Solar Power Calculator

Enter your panel array details — see exactly how much electricity it produces per day, month, and year.

Your solar array

Number of panels?

panels

Panel wattage?

Peak sun hours?

hrs/day

System efficiency?

Electricity rate (for savings)?

$/kWh

Your 20-panel 8.0 kW system produces

11,300 kWh per year

Daily production31.0 kWh

Monthly production942 kWh

System size8.00 kW

Est. annual savings$1,695

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How to Use This Calculator

Enter your panel count and wattage

Start with the number of panels you have or are considering, and their rated wattage. You'll find the wattage on the panel label, datasheet, or installer quote. Common residential panels are 350W–450W. Multiply these together to get your total system capacity in watts (or divide by 1,000 for kilowatts).

Set your peak sun hours

Peak sun hours is the critical multiplier. It's not how many hours the sun is up — it's the equivalent hours of full-intensity (1,000 W/m²) sunlight your location receives per day. A partly cloudy city might have 12 hours of daylight but only 3.5 peak sun hours. Use our Peak Sun Hours Calculator to find your exact value.

Set system efficiency

The system efficiency (default 0.86 = 86%) accounts for all the energy your panels generate that doesn't make it to the meter: inverter conversion losses (3–6%), temperature derating (panels produce less power when hot), wiring resistance, soiling (dust, bird droppings), and any shading. A well-maintained micro-inverter system in a low-temperature climate might achieve 92%. A string-inverter system with partial afternoon shading might only reach 78%.

Add your electricity rate

Enter your electricity rate to see estimated annual dollar savings. This tells you how much money your system's production is worth, assuming all production offsets grid electricity at that rate. If you export excess to the grid at a lower buyback rate, your actual savings will be slightly lower.

The Formula

Solar power output calculation uses this standard formula:

System kW = Number of panels × Panel wattage (W) ÷ 1000 Daily kWh = System kW × Peak sun hours × System efficiency Monthly kWh = Daily kWh × 30.44 (avg days/month) Annual kWh = Daily kWh × 365 Annual savings ($) = Annual kWh × Electricity rate ($/kWh)

This is the reverse calculation from a panel sizing calculator. Instead of starting from consumption and working backwards to panel count, you start with a panel array and calculate its production. Both use the same underlying formula — just solved for different unknowns.

The key to understanding this formula: your 400W panels don't produce 400W for 8 hours. They produce the equivalent of 400W for only peak sun hours worth of time, then that's reduced by system efficiency losses. A 400W panel in Phoenix (6.5 PSH, 86% eff) produces 400 × 6.5 × 0.86 = 2,236 Wh = 2.24 kWh per day.

Example

A 25-panel rooftop system — Denver, CO

A homeowner in Denver has a 25-panel system with 400W panels. Denver gets 5.0 peak sun hours per day and the system runs at 87% efficiency.

Number of panels25

Panel wattage400W

System size10.0 kW

Peak sun hours5.0 hrs/day

System efficiency87%

Result

Daily production43.5 kWh

Monthly production1,323 kWh

Annual production15,878 kWh

Annual savings at $0.14/kWh$2,223

At Xcel Energy's ~$0.14/kWh rate, this 10 kW Denver system saves roughly $2,200/year. At a typical installed cost of $28,000–$35,000 before incentives ($17,600–$24,500 after 30% ITC), the payback period is approximately 8–11 years — well within the 25–30 year panel lifespan.

FAQ

How much power does a 400W solar panel produce per day? ▼

A single 400W panel produces between 1.1 kWh/day (Seattle, 3.5 PSH, 86% efficiency) and 2.2 kWh/day (Phoenix, 6.5 PSH, 86% efficiency). At the US average of 4.5 PSH, a 400W panel produces about 1.55 kWh/day (or 47 kWh/month, 566 kWh/year). The rated wattage is the maximum output under ideal lab conditions — real-world production is always lower due to location, heat, and losses.

How much power does a 10 kW solar system produce per day? ▼

A 10 kW system produces 27–50 kWh/day depending on location: about 27 kWh in Seattle (3.5 PSH), 39 kWh at the US average (4.5 PSH), and 56 kWh in Phoenix (6.5 PSH), all at 86% efficiency. Monthly production ranges from 820–1,700 kWh. Enter your specific panel count and location above for an exact number.

Why does my solar system produce less than the calculator says? ▼

Common reasons for lower-than-expected production: (1) Shading from trees, chimneys, or neighboring buildings — even partial shading on one panel can drop production by 20%+ with string inverters. (2) Panel soiling — dust and bird droppings can reduce output by 5–10% without regular cleaning. (3) Higher temperatures than assumed — panels lose ~0.4%/°C above 25°C. (4) Sub-optimal tilt or orientation. (5) Inverter issues. Check your monitoring system for clues. Micro-inverters or power optimizers can recover much of the shading loss.

Do solar panels produce power on cloudy days? ▼

Yes — solar panels produce power from diffuse light even on overcast days, typically at 10–25% of rated capacity. Light cloud cover might allow 50–80% of normal production. This is already factored into the peak sun hours calculation, which averages sunny, cloudy, and rainy days over the year. A day with heavy rain might deliver only 0.3 PSH while a clear day might deliver 8.0 PSH — the annual average smooths these out.

How much does solar panel output degrade over time? ▼

Modern monocrystalline panels degrade at about 0.5% per year (the industry standard warranty guarantees no more than 0.7%/year). After 25 years, a panel rated at 400W will produce about 88–91% of its original output (~352–364W). This is why solar is still economically attractive after 20+ years — the production decline is very gradual. The degradation is already accounted for in the annual production figures used by most solar proposals.