Your solar tally
–panels of – W
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- Yearly production (year 1)
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- Share of your use covered
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- Roof area needed
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- Suggested inverter (AC)
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- System cost after incentives
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- First-year savings
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- Simple payback
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- 25-year net savings
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- CO₂ avoided per year
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How solar system size is calculated
Every solar estimate – from a quick online quote to an installer's design software – starts with the same three numbers: how much electricity you use, how much sunshine your roof receives and how much is lost between the panel and your wall socket.
System size (kW) = yearly kWh needed ÷ (peak sun hours × 365 × derate factor)
1. Your electricity use
Find the kWh figure on your utility bill, not the dollar amount. Twelve months is ideal because air conditioning and heating cause big seasonal swings. The average U.S. household uses roughly 850 – 900 kWh per month; homes in Europe often use 250 – 400 kWh. If you plan to buy an electric car (about 250 – 350 kWh per month for 1,000 miles) or a heat pump, raise the offset target accordingly.
2. Peak sun hours
A "peak sun hour" is one hour of sunlight at an intensity of 1,000 W/m², the condition under which panels are rated. Phoenix receives about 6 – 6.5 peak sun hours on an average day, Chicago about 4.5, London about 2.7. Our regional presets are yearly averages for a well-tilted roof; for a precise figure use a solar resource map or the free PVWatts tool from NREL.
3. Losses and orientation
Real systems never deliver their nameplate rating. Inverter conversion (3 – 4 %), wiring, heat (panels lose ~0.3 – 0.4 % output per °C above 25 °C), dust, snow and mismatch typically add up to 12 – 16 %. A south-facing roof at a tilt close to your latitude produces the most energy; east- or west-facing roofs produce around 15 % less but spread production across the day, which can be valuable with time-of-use tariffs.
Choosing solar panels: what the specs mean
| Specification | What to look for | Why it matters |
|---|
| Power (W) | 400 – 450 W residential | Fewer, higher-wattage panels reduce racking and labour cost |
| Efficiency | 20 – 23 % (monocrystalline) | Determines how much roof area you need |
| Temperature coefficient | −0.26 to −0.35 %/°C | Lower is better in hot climates |
| Degradation | ≤ 0.4 – 0.5 % per year | Affects 25-year production |
| Product warranty | 15 – 25 years | Covers manufacturing defects |
| Performance warranty | ≥ 85 – 90 % at 25 – 30 years | Guarantees output over time |
| Cell technology | TOPCon, HJT, back-contact | Newer cells degrade less and handle heat better |
Inverters: string, micro or hybrid?
A string inverter is the cheapest option for simple, unshaded roofs. Microinverters or power optimizers manage each panel individually, so one shaded panel does not drag down the rest – ideal for complex roofs. A hybrid inverter can charge a home battery for backup. Systems are commonly designed with a DC-to-AC ratio of about 1.1 – 1.3, which is why our suggested inverter size is smaller than the panel array.
Understanding payback
Payback is the number of years until bill savings equal what you paid. Our model increases electricity prices by 3 % a year, reduces panel output by 0.5 % a year and assumes you are credited for solar energy at your full retail rate (net metering). If your utility pays less for exported energy, payback will be longer unless you use most of the power at home or add a battery. Most residential systems in sunny regions pay back in 6 – 10 years and keep producing for 25 – 30.
Before you sign a contract
- Get at least three quotes and compare them in price per watt, not total price.
- Ask for the projected yearly kWh and check it against the estimate above.
- Check whether your roof needs replacing within 10 years – removing and reinstalling panels is expensive.
- Confirm the installer handles permits, interconnection and inspection.
- Read the financing terms carefully; loans and leases change the real payback.
Frequently asked questions
How many solar panels does it take to power a house?
A typical U.S. home using 900 kWh per month needs about 6 – 8 kW of panels, or roughly 15 – 20 panels of 400 W, depending on sunshine. Homes in sunny states need fewer; homes in cloudy northern climates need more.
How much roof space do I need?
A 400 W panel measures about 1.7 × 1.1 m (around 21 sq ft). Including spacing and fire-code setbacks, plan for about 2.2 m² (24 sq ft) per panel.
How much does one solar panel produce per day?
A 400 W panel produces roughly 400 W × peak sun hours × 0.85, i.e. about 1.5 kWh per day with 4.5 sun hours and 2.1 kWh with 6.2 sun hours.
Do I need a battery?
Not necessarily. With net metering the grid acts as your "battery". A battery makes sense for backup during outages, for time-of-use tariffs or where exported energy earns very little.
Do solar panels work on cloudy days and in winter?
Yes, but at reduced output – roughly 10 – 25 % of normal on heavily overcast days. Cold temperatures actually improve panel efficiency; the main winter losses are shorter days and snow cover.
About SunTally
SunTally is an independent, free calculator that sizes residential solar panel systems from your electricity usage, local sun hours and panel specifications, and estimates roof area, yearly production and payback. We are not affiliated with any solar manufacturer, installer or financing company.
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The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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