Solar Panel Calculator

How the solar calculator works

The calculator runs the same four steps an installer's sizing sheet does, using the numbers you enter instead of national averages. Every input is editable, so if you know your real usage or have a quote in hand, replace the defaults and the results update immediately.

Step 1: Usage from your bill

monthly kWh = monthly bill ÷ rate per kWh
yearly kWh = monthly kWh × 12

Dividing your bill by your rate gives monthly consumption. Compare that figure with the kWh printed on your statement. If it is far off, your rate input is probably a supply-only rate that leaves out delivery charges; use your total bill divided by total kWh instead.

Step 2: System size from sun hours

system kW = (yearly kWh × offset) ÷ (sun hours × 365 × 0.78)

Offset is the share of your usage you want solar to cover; the default is 90%. Peak sun hours are the daily equivalent of full-strength sunlight for your area, typically 3.5 in the Northeast and Pacific Northwest and over 6 in the desert Southwest. The 0.78 performance ratio accounts for inverter losses, wiring, heat, dust, and panel mismatch, so the size you get is the DC rating you will see on a quote, not an ideal-conditions number.

Step 3: Panel count and roof area

panels = system kW ÷ panel kW, rounded up
roof area = panels × 20 sq ft

Panel count is rounded up to the next whole panel. Each residential panel is roughly 17.5 square feet, and the 20 square feet allowance adds room for racking gaps and fire setbacks. If you enter your usable roof area, the calculator tells you whether the array fits. For a panel-by-panel breakdown, use the how many solar panels do I need calculator.

Step 4: Cost, tax credit, savings, and payback

gross cost = system kW × 1,000 × cost per watt
tax credit = gross cost × 30%
net cost = gross cost − tax credit
yearly savings = (yearly kWh × offset) × rate per kWh
payback years = net cost ÷ yearly savings

The default $2.75 per watt is a national average for a turnkey residential install in 2026, and the 30% figure is the federal residential clean energy credit. Savings assume every kilowatt-hour you produce replaces one you would have bought at your retail rate. The solar panel cost calculator and solar payback calculator go deeper on the last two lines.

Worked example with the default inputs

Take a $180 monthly bill at $0.17 per kWh, 4.5 sun hours, a 90% offset, 400 W panels, and $2.75 per watt. Usage is 180 ÷ 0.17 = 1,059 kWh per month, or 12,706 kWh per year. The 90% target is 11,435 kWh. System size is 11,435 ÷ (4.5 × 365 × 0.78) = 8.93 kW, which rounds up to 23 panels covering about 460 square feet. Gross cost is 8.93 kW × 1,000 × $2.75 = $24,546; the credit is $7,364, leaving a net cost of $17,182. Yearly savings are 11,435 kWh × $0.17 = $1,944, so payback is $17,182 ÷ $1,944 = 8.8 years, which the calculator labels a moderate solar candidate.

What to have ready

Three numbers do most of the work. The defaults suit a typical US household, but your own figures make the estimate considerably more useful.

A recent utility bill

Look for the kWh used and the rate per kWh. Usage swings with the seasons, so a bill from a mild month, or better, the 12-month usage chart most utilities print, gives a fairer annual picture than a July or January statement. If your bill does not show a single rate, divide the total amount due by the kWh used. For a state benchmark, the EIA state electricity profiles list average residential prices.

A roof area estimate

Count only the unshaded, south-, east-, or west-facing sections where panels could actually go, not the whole footprint of the house. A satellite view with a measuring tool gets you close enough; a 23-panel array needs about 460 square feet.

Local peak sun hours

The most reliable source is NREL PVWatts, which reports solar radiation in kWh per square meter per day for any US address; that number is your peak sun hours. If you skip this step, 4.5 is a fair middle value for most of the country outside the Southwest and the far north.

Assumptions and limits

The model is deliberately simple so every line of the result can be checked by hand. A few things are fixed or left out as a result.

• The performance ratio is fixed at 78%. Heavy shading, a north-facing roof, or a very flat pitch would push real output below that, and a well-sited system in a cool, sunny climate can slightly exceed it.
• Savings are valued at your full retail rate, which assumes one-to-one net metering. Where utilities credit exports below retail, such as California under NEM 3.0 and a growing number of other states, savings will be lower unless you shift usage or add storage. The solar battery calculator covers that case.
• No panel degradation or utility rate escalation is modeled. Panels lose roughly 0.5% of output per year while rates have historically risen 2 to 3% per year, so over a 25-year life the two effects roughly cancel, but payback in any single year will differ.
• Installer pricing varies by region, roof complexity, and equipment. See solar panel cost by state for typical per-watt ranges, and the methodology page for the sources behind every default.

Quick reference: bill to system size

At $0.17 per kWh, 4.5 sun hours, 100% offset, 400 W panels, and $2.75 per watt after the 30% credit. Net cost is based on the exact kW figure, so it is slightly under what the rounded-up panel count would cost.