a reasonable first-pass approximation, not a precise annual forecast.
Multiply your array's total wattage (Wp) by your location's peak sun hours and by your system's overall efficiency, then divide by 1000 to get daily kWh: Daily kWh = Wp × Sun Hours × Efficiency / 1000.
The Wp rating is measured under fixed laboratory Standard Test Conditions (1000 W/m²), while real sunlight intensity varies throughout the day and with weather, season, and panel angle — "peak sun hours" is exactly the concept used to translate this variable real-world exposure into an equivalent full-intensity time.
It is a reasonable first-pass approximation but not precise, since sun hours vary meaningfully by season. A more accurate yearly figure sums each month's generation using that month's own average sun-hours value rather than a single flat daily number.
75-85% is typical for a well-installed system, covering inverter conversion losses, wiring resistance, panel soiling, and elevated operating temperature effects — use the lower end of that range for a conservative estimate or if any of these loss factors are expected to be worse than typical.
Compare the calculated daily generation (kWh) against your actual daily energy consumption (from your utility bill or load calculation) — the "Your Daily Load" field above does this comparison automatically and reports the surplus or shortfall.
You have a shortfall that must be covered by grid import (grid-tied systems), draining your battery bank without full recharge (off-grid systems, unsustainable long-term), or you need a larger array/panel wattage — use the Solar Panel Sizing calculator to find the array size that actually covers your load.
For off-grid or backup-critical systems, size for your worst realistic month so the system remains adequate year-round; for grid-tied systems offsetting an annual bill, average annual sun hours is usually an acceptable basis since the grid absorbs any shortfall.
Yes — panels typically lose about 0.5-0.8% of their rated output per year from gradual degradation, so a system's generation 10-20 years into its life will be somewhat lower than this calculator's day-one estimate; many installers build this into long-term production guarantees.
They are inverses of each other: Solar Panel Sizing asks "how big an array do I need for my load?", while this calculator asks "how much will a specific array actually generate, and does it cover my load?" — useful for double-checking a proposed system size from both directions.
No — solar panels still produce some power from diffuse (scattered) sunlight on cloudy days, typically 10-25% of clear-sky output depending on cloud thickness, though this calculator's "sun hours" figure is meant to already be an average that accounts for a location's typical mix of sunny and cloudy days over time.
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