What are peak sun hours?

Peak sun hours measure the total daily solar irradiance at a location, expressed as the number of equivalent hours at 1,000 W/m2 (full intensity). A location receiving 5 peak sun hours per day accumulates 5 kWh of solar energy per square meter. It is not the hours of daylight -- a 14-hour summer day in Minnesota might deliver only 5 peak sun hours because morning and evening sun is at low angles with reduced intensity. NREL's PVWatts database provides average peak sun hours by ZIP code for accurate location-specific estimates.

What does 14% system losses mean?

The 14% default loss accounts for real-world efficiency reductions between rated panel output and actual delivered energy. The main components are inverter conversion losses (3-5%), wiring and connection losses (1-2%), temperature derating (panels produce less as they heat above 25 degrees C, typically 0.35-0.5% per degree above rating, accounting for 3-6% in hot climates), soiling and shading (1-3%), and mismatch losses between panels (1-2%). Systems in hot climates or with occasional shading should use 18-20% losses. Exceptionally clean, cool, well-installed systems may achieve losses as low as 10%.

How many panels do I need to power my home?

Divide your average daily electricity consumption (in kWh) by the expected daily output of a single panel. If your home uses 30 kWh/day and a 400W panel in your location produces 1.6 kWh/day (400W x 5 peak sun hours x 0.80 efficiency), you need roughly 19 panels. The U.S. average household uses about 29 kWh/day. Most residential systems run 6-12 kW (15-30 panels at 400W each). Add panels if you plan to charge an electric vehicle or switch from gas to electric heating.

How does tilt angle affect solar panel output?

Panels tilted to equal the site's latitude capture the maximum annual energy. At 35 degrees latitude, a 35-degree tilt is optimal for full-year production. Steeper tilts favor winter production; shallower tilts (0-10 degrees) favor summer. The difference between optimal tilt and a flat (0-degree) installation is about 10-15% annual output loss at mid-latitudes. Roof-mounted systems are typically fixed at whatever pitch the roof provides, which is acceptable -- the output penalty for a reasonable roof pitch vs. optimal tilt is usually under 5-8% at mid-latitudes.

How should I configure panels in series vs. parallel (string sizing)?

Panels connected in series add their voltages while current stays constant; panels in parallel add current while voltage stays constant. String inverters require the string voltage to fall within the inverter's MPPT (maximum power point tracking) input range -- typically 200-600V for residential systems. A common string at 40V panel Voc would have 8-12 panels per string (320-480V). Microinverters and DC optimizers eliminate string sizing constraints by handling each panel independently, making them better for roofs with shading or multiple orientations. String sizing errors are a top cause of underperformance in DIY installs.

How do I calculate solar payback period?

Payback period = total installed cost after incentives divided by annual electricity savings. For a $20,000 system claiming the 30% federal ITC (Investment Tax Credit), the net cost is $14,000. If the system generates 8,000 kWh/year and you pay $0.15/kWh, annual savings are $1,200, giving a simple payback of 11.7 years. Add net metering credit if your utility buys excess generation. The 30% federal ITC is available through 2032 under the Inflation Reduction Act. State incentives and SREC (solar renewable energy credit) markets can reduce payback to 6-8 years in favorable states.

Solar Panel Output Estimator

Estimate daily and annual solar production, bill savings, payback, and 25-year ROI from panel count, location, and system losses.

100% client-side. Inputs stay in your browser (ons-solar-panel-inputs).

Inputs

LocationNumber of panelsPanel wattage (W)System typeTilt (degrees)Azimuth (south = 180)System losses (%)Electricity rate ($/kWh)System cost ($)Annual degradation (%)

Peak sun hours assume about 30° tilt and 180° azimuth (south). Tilt and azimuth fields are for your records; adjust location for climate zone changes.

Results

Daily output

37.8 kWh

Annual output (yr 1)

13,811.6 kWh

Annual savings (yr 1)

$1,795.51

Array size

8 kW

Peak sun hours

5.5 h/day

Payback period

12 years

25-yr ROI

111.5%

CO2 offset (year 1): 5.33 tons/yr. CO2 offset uses EPA eGRID US average (~386 g CO2/kWh). Verified 2026-05-25.

Peak sun hour data from NREL PVWatts. Actual output varies with weather, shading, and installation quality.

How this tool works

We multiply your array size in kW by location peak sun hours (kWh per kW per day), then subtract system losses for inverter, wiring, soiling, and temperature. Daily kWh times 365 gives year-one production. Each year compounds panel degradation on energy, not on the rate you pay. Cumulative bill savings cross your installed cost in the payback year. CO2 offset uses the EPA US grid average of about 386 grams per kWh.

Worked example

Twenty 400 W panels in California make an 8 kW array. At 5.5 peak sun hours and 14% losses you get about 37.8 kWh per day, or roughly 13,800 kWh in year one. At $0.13 per kWh that is about $1,790 in year-one savings. On a $20,000 install, cumulative savings typically pass system cost around year 11 with 0.5% yearly degradation.

Frequently asked questions

What are peak sun hours?
Peak sun hours measure the total daily solar irradiance at a location, expressed as the number of equivalent hours at 1,000 W/m2 (full intensity). A location receiving 5 peak sun hours per day accumulates 5 kWh of solar energy per square meter. It is not the hours of daylight -- a 14-hour summer day in Minnesota might deliver only 5 peak sun hours because morning and evening sun is at low angles with reduced intensity. NREL's PVWatts database provides average peak sun hours by ZIP code for accurate location-specific estimates.
What does 14% system losses mean?
The 14% default loss accounts for real-world efficiency reductions between rated panel output and actual delivered energy. The main components are inverter conversion losses (3-5%), wiring and connection losses (1-2%), temperature derating (panels produce less as they heat above 25 degrees C, typically 0.35-0.5% per degree above rating, accounting for 3-6% in hot climates), soiling and shading (1-3%), and mismatch losses between panels (1-2%). Systems in hot climates or with occasional shading should use 18-20% losses. Exceptionally clean, cool, well-installed systems may achieve losses as low as 10%.
How many panels do I need to power my home?
Divide your average daily electricity consumption (in kWh) by the expected daily output of a single panel. If your home uses 30 kWh/day and a 400W panel in your location produces 1.6 kWh/day (400W x 5 peak sun hours x 0.80 efficiency), you need roughly 19 panels. The U.S. average household uses about 29 kWh/day. Most residential systems run 6-12 kW (15-30 panels at 400W each). Add panels if you plan to charge an electric vehicle or switch from gas to electric heating.
How does tilt angle affect solar panel output?
Panels tilted to equal the site's latitude capture the maximum annual energy. At 35 degrees latitude, a 35-degree tilt is optimal for full-year production. Steeper tilts favor winter production; shallower tilts (0-10 degrees) favor summer. The difference between optimal tilt and a flat (0-degree) installation is about 10-15% annual output loss at mid-latitudes. Roof-mounted systems are typically fixed at whatever pitch the roof provides, which is acceptable -- the output penalty for a reasonable roof pitch vs. optimal tilt is usually under 5-8% at mid-latitudes.
How should I configure panels in series vs. parallel (string sizing)?
Panels connected in series add their voltages while current stays constant; panels in parallel add current while voltage stays constant. String inverters require the string voltage to fall within the inverter's MPPT (maximum power point tracking) input range -- typically 200-600V for residential systems. A common string at 40V panel Voc would have 8-12 panels per string (320-480V). Microinverters and DC optimizers eliminate string sizing constraints by handling each panel independently, making them better for roofs with shading or multiple orientations. String sizing errors are a top cause of underperformance in DIY installs.
How do I calculate solar payback period?
Payback period = total installed cost after incentives divided by annual electricity savings. For a $20,000 system claiming the 30% federal ITC (Investment Tax Credit), the net cost is $14,000. If the system generates 8,000 kWh/year and you pay $0.15/kWh, annual savings are $1,200, giving a simple payback of 11.7 years. Add net metering credit if your utility buys excess generation. The 30% federal ITC is available through 2032 under the Inflation Reduction Act. State incentives and SREC (solar renewable energy credit) markets can reduce payback to 6-8 years in favorable states.

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