Free Solar Calculator
Estimate first-year solar energy production from a known DC array, or calculate a preliminary solar system size from annual electricity use. The results include panel count, annual and monthly kWh, roof area, capacity factor, a planning range and estimated year-25 output.
Last Updated: July 25, 2026. Method reviewed against current PVWatts V8 documentation and U.S. Department of Energy solar guidance.
Solar Panel Size & Output Calculator
Use an annual-average peak-sun-hours value for the proposed array plane. Every assumption is editable, and no price, tariff or incentive is built in.
The module footprint excludes walkways, fire setbacks, row spacing, obstructions and unusable roof sections. “Offset” compares annual totals and does not measure hourly self-consumption or backup capability.
How to Use This Solar Calculator
- Choose a calculation goal. Select “energy output” when you know the array's DC capacity. Select “system size” when you know annual electricity use and want a preliminary capacity.
- Enter annual electricity use. Find total kWh—not the bill amount—from 12 months of utility statements. In output mode this is optional; in sizing mode it is required.
- Set the target offset. A 100% annual target means estimated generation equals annual consumption. It does not mean the home can operate without the grid at every hour.
- Use local peak sun hours. Enter an annual-average value for the array's location, tilt and orientation. Peak sun hours are energy-equivalent hours, not simply daylight hours.
- Review losses and module details. Change the default screening assumptions to match a site survey, equipment datasheet or qualified design.
- Read the complete result. Check the whole-panel size, panel count, energy range, module footprint, capacity factor and long-term degradation scenario—not only the headline annual kWh.
Solar Calculator Formula
This tool uses a transparent annual energy approximation. It multiplies DC array capacity by average daily peak sun hours and days per year, then applies the remaining share after total system losses.
When solving for required capacity, the formula is rearranged:
The sizing mode then rounds the module count up. The displayed whole-panel capacity equals panel count multiplied by panel wattage and divided by 1,000. Production is recalculated from that installable whole-panel capacity, so the estimated offset may be slightly above the selected target.
Capacity factor compares first-year production with the energy the DC nameplate capacity would produce if it ran at full power during all 8,760 hours of a non-leap year:
For the long-term scenario, year-25 output equals first-year output multiplied by (1 − annual degradation rate)24. It is a simple compounding scenario; a manufacturer warranty, site history or engineering model should replace the sample degradation rate where available.
Worked Solar Panel Example
Suppose a proposed array is 6 kW DC, the site receives 4.5 average peak sun hours per day and total losses are 14%. A 400-watt module is assumed.
- Panel count: 6,000 W ÷ 400 W = 15 panels.
- Average monthly production: 8,475.3 ÷ 12 = about 706.3 kWh.
- Average daily production: 8,475.3 ÷ 365 = about 23.2 kWh.
- Capacity factor: 8,475.3 ÷ (6 × 8,760) = about 16.1%.
- Year-25 scenario at 0.5% annual degradation: about 7,515 kWh.
If the home uses 10,000 kWh per year, the estimated annual energy offset is about 84.8%. Actual bills can show a different percentage because retail rates, export compensation, fixed charges, time-of-use periods, taxes, curtailment and self-consumption timing are not part of this production formula.
Peak Sun Hours and Estimated Output
Peak sun hours convert the day's solar irradiation into equivalent hours at 1,000 watts per square metre. They are not the hours between sunrise and sunset. Two roofs with the same daylight duration can have different solar resources because of latitude, weather, tilt, azimuth, haze and shading.
The table shows how strongly this one input changes the annual estimate for 1 kW DC with 14% total losses. It is a sensitivity comparison, not a location lookup.
| Peak sun hours/day | Year-1 kWh per 1 kW DC | Approximate DC capacity factor | Interpretation |
|---|---|---|---|
| 3.0 | 942 kWh | 10.8% | Lower-resource or less-favourable annual scenario |
| 4.0 | 1,256 kWh | 14.3% | Moderate annual scenario |
| 5.0 | 1,570 kWh | 17.9% | Strong annual solar-resource scenario |
| 6.0 | 1,883 kWh | 21.5% | High annual solar-resource scenario |
For a consequential decision, obtain plane-of-array solar resource data for the exact location. The current PVWatts V8 model goes far beyond this calculator by using weather data and detailed models for array geometry, modules, inverters and thermal effects.
Choosing Reliable Solar Inputs
System size: use DC nameplate capacity
Add the rated watts of all modules and divide by 1,000. Fifteen 400-watt modules equal 6 kW DC. Do not substitute inverter AC capacity unless the quote explicitly identifies the system that way. DC-to-AC ratio and inverter clipping require a more detailed model and are not separately calculated here.
Annual electricity use: total 12 months of kWh
Use a full recent year to capture seasonal heating, cooling and occupancy. Adjust the forecast if you expect an electric vehicle, heat pump, home extension, efficiency upgrade or occupancy change. A monthly bill's currency amount cannot replace kWh because tariffs and fixed charges vary.
Total system losses: avoid double counting
The editable 14% example is a screening assumption informed by the familiar PVWatts loss input, not a claim that every project loses exactly 14%. Real losses may include soiling, shading, snow, mismatch, wiring, connections, nameplate tolerance, availability and other effects. If a solar-resource value or professional model already incorporates a factor, do not subtract it again.
Panel rating and area: use the exact module
Nameplate wattage affects panel count. Module dimensions affect only the footprint result. The calculator multiplies whole panels by area per panel; it does not design the roof. Fire setbacks, walkways, hips, valleys, vents, skylights, chimneys, structural zones and row spacing can require substantially more usable area.
Degradation and the planning range
The 0.5% degradation example is editable and broadly consistent with published research medians, but technology, climate, equipment quality and operating conditions differ. The ± range is controlled by you. It is a scenario band for sensitivity testing—not a warranty, probability distribution or weather confidence interval.
What Changes Real Solar Production?
The U.S. Department of Energy also recommends checking roof size, shape, slope, age and shade. Its homeowner guidance notes that south-facing roofs with slopes between 15 and 40 degrees often perform well in the Northern Hemisphere, but other roofs may still be suitable. Structural condition, electrical service, permits, interconnection and local code require qualified review.
Annual Energy Offset Is Not Energy Independence
A 100% annual offset means estimated yearly solar generation equals yearly electricity consumption. Solar output and household demand do not occur at the same times. Without adequate storage or another supply, the home can export surplus power during sunny periods and import energy at night or during poor weather.
This distinction also matters for savings. One exported kWh may not receive the same value as one retail kWh avoided. Fixed customer charges, demand charges and time-of-use prices can remain even when annual generation equals annual usage. Use the Electricity Bill Calculator with the current tariff, then evaluate self-consumption and export rules separately.
Simple Calculator vs Detailed Solar Model
| Question | This solar calculator | Detailed location model or professional design |
|---|---|---|
| Annual production screening | Yes, from entered sun hours and losses | Yes, using weather and equipment models |
| Preliminary system size | Yes, including whole-panel rounding | Yes, with layout and electrical constraints |
| Monthly or hourly output | Monthly average only | Monthly and often hourly profiles |
| Tilt, azimuth and shade | Represented only through your inputs | Modelled explicitly when data is supplied |
| Inverter clipping and thermal effects | Not separately modelled | Usually modelled |
| Permits, structure and interconnection | Not evaluated | Requires qualified local review |
Related Solar and Energy Calculators
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Solar Calculator FAQs
Quick guidance for interpreting solar system size, panel count, production and energy-offset results.
How many solar panels do I need?
Choose “system size from annual electricity use,” enter 12 months of kWh, the target offset, local peak sun hours, losses and module wattage. The calculator solves for minimum DC capacity and rounds up to a whole-panel count. Roof layout, electrical limits and local rules can change the installable number.
How much energy does a 1 kW solar system produce?
It depends on solar resource and losses. At 4.5 peak sun hours per day and 14% losses, the simplified estimate is 1 × 4.5 × 365 × 0.86, or about 1,413 kWh in the first year.
What are peak sun hours?
Peak sun hours express daily solar irradiation as equivalent hours at 1,000 watts per square metre. They are not ordinary daylight hours. Use an annual-average value for the proposed array plane and location.
What system losses should I enter?
Use a site- and design-specific total where possible. Losses can include shading, soiling, snow, mismatch, wiring, connections, nameplate tolerance and availability. The 14% sample is editable and should not be treated as universal.
Does 100% solar offset mean I can go off-grid?
No. It compares estimated annual generation with annual consumption. Hourly and seasonal mismatches remain, and an off-grid design also requires load analysis, storage, reserve capacity and equipment sized for difficult conditions.
Does the roof-area result include setbacks and spacing?
No. It is panel count multiplied by entered module area. Add required fire access, edge setbacks, service paths, row spacing and unusable areas around roof features, and obtain structural and code review.
Why might PVWatts or an installer give a different result?
Detailed models can use location-specific weather, tilt, azimuth, array type, module behaviour, inverter efficiency, DC-to-AC ratio, temperature, shade and hourly conditions. This page intentionally uses a simpler annual formula with visible assumptions.
Does this calculator estimate solar cost, payback or tax credits?
No. It estimates capacity and energy only. Installed price, financing, incentives, maintenance, tariffs, export compensation and taxes require current local inputs. As of July 2026, the IRS says the U.S. Residential Clean Energy Credit is unavailable for property placed in service after December 31, 2025.
Method and Reference Sources
- National Laboratory of the Rockies — PVWatts V8 API inputs and outputs
- PVWatts Calculator — current Version 8 model information
- U.S. Department of Energy — Homeowner's Guide to Solar
- NLR research — Compendium of Photovoltaic Degradation Rates
- Internal Revenue Service — current Residential Clean Energy Credit guidance
Financial and technical disclaimer: This calculator provides a preliminary educational estimate, not a generation guarantee, financial forecast, tax opinion, engineering design, structural assessment, electrical plan, permit approval, interconnection decision, installer quote or safety instruction. Verify solar resource, shade, roof condition, equipment, layout, tariffs, export rules, incentives and local requirements with current official sources and qualified professionals before purchasing, financing or installing a system. Never work on an energized electrical system or access a roof without appropriate training and protection.