What is the PVWatts calculator?

PVWatts is NREL's free online calculator at pvwatts.nrel.gov that estimates the energy production of grid-connected solar PV systems from location weather data and system inputs. This page is a separate PVWatts-style estimator that applies a transparent peak-sun-hours formula to the values you enter.

Is this the official NREL PVWatts calculator?

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. Use the official NREL PVWatts link in the calculator panel for that model.

How do I estimate solar kWh with a PVWatts-style calculation?

Multiply DC system kW by plane-of-array peak sun hours and 365, then by (1 − system losses) and inverter efficiency. A 5 kW array at 4.5 sun hours with 14% losses and a 96% inverter gives about 6,780 kWh per year.

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.

What are typical system losses?

NREL PVWatts defaults to 14.08%: soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0% and availability 3%. The categories are combined multiplicatively, 1 − (0.98 × 0.97 × 1 × 0.98 × 0.98 × 0.995 × 0.985 × 0.99 × 1 × 0.97) = 14.08%, so they are not simply added. Open the loss breakdown above to change any category. Inverter efficiency is applied separately, so do not include inverter loss again.

How big a solar system do I need for my annual usage?

Divide annual kWh by the AC yield per kW: kW = annual kWh ÷ (sun hours × 365 × (1 − losses) × inverter efficiency). For 10,800 kWh a year at 4.5 sun hours, 14% losses and a 96% inverter, that is 10,800 ÷ 1,356 ≈ 7.96 kW, or 20 modules of 400 W. Enter your usage in the calculator to get the size and module count.

How do I get monthly production like PVWatts?

Run NREL PVWatts for your address and copy the 12 values in its "Solar Radiation (kWh/m²/day)" column, then paste them into the monthly sun hours box here. Each value is plane-of-array peak sun hours, so this calculator multiplies it by the days in that month, your system size and the loss and inverter factors. PVWatts itself also models cell temperature and inverter clipping, so its AC totals will usually be a few percent lower.

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 × days × loss factors, with a month-by-month table, the PVWatts 14.08% loss breakdown and the system size needed for your annual kWh. For location-based weather modeling, use official NREL PVWatts.

A PVWatts calculator estimates how many kWh a solar array produces.

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.

Loss breakdown (NREL PVWatts defaults)

Combined loss = 1 − Π(1 − loss) = 14.08%

Peak Sun Hours (h/day)

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

Monthly sun hours (optional)

Use monthly values instead of the annual average

Paste the 12 "Solar Radiation (kWh/m²/day)" values from NREL PVWatts into any box; they fill January to December.

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 Electricity Use (kWh) (optional)

Gives the DC system size needed to produce this much AC energy per year.

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.

Monthly Production

Month	Sun h/day	AC kWh	Bill value	Bar

Performance Metrics

Results will appear here after calculation.

Formula Breakdown

The step-by-step calculation will appear here.

Quick answer

A PVWatts calculator estimates how many kWh a solar array produces. This independent PVWatts-style estimator, not NREL's official PVWatts, uses AC kWh/year = system kW × peak sun hours × 365 × (1 − losses) × inverter efficiency; NREL's tool adds location weather data.

Example: 5 kW × 4.5 sun hours × 365 = 8,212.5 kWh DC; × 0.86 (14% losses) × 0.96 inverter = about 6,780 kWh a year, 565 kWh a month or 18.6 kWh a day.

Enter your system kW and sun hours for daily, monthly and yearly kWh.

Summary

This independent solar output estimator uses a transparent peak-sun-hours calculation rather than claiming to reproduce the NREL PVWatts model. It estimates annual, monthly and daily AC generation, bill value and equivalent panel quantity. Tilt and azimuth are recorded for scenario documentation but do not alter output: your plane-of-array sun-hours input must already reflect that orientation. For weather-file, temperature, clipping and location-based modeling, use official NREL PVWatts.

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.
Optionally enter 12 monthly plane-of-array sun-hour values (for example the Solar Radiation column from NREL PVWatts) to get a month-by-month table using each month's real number of days.
The loss breakdown combines the ten PVWatts loss categories multiplicatively: total = 1 − Π(1 − loss), so the defaults give 14.08%, not their 15% sum.
Enter annual electricity use to get the DC system size and module count needed to offset it.
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: 2026-10-09 · Reviewed by Nham Vu