March 26, 20264 min read

Solar Panel Sizing Calculator

Calculate how many solar panels you need, estimated system cost, payback period, and annual savings. Plan your residential or commercial solar installation.

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Solar panel sizing sounds technical but it comes down to a handful of numbers: how much electricity you use, how much sun your location gets, and how efficiently your panels convert that sun into usable power. Get those three inputs right and the math is straightforward. The CalcHub Solar Panel Sizing Calculator handles all of it, including the shading adjustments and inverter losses that installers sometimes gloss over in their proposals.

The Core Calculation

System Size (kW) = Annual kWh Consumption / (Peak Sun Hours × 365 × System Efficiency)

System efficiency is typically 75–85% accounting for inverter losses, wiring losses, and temperature derating. Using 80%:

For a home using 10,000 kWh/year in Phoenix, AZ (5.5 peak sun hours):

System size = 10,000 / (5.5 × 365 × 0.80) = 6.23 kW

At 400W per panel: roughly 16 panels

The same home in Seattle (3.5 peak sun hours):

System size = 10,000 / (3.5 × 365 × 0.80) = 9.80 kW

Roughly 25 panels

Location matters enormously.

Peak Sun Hours by Region (US)

Region	Peak Sun Hours/Day	Example Cities
Southwest	5.5–7.0	Phoenix, Las Vegas, LA
Southeast	4.5–5.5	Atlanta, Orlando, Dallas
Midwest	3.5–4.5	Chicago, Kansas City
Northeast	3.5–4.5	New York, Boston
Northwest	2.5–4.0	Seattle, Portland
Hawaii	5.5–6.5	Honolulu

For international locations, NASA's POWER database provides historical solar irradiance data that the CalcHub calculator queries automatically when you enter your location.

Cost and Payback Estimation

System Size	Installed Cost (US 2026)	After Federal ITC (30%)	Annual Savings*	Payback
4 kW	$12,000	$8,400	~$900	~9 years
6 kW	$17,400	$12,180	~$1,350	~9 years
8 kW	$22,400	$15,680	~$1,800	~8.7 years
10 kW	$27,000	$18,900	~$2,250	~8.4 years

*Assuming $0.15/kWh average retail electricity rate. Savings increase with higher utility rates.

The federal Investment Tax Credit (ITC) at 30% is currently extended through 2032. Many states add additional incentives that can reduce payback to 5–7 years in high-electricity-cost states like California, Hawaii, and New York.

Battery Storage Consideration

Solar-only systems export excess power during the day and draw from the grid at night. Adding battery storage (like a Tesla Powerwall or LG Energy storage) lets you:

Store midday excess for evening use
Provide backup power during outages
Maximize self-consumption (especially in states with unfavorable net metering)

A 13.5 kWh battery (Powerwall) costs roughly $10,000–12,000 installed. For most homes with time-of-use rates, payback on storage alone is 7–12 years. The calculator includes optional battery sizing and cost analysis.

Tips

Roof orientation matters. South-facing is ideal in the northern hemisphere. East/west split installations work well too — they produce more in morning/evening and less midday, which can be better for self-consumption if your household uses power in mornings and evenings.
Shading kills production. Even one shaded cell reduces the entire panel string's output. Trees, chimneys, and neighboring buildings need careful assessment.
Net metering rules vary. In some states, surplus electricity sells back to the grid at full retail rate (1:1 net metering). Others pay wholesale rates (far less valuable). This dramatically changes the financial case.

How long do solar panels last?

Modern panels are typically warranted for 25 years at 80% of rated output. Real-world degradation is about 0.5% per year, meaning a panel rated 400W produces roughly 350W after 25 years. Most panels outlast their warranty and continue producing at reduced output for 30–40 years.

Should I wait for prices to keep falling?

Solar prices have fallen 90% over the past 15 years, but the rate of decline has slowed significantly. The federal tax credit at 30% is available now and is scheduled to step down after 2032. Waiting costs you the savings you would have generated during the delay.

What's the difference between monocrystalline and polycrystalline panels?

Monocrystalline (mono-Si) panels are cut from single-crystal silicon — higher efficiency (20–24%), better performance in low light and heat, more expensive. Polycrystalline (poly-Si) are made from cast silicon fragments — slightly lower efficiency (15–18%), cheaper. Most new residential installations use monocrystalline due to the efficiency advantage on limited roof space.

Electricity Cost Calculator — understand your baseline before sizing solar
Energy Savings Calculator — home energy improvements to reduce system size needed
EV Savings Calculator — pair solar with EV charging for maximum savings