Solar Panel Output Calculator: Estimate Your Daily Energy Yield

Solar panels are rated in kilowatts peak (kWp) under ideal lab conditions. What they actually produce each day depends on how much sun your site receives and how efficiently your system converts that sun into usable electricity. The formula connecting all three is simple:

kWh/day ≈ kWp × peak sun hours × (system performance % ÷ 100)

---

What kWp means

kWp (kilowatts peak) is the nameplate DC power of your array under Standard Test Conditions (STC): 1,000 W/m² irradiance, 25 °C cell temperature, and a standard air mass. In practice, real output varies with temperature, angle, and cloud cover.

You can enter kWp directly if you know your total array size, or let the calculator derive it from per-panel wattage and panel count:

kWp = (W per panel × number of panels) ÷ 1,000

For example, 16 panels at 400 W each = 6.4 kWp.

---

What peak sun hours means

Peak sun hours are not hours of daylight. One peak sun hour equals the energy from one hour of sunlight at the reference intensity of 1 kW/m². A day with 6 hours of partial cloud may only deliver 4 peak sun hours of actual energy.

Your location’s average is available from solar irradiance maps (NASA POWER, PVGIS, SolarAtlas). Typical ranges:

• Northern Europe / UK / Pacific Northwest: 2.5–3.5 h/day annual average
• Central Europe / Northern US: 3.5–4.5 h/day
• Southern Europe / Southern US / Australia: 4.5–6.0 h/day
• Middle East / North Africa / SW USA desert: 6.0–7.5 h/day

Use your annual average for a full-year yield estimate, or seasonal averages to model summer vs winter output separately.

---

What system performance covers

No system delivers 100% of its DC nameplate capacity as usable AC energy. A single performance percentage (also called performance ratio, or PR) rolls up all the real-world losses:

• Inverter efficiency — typically 96–98% for a quality string inverter
• Wiring and combiner losses — 1–3%
• Module temperature — hot cells produce less; a 25 °C day is fine but a summer roof in full sun can reach 60–70 °C
• Tilt and azimuth penalty — non-optimal roof angles reduce yield
• Soiling — dust, pollen, and bird droppings shade cells
• Shading — neighbouring buildings, trees, chimneys
• Module mismatch — slight differences in cell characteristics across a string

For a well-designed, lightly shaded system: 75–85% is a realistic planning range. Heavily shaded or older systems may be lower. Entering 100% gives an upper-bound DC estimate before any losses.

---

The formula

kWh/day ≈ kWp × peak sun hours × (system performance % ÷ 100)

Monthly ≈ daily × 30.4 | Annual ≈ daily × 365

Example: 6.4 kWp × 4.5 h × 0.80 = 23.0 kWh/day → ~8,400 kWh/year.

---

Quick reference

• 3.0 kWp, 4.5 h/day, 80% perf. → 10.8 kWh/day — Small rooftop, moderate sun
• 6.6 kWp, 5.0 h/day, 78% perf. → 25.7 kWh/day — Common AU residential, decent site
• 10 kWp, 3.5 h/day, 85% perf. → 29.8 kWh/day — Larger array, cloudier climate
• 4.0 kWp, 4.0 h/day, 100% perf. → 16.0 kWh/day — Upper bound before losses

---

Real-world examples

Sanity-checking an installer quote

An installer quotes a 7.2 kWp system and estimates 4.2 peak sun hours with 80% performance:

7.2 × 4.2 × 0.80 = 24.2 kWh/day → ~8,800 kWh/year.

Compare this to their written production estimate. If they say 10,000+ kWh/year, ask what assumptions drive the difference.

Summer vs winter planning

The same 5 kWp roof in Germany might see 5.5 h/day in June but only 1.5 h/day in December. Running the calculator twice shows you the swing between seasons and helps size storage or set import/export expectations.

EV charging from solar

If your system produces an estimated 18 kWh/day on an average day and your EV averages 3.5 miles per kWh, that is roughly 63 miles of solar-powered driving per day — before charger efficiency losses.

Feed-in vs self-consumption

Knowing daily yield lets you compare it to your household consumption profile. If you use 10 kWh/day and produce 20 kWh, you are likely exporting half — useful for deciding whether battery storage makes sense.

---

What this tool does not cover

This is a simple average-day model. It does not:

• Model hourly irradiance or shading throughout the day
• Account for MPPT clipping when the array DC exceeds inverter AC capacity
• Handle battery round-trip losses for storage systems
• Simulate time-of-use tariffs or feed-in rate calculations
• Replace a full PV simulation (PVGIS, PVsyst, SAM) for financial models or warranty claims

For a quick quote comparison, planning conversations, or “will this cover my bill?” questions, the formula is exactly what you need.

---

FAQ

What are peak sun hours?

They are not hours of daylight. One peak sun hour = the same solar energy as one full hour at the reference 1 kW/m² irradiance. A 10-hour day with partial cloud might deliver only 4 peak sun hours.

What system performance should I use?

75–85% is a typical planning range for residential systems. Use 100% to see an ideal upper bound, or your installer’s stated performance ratio if they provide one.

kWp is DC — does this give AC output?

The performance % folds in inverter efficiency, so your result is approximately AC output at the meter. For pure DC estimates (off-grid battery charging), use a higher performance % that excludes inverter losses.

I entered kWp and panel watts — which wins?

If the kWp field has a valid number, it takes priority. Per-panel inputs are ignored until you clear kWp.

How does this relate to other Tooladex tools?

Use the Electricity Cost Calculator to see what your estimated kWh production would save or earn, and the Battery Runtime Calculator to see how long stored solar energy would power a load overnight.

---

Try the Tooladex Solar Panel Output Calculator — enter your kWp (or panel count and wattage), peak sun hours, and system performance to get an instant daily, monthly, and annual energy estimate.