Solar Panel Output Calculator

How to use this calculator

Enter six values. The calculator returns average daily, 30.4-day monthly, 365-day annual, and forward 25-year energy scenarios, plus gross energy-value scenarios.

1. Array size (W DC) — sum the modules’ STC nameplate wattages. DOE describes STC ratings as 1,000 W/m² and 25°C cell temperature (DOE FEMP).
2. Peak sun hours per day — enter location- and plane-specific average daily solar radiation expressed as equivalent hours at 1 kW/m². PVWatts V8 is a primary site-resource option. The 4.8 default is illustrative.
3. System efficiency (%) — a single aggregate performance factor. The 78% default is a scenario assumption, not the PVWatts loss setting.
4. Panel age (years) — the code compounds a fixed 0.5% annual degradation assumption before calculating current output.
5. Energy value scenario ($/kWh) — a single hypothetical value applied to every generated kWh. Actual self-consumed and exported energy commonly have different values. The $0.165 default matches EIA’s 2024 US residential average and is not a 2026 forecast (EIA).
6. Annual value-rate change (%) — the 3% default is a hypothetical compounding rate, not a price forecast. Enter 0% for a constant nominal-rate scenario.

Formula used by the calculator

The implementation uses these equations, with n running from 0 through 24:

age_factor           = (1 - 0.005)^age
daily_kWh            = array_W × peak_sun_hours × efficiency_decimal × age_factor / 1000
monthly_kWh          = daily_kWh × 30.4
annual_kWh           = daily_kWh × 365
year_n_kWh           = annual_kWh × (1 - 0.005)^n
forward_25_year_kWh  = sum(year_n_kWh), n = 0...24
year_1_gross_value   = annual_kWh × value_rate
forward_25_year_gross_value =
  sum(year_n_kWh × value_rate × (1 + rate_change_decimal)^n), n = 0...24

The 25-year period starts from the age-adjusted annual output. The code then applies another 0.5% reduction in each future year after year 1.

Reproducible verification example

Use the prefilled US values: 6,000 W, 4.8 peak sun hours, 78%, age 0, $0.165/kWh, and 3% escalation.

• Daily: 6,000 × 4.8 × 0.78 ÷ 1,000 = 22.464 kWh, displayed as 22.46 kWh
• Monthly: 22.464 × 30.4 = 682.9056, displayed as 683 kWh
• Annual: 22.464 × 365 = 8,199.36, displayed as 8,199 kWh
• Forward 25-year energy: 8,199.36 × sum(0.995^n), n = 0 through 24, = 193,143.73, displayed as 193,144 kWh
• Year-1 gross energy value: 8,199.36 × $0.165 = $1,352.89, displayed as $1,353
• Forward 25-year gross energy value: sum(8,199.36 × 0.995^n × $0.165 × 1.03^n) = $46,122.17, displayed as $46,122

Output table for the default scenario

Dollar values are gross scenarios that apply the entered rate to every generated kWh. They are not predicted bill savings and ignore self-consumption, export compensation, fixed charges, tariff periods, taxes, financing, and discounting.

The energy equation is linear in array size when every other input is held constant.

What the simple model leaves to the inputs

Location, tilt, and azimuth

Solar resource varies by location, array plane, season, and weather year. PVWatts V8 uses NSRDB typical-year data where available and accepts explicit tilt and azimuth (NREL API). This calculator instead applies one daily average to all 365 days. The solar panel tilt calculator supplies only latitude-based heuristics, not a production simulation.

Temperature and STC nameplate power

STC nameplate power is not an operating forecast. DOE notes that actual irradiance is usually below 1,000 W/m² and cell temperature is typically above the 25°C rating condition (DOE FEMP). Use the quoted module’s temperature coefficient or a model that represents module and mounting type.

Shading, soiling, and snow

Shade impact depends on timing, bypass-diode operation, string layout, and power electronics. Soiling and snow are site- and maintenance-dependent. No universal percentage is defensible from these six inputs. PVWatts lets users edit shading, soiling, snow, and other losses separately (NREL PVWatts).

How to interpret the 78% default

The 78% value is a coarse, editable factor retained to define the default scenario. It is not a safety margin and is not NREL’s residential-rooftop standard. The current PVWatts web calculator starts system losses at 14%, while the V8 API separately lists a 96% rated inverter efficiency default and models irradiance, temperature, and inverter behavior (PVWatts calculator; V8 API). Those settings are not equivalent to replacing the full model with 78%.

For an existing system, a historical aggregate factor can be calibrated as measured annual AC kWh ÷ (array DC kW × resource-equivalent hours/year), using complete quality-controlled data. For a proposed system, use a site-specific simulation and its uncertainty range.

Limitations and common errors

• Multiplying nameplate power by 24 hours: a PV array is not at nameplate output continuously.
• Treating defaults as local facts: 4.8 hours, 78%, $0.165, and 3% define a scenario only.
• Double-counting losses: do not apply 78% to an input already derived from delivered AC kWh per installed kW.
• Treating gross energy value as bill savings: self-consumption, exports, fixed charges, and time-varying prices can materially change actual savings.
• Reading 25-year output as a warranty or forecast: it is a deterministic 0.5% scenario. The NREL review found a median, not a universal rate (NREL).

This calculator does not model hourly weather, tilt, azimuth, temperature, module technology, DC/AC ratio, clipping, shading geometry, snow timing, outages, curtailment, battery dispatch, or tariff intervals. NREL cautions that even PVWatts predictions contain inherent assumptions and uncertainties and omit unrepresented site characteristics (NREL). Use this simpler result as a scenario, not a production guarantee or financial quote.

Sources

• NREL PVWatts V8 API — current resource, array, loss, and inverter inputs
• NREL PVWatts calculator — web defaults, weather variability, and model limitations
• DOE FEMP: Optimizing Solar Photovoltaic Performance for Longevity — STC and field-performance factors
• NREL Photovoltaic Degradation review — evidence behind the 0.5% scenario assumption
• EIA 2024 residential electricity statistics — source and date for the $0.165 default