Solar System Size Calculator
Turn your monthly electricity consumption and a target solar offset into an approximate system size in kilowatts and an approximate panel count. Panel wattage and expected production are both adjustable.
Assumption — depends on orientation, tilt, shading and equipment.
Assumption — 400 W is a common residential module rating.
Estimated results
Assumptions used
- Production: 1,500 kWh per kW per year
- Panel rating: 400 W per module
- No allowance made for usable roof area or shading losses
Planning estimate only. Actual production, utility rates, system cost, incentives and savings vary by property and provider.
Want a property-specific solar estimate?
Get My Solar QuotePlanning estimate only. Actual production, utility rates, system cost, incentives and savings vary by property and provider. This calculator does not evaluate usable roof area, shading or structural capacity.
Reading the Result
System size is an electrical answer
Kilowatts describe the rated direct current capacity of the array under standard test conditions. That rating is what quotes, permits and interconnection paperwork refer to, and it is the number to hold constant when comparing proposals from different providers.
Panel count is a physical answer
Once you fix a system size, panel count falls out of module wattage. Fewer, higher-wattage modules take less roof area for the same capacity. That trade-off matters most on roofs where usable unshaded area — not budget — is the limiting factor.
What the calculator cannot see
It does not know your roof. Setbacks, vents, skylights, chimneys, HVAC equipment, roof pitch, plane orientation and shading all reduce how much of a roof is actually usable. Read the roof suitability guide, then have the layout confirmed during a site assessment.
Then price it
A kilowatt figure is the starting point for a quote, not the end of one. Equipment tier, inverter architecture, mounting method, electrical work and whether battery storage is included all move the total. The solar panel cost guide covers each variable.
Before You Request Quotes
- Average twelve months of kilowatt-hour usage, not one bill.
- Decide your target offset and why — full offset is not always the goal.
- Note any upcoming load changes such as EV charging or an HVAC replacement.
- Replace the default production assumption with a designer's modeled figure.
- Ask every provider to quote the same system size for a fair comparison.
- Confirm whether the proposed layout fits on unshaded roof planes.
- Ask whether the inverter allows expansion or a battery addition later.
- Get the panel model, quantity and inverter model in writing.
System Sizing FAQs
How is solar system size calculated?
Multiply monthly kilowatt-hour consumption by twelve to get annual usage, multiply by the share you want solar to cover, then divide by the kilowatt-hours a single kilowatt of solar is expected to produce at your site each year. The result is system size in kilowatts. Dividing that by panel wattage gives an approximate module count, before any adjustment for usable roof area.
What panel wattage should I assume?
Residential photovoltaic modules commonly sit in the 350 to 450 watt range, and 400 watts is a reasonable round assumption for planning. The wattage matters mainly for panel count and physical area: a higher-wattage module produces the same system size with fewer panels, which helps when usable roof area is the binding constraint rather than budget.
Should I size for 100 percent offset?
Not automatically. A full-offset system needs enough unshaded roof area to hold it, and the value of production beyond what you consume depends on the buyback or interconnection terms on your account, which vary by provider and can change. Many designs land below full offset because of roof geometry, budget or a deliberate decision to consume nearly everything on site.
Does the calculator account for my roof area?
No. It answers the electrical question — how many kilowatts would meet your target — not the geometric one. Actual capacity depends on usable roof planes, setbacks, obstructions like vents and skylights, pitch, and shading. A site assessment can confirm whether the calculated size physically fits, and on which planes.
What if my electricity usage is about to change?
Size to the usage you expect going forward, not just history. Adding electric vehicle charging, a pool pump, an HVAC replacement or a home addition all shift annual consumption. Note the change in your quote request so the design accounts for it, and ask whether the inverter and electrical layout leave room to expand the array later.
How does this relate to production per kilowatt?
Production per kilowatt is the single most influential assumption in the calculation. It bundles roof orientation, tilt, shading, module efficiency, inverter losses, temperature effects and soiling into one number. A designer modeling your actual roof planes will produce a more accurate figure than any default, so replace the assumption once you have theirs.
Related Solar Resources
- Laredo Solar Savings CalculatorModel production, offset and savings.
- Solar Panel Cost in Laredo TXWhat moves a solar quote.
- Is My Roof Good for Solar?Whether the array physically fits.
- How Home Solar Panels WorkPanels, inverter, panel, grid and battery.
- Residential Solar Laredo TXPlanning a home solar project.
- Solar Inverter InstallationInverter choices and what they affect.
Want the sizing confirmed against your actual roof? Call 956-395-1662 or start a quote request.
Ready to Explore Solar for Your Property?
Start with a quick quote request and provide a few details about your property and current electricity use.