Solar System Size Calculator
Estimate the solar PV system size in kW needed to offset your annual electricity usage from kWh and sun hours.
Common conversions
| Input | Result |
|---|---|
| 4 kW DC array, 400 W panels | 10 panels |
| 7.2 kW DC array, 400 W panels | 18 panels |
| 10 kW DC array, 400 W panels | 25 panels |
| 12 kW DC array, 400 W panels | 30 panels |
| 7.2 kW array, Phoenix (5.8 sun h/day) | ≈ 11,500 kWh/yr |
| 7.2 kW array, Seattle (3.9 sun h/day) | ≈ 7,800 kWh/yr |
| Typical residential array size | 4-12 kW DC |
| Typical string inverter sizing (vs array kW) | 0.8-1.0× DC size |
| Average US residential annual usage | ≈ 10,800 kWh |
The math behind it
- Panel count = 7.2 × 1000 / 400 = 18 panels
- Phoenix production = 7.2 × 5.8 × 365 × 0.78 ≈ 11,890 kWh/yr
- Seattle production = 7.2 × 3.9 × 365 × 0.78 ≈ 7,995 kWh/yr
Everything you need to know
A solar quote full of numbers is easier to evaluate once you separate two things that get blended together constantly: the system's nameplate kW (how much it can produce under lab test conditions) and its actual kWh production (how much energy it delivers over a year in your climate).
kW nameplate vs. kWh production
A panel's wattage rating is measured under Standard Test Conditions: 1000 W/m² of sunlight, 25°C cell temperature, and no shading. Real rooftops rarely match that. A 7.2 kW DC array in sunny Phoenix, with roughly 5.8 peak sun hours a day, produces around 11,500-11,900 kWh per year. The identical 7.2 kW array in cloudier Seattle, with about 3.9 peak sun hours a day, produces closer to 7,800-8,000 kWh per year, a difference of nearly 35% from location alone. This is why two homeowners can have the same size system and completely different electric bills, and why sizing should always start from your annual kWh usage and local sun hours, not just a target kW number a salesperson suggests.
Panel wattage × count, and inverter sizing
Once you know the target system size in kW, dividing by an individual panel's wattage tells you how many panels you need. A 7.2 kW array built from 400 W panels needs 18 panels; the same 7.2 kW built from 320 W panels needs 23. The inverter, which converts the panels' DC output to usable AC, is typically sized at 80-100% of the array's DC rating rather than a 1:1 match, because the array rarely hits its full rated output at once (this is called the DC-to-AC ratio, and mild oversizing of the panels relative to the inverter is normal and improves production during low-light hours).
Net metering basics
Net metering lets excess daytime production run your utility meter backward, crediting you for power you send to the grid and letting you draw it back at night. Policies vary widely: some utilities credit exported power at the full retail rate, others at a lower wholesale rate, and a growing number use time-of-use rates that make evening consumption more expensive than the credit you earned at noon. Before sizing an array to 100% of annual usage, check your utility's specific net metering or net billing rules, since a program with poor export credits can make a smaller array paired with a battery more cost-effective than an oversized array alone.
Common applications
Most homeowners target 100% offset of annual kWh. Use 12 months of utility bills, not a single month, since usage and sun hours both swing seasonally.
Multiply daily kWh need by 2× to cover winter shortfalls and battery charge/discharge losses, then size the PV array against that derated production figure.
An EV driven 12,000 mi/yr at 3.5 mi/kWh needs about 3,430 kWh/yr, roughly a 1-1.5 kW DC addition to an existing array in a moderately sunny climate.
Switching from gas to an electric heat pump can add 3,000-6,000 kWh of annual usage in cold climates, often justifying a 2-4 kW increase in array size.
Common mistakes
The same kW array can produce 35% more or less energy per year depending on local sun hours, roof tilt, and shading. Always check projected annual kWh, not just the kW label.
PVWatts and similar tools give very different output for a south-facing 25° roof than for a flat commercial roof or a west-facing installation. Match the inputs to your actual roof.
Adding a heat pump or EV after your solar install can push usage up 30-50%. Plan for at least 20-25% headroom if roof space allows it.
Matching inverter capacity 1:1 to array DC rating wastes money and slightly reduces early-morning and late-afternoon production compared to a properly undersized inverter.
Frequently asked questions
What are peak sun hours?+
Peak sun hours are the equivalent number of hours per day at 1000 W/m² of solar irradiance. Phoenix averages around 5.8 peak sun hours a day, while Seattle averages closer to 3.9, which is why identical systems produce very different annual kWh in each city.
How many panels do I need for a 7.2 kW system?+
With 400 W panels, a 7.2 kW DC array needs 18 panels (7.2 × 1000 / 400 = 18). With smaller 320 W panels the same system needs 23 panels instead.
Is a bigger kW rating always better?+
No. A 10 kW array in a low-irradiance climate can produce less annual energy than a well-placed 7 kW array in a sunnier location with better roof orientation. Annual kWh production, not nameplate kW, is what should be compared.
What performance ratio should I use for sizing?+
Use 0.75-0.82 for most residential estimates. This factor accounts for inverter losses, wiring losses, temperature derating (panels lose efficiency as they heat up), soiling, and partial shading.
Should my inverter match my array's kW rating exactly?+
No. Inverters are typically sized at 80-100% of the array's DC rating, since the panels rarely produce their full rated output simultaneously. A 7.2 kW DC array commonly pairs with a 6-7.2 kW inverter.
Does net metering change how big my system should be?+
Yes. If your utility credits exported power at a reduced rate, oversizing your array to fully offset usage becomes less financially attractive than sizing closer to your daytime consumption and adding a battery for the rest.
How much does an EV or heat pump change my solar sizing?+
Significantly. An EV driven 12,000 miles a year at 3.5 miles per kWh adds about 3,430 kWh of annual usage, which can add roughly 1-1.5 kW to your recommended array size depending on your local sun hours.
What's a typical residential solar system size?+
Most US homes install between 4 kW and 12 kW DC, with 6-8 kW being the most common range for a household using around 10,000-11,000 kWh per year.
Why did my installer size my system smaller than my full usage?+
Roof space, shading, panel orientation, and local permitting or net-metering caps can all limit array size below 100% offset. Some installers also intentionally undersize slightly since usage often grows year to year.