Does this reproduce the official NREL PVWatts model?

No. This independent estimator multiplies DC size by user-supplied plane-of-array peak sun hours and applies explicit loss factors. Official NREL PVWatts runs a weather-driven model using irradiance, temperature, wind and system inputs.

What tilt angle should I use?

There is no universally optimal tilt. It depends on latitude, seasonal solar resource, horizon, snow and soiling, roof constraints and the production period you value. Use official NREL PVWatts or a site assessment to compare orientations.

What azimuth angle is best?

Equator-facing arrays often maximize annual exposure, but the best azimuth depends on local solar resource, shading, roof constraints and when electricity is most valuable. This estimator records azimuth but does not calculate its effect.

Where can I find plane-of-array peak sun hours?

Use official NREL PVWatts or another documented solar-resource source for your location and array plane. Do not use generic daylight duration: peak sun hours are equivalent full-sun energy hours.

How accurate is this calculator?

It is a transparent screening estimate, not a weather simulation or bankable forecast. Results depend directly on the plane-of-array sun-hours and loss assumptions you enter and do not model hourly temperature, inverter clipping or year-to-year weather.

PVWatts-Style Solar Output Estimator

Estimate solar output as system kW × plane-of-array peak sun hours × 365 × loss factors, with monthly energy and equivalent panel count. For location-based weather modeling, use official NREL PVWatts.

System Parameters

DC System Size (kW)

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

Module Nameplate Rating (W)

Used only for whole-module count: ceiling(system watts ÷ module watts).

Optional output scenario

Optional user-selected derate; leave at 100% unless you intentionally want a conservative scenario.

Tilt Angle (degrees)

0° = horizontal, 90° = vertical. Recorded only; your sun-hours input must already reflect tilt.

Azimuth Angle (degrees)

180° = true south; 0°/360° = true north. Recorded only; it does not change this estimate.

System Losses (%)

Default 14% covers DC-side wiring, soiling, shading, mismatch, and aging; inverter efficiency is applied separately.

Peak Sun Hours (h/day)

Use plane-of-array sun hours that already reflect the array's location and orientation.

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.

Open official NREL PVWatts for location-based modeling ↗ Read the NREL PVWatts technical model reference (PDF) ↗

PVWatts® is a registered trademark of Alliance for Sustainable Energy, LLC, the operator of NREL. This independent tool is not affiliated with or endorsed by NREL.

Privacy: calculations stay in this browser; system and rate inputs are not uploaded.

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

How it works

Enter your DC system size in kilowatts (e.g. 5 kW for a typical home system).
Enter one module nameplate rating to estimate how many whole modules meet or exceed that DC size.
Record panel tilt and azimuth so scenarios can be compared consistently.
Enter DC-side system losses as a percentage (wiring, soiling, shading, mismatch, and aging); inverter efficiency is entered separately.
Enter plane-of-array peak sun hours per day for the same tilt and azimuth.
The calculator shows annual, monthly and daily output, equivalent module count and estimated bill value.

Use cases

Estimate annual energy yield before purchasing rooftop solar panels.
Compare monthly average production with a monthly electricity bill.
Estimate the whole-module count needed to meet or exceed a target DC nameplate capacity.
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: 2026-09-19 · Reviewed by Nham Vu