What is the PVWatts formula?

PVWatts calculates DC energy as: E_DC = System_Size_kW × Peak_Sun_Hours × 365 × Tilt_Factor. AC energy is then E_AC = E_DC × (1 - Losses/100) × Inverter_Efficiency. This matches the simplified NREL PVWatts v5 model.

What tilt angle should I use?

For maximum annual production, set your tilt equal to your latitude. For example, at 35° latitude, a 35° tilt is optimal. Flat roofs often use 10–15° for drainage. Steeper tilts favor winter production; shallower tilts favor summer.

What azimuth angle is best?

In the northern hemisphere, 180° (true south) is optimal. In the southern hemisphere, 0° (true north) is optimal. East (90°) and west (270°) orientations reduce output by roughly 15–20%.

What are typical system losses?

NREL recommends 14% as the default total system loss, which accounts for wiring losses (~2%), soiling (~2%), shading (~3%), module mismatch (~2%), inverter losses (~4%), and aging (~1%). Adjust upward if shading is significant.

Where can I find peak sun hours for my location?

Use the NREL PVWatts website (pvwatts.nrel.gov) or the Global Solar Atlas (solargis.com) to find peak sun hours for your address. Typical values range from 3.5 h/day (northern climates) to 6.5 h/day (desert southwest).

How accurate is this calculator?

This tool uses a simplified version of the NREL PVWatts v5 algorithm. Real-world output can vary ±10–20% due to local weather, exact shading conditions, panel degradation, and other site-specific factors. For a bankable energy estimate, use the full NREL PVWatts API with your exact location.

PVWatts Solar Calculator

Name: PVWatts Solar Calculator
Availability: InStock
Author: Nham Vu

Estimate annual solar energy output (kWh/year) from your PV system using the NREL PVWatts formula.

System Parameters

DC System Size (kW)

Total nameplate DC capacity of your solar array (e.g. 5 kW for a typical home).

Module Type

Module temperature and irradiance correction factor.

Tilt Angle (degrees)

0° = flat/horizontal, 90° = vertical. Set to your latitude for maximum annual output.

Azimuth Angle (degrees)

180° = true south (optimal for northern hemisphere). 0°/360° = north.

System Losses (%)

NREL default is 14%. Includes wiring, soiling, shading, mismatch, and inverter losses.

Peak Sun Hours (h/day)

Annual average daily peak sun hours for your location (3.5–6.5 h/day for most of the US).

Inverter Efficiency (%)

Typical inverters are 95–97% efficient (CEC weighted average).

Electricity Rate ($/kWh) (optional)

Used to estimate annual savings. Leave at 0 to skip.

Annual Energy Production

Enter parameters on the left and click Calculate.

Performance Metrics

Results will appear here after calculation.

Formula Breakdown

The step-by-step calculation will appear here.

Summary

Estimate annual solar energy output (kWh/year) from your PV system using the NREL PVWatts formula.

How it works

Enter your DC system size in kilowatts (e.g. 5 kW for a typical home system).
Set the panel tilt angle in degrees — 0 is flat/horizontal, 90 is vertical.
Set the azimuth angle — 180 degrees is true south (optimal in the northern hemisphere).
Enter total system losses as a percentage (wiring, soiling, shading, inverter, etc.).
Enter peak sun hours per day for your location (use NREL maps or local data).
The calculator instantly shows DC output, AC output, and estimated annual kWh production.

Use cases

Estimate annual energy yield before purchasing rooftop solar panels.
Compare different tilt and azimuth orientations for optimal panel placement.
Calculate expected savings by multiplying kWh/year by your electricity rate.
Size a battery storage system by knowing daily and annual energy production.
Validate contractor quotes by independently estimating expected system output.
Assess the impact of shading losses on overall system performance.
Plan off-grid solar systems by estimating available energy per year.
Conduct feasibility studies for commercial or utility-scale PV installations.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu