Solar system size calculator
Estimate recommended kW, panel count, generation, roof area, and cost.
Updated 2026-07-17 · Formula version appears with every result.
Preparing the calculator…
This should only take a moment. You can retry if it does not finish.
Data confidence
Early estimate
Evidence used
Uses monthly electricity use, target offset, panel wattage, and a planning production yield.
What can vary
Site-specific solar resource, shade, roof geometry, inverter choice, and utility rules can change size.
Planning use
Educational estimate only. Confirm the final design and quotation with qualified professionals.
How this estimate works
Calculations are versioned, reviewed, and reproducible. Inputs are validated and planning ranges are used instead of false precision.
Results support early decisions. Confirm utility rates, equipment specifications, site conditions, and professional requirements before committing money. Review the methodology and source policy before treating any estimate as decision-ready.
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Before using the calculator
What this calculator estimates
Estimate a solar capacity range, panel count, annual production, and roof-area requirement from monthly kWh and target offset.
Who it is for
Buyers checking whether an installer-recommended kW size is plausible before a site-specific design.
Inputs and methodology
The target monthly energy is monthly kWh multiplied by the selected offset. The model converts this target into a capacity range using documented solar-yield and system-loss assumptions, then divides by panel wattage for an indicative count.
Required inputs
- Monthly electricity use in kWh
- Target percentage to offset
- Proposed panel wattage
Philippine planning assumptions
- Monthly use represents a stable baseline.
- Regional solar resource, shading, orientation, losses, and curtailment can change production.
Worked example
For 450 kWh per month and an 80% target, the energy target is 360 kWh per month. The required kW depends on local yield and system losses; the panel count then depends on the selected panel wattage.
How to interpret the result
Use the range to challenge proposals that are much larger or smaller. Ask the installer to show the same monthly kWh, target offset, yield, shading, losses, and exact panel count.
Use the result as a comparison baseline: change one assumption at a time, keep the original inputs, and ask installers to explain why their written estimate differs.
Common mistakes
- Sizing from the peso bill without checking kWh and rate
- Ignoring roof fit and daytime consumption
Related questions
Should the system offset 100%?
Not automatically. Roof limits, daytime use, export treatment, budget, future loads, and operating goals can make another target more sensible.
Why can an installer recommend a different size?
A measured roof layout, shading study, current equipment choices, circuit limits, daytime load profile, and utility requirements can justify a different written recommendation.
Limitations and disclaimer
- The result is an educational planning estimate, not a quotation, site survey, electrical design, or engineering approval.
- Verify current rates, equipment, roof conditions, utility rules, and written installer assumptions before spending.
Last reviewed 2026-07-20