Solar Panel Calculator
Size a rooftop solar array from your electricity bill, sun hours and panel rating.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
About this calculator
This is a rough sizing scenario based on average peak sun hours and an entered loss factor. Savings assume every estimated kWh offsets usage at the entered USD/kWh rate; actual self-use and export credits vary. It excludes fixed fees, maintenance, degradation, taxes and incentives. Results are not a site-specific quote.
Worked example
calculated with the default values below
Monthly electricity use 900 kWh · Peak sun hours per day 4.5 hours · Panel rating 400 W · Panel footprint 18 sq ft · System losses (wiring, heat, inverter) 18 % · Electricity price 0.16 $/kWh · Installed cost 2.8 $/W Array size 8.40 kW · Panels needed 21 panels · Roof area required 378 sq ft · Annual production 11,314 kWh · Modeled annual savings 1,810.17 USD · Share of use covered 105 % · Simple payback (model estimate) 13.0 years
Method and assumptions
n = ⌈(M×12÷365) ÷ (h×(1−l÷100)) × 1000 ÷ P⌉; E = n×P÷1000×h×(1−l÷100)×365 Array sizing divides average daily kWh by peak sun hours adjusted for entered system loss, then rounds panel count upward. Savings apply the entered USD/kWh rate to all modeled generation; this assumes full retail offset and excludes many site-specific costs and credits. Payback is undefined when savings are zero. This is a rough sizing scenario based on average peak sun hours and an entered loss factor. Savings assume every estimated kWh offsets usage at the entered USD/kWh rate; actual self-use and export credits vary. It excludes fixed fees, maintenance, degradation, taxes and incentives. Results are not a site-specific quote. The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
Data sources
The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
n = ⌈(M×12÷365) ÷ (h×(1−l÷100)) × 1000 ÷ P⌉; E = n×P÷1000×h×(1−l÷100)×365
- M
- Monthly electricity use (kWh)
- h
- Peak sun hours/day
- l
- System losses (%)
- P
- Panel rating (W)
Array sizing divides average daily kWh by peak sun hours adjusted for entered system loss, then rounds panel count upward. Savings apply the entered USD/kWh rate to all modeled generation; this assumes full retail offset and excludes many site-specific costs and credits. Payback is undefined when savings are zero.
Frequently asked questions
How should I interpret the savings and payback?
Savings apply the entered retail electricity rate to all modeled generation, which can overstate value when export credits differ. Payback is undefined if modeled savings are zero. The estimate excludes fixed fees, maintenance, degradation, taxes and incentives; ask an installer for a site-specific quote.
Which inputs and limits apply to Solar Panel Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. Monthly electricity use: 10–20,000 kWh; Peak sun hours per day: 1–8 hours; Panel rating: 50–800 W; Panel footprint: 3–60 sq ft; System losses (wiring, heat, inverter): 0–50 %; Electricity price: 0–2 $/kWh; Installed cost: 0.3–10 $/W n = ⌈(M×12÷365) ÷ (h×(1−l÷100)) × 1000 ÷ P⌉; E = n×P÷1000×h×(1−l÷100)×365 Array sizing divides average daily kWh by peak sun hours adjusted for entered system loss, then rounds panel count upward. Savings apply the entered USD/kWh rate to all modeled generation; this assumes full retail offset and excludes many site-specific costs and credits. Payback is undefined when savings are zero. This is a rough sizing scenario based on average peak sun hours and an entered loss factor. Savings assume every estimated kWh offsets usage at the entered USD/kWh rate; actual self-use and export credits vary. It excludes fixed fees, maintenance, degradation, taxes and incentives. Results are not a site-specific quote. The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
What is a complete worked example for Monthly electricity use = 900 kWh; Peak sun hours per day = 4.5 hours?
Inputs: Monthly electricity use = 900 kWh; Peak sun hours per day = 4.5 hours; Panel rating = 400 W; Panel footprint = 18 sq ft; System losses (wiring, heat, inverter) = 18 %; Electricity price = 0.16 $/kWh; Installed cost = 2.8 $/W. Calculation: n = ⌈(M×12÷365) ÷ (h×(1−l÷100)) × 1000 ÷ P⌉; E = n×P÷1000×h×(1−l÷100)×365. Results: Array size = 8.40 kW; Panels needed = 21 panels; Roof area required = 378 sq ft; Annual production = 11,314 kWh; Modeled annual savings = 1,810.17 USD; Share of use covered = 105 %; Simple payback (model estimate) = 13.0 years.