System size is your annual usage divided by peak sun hours × 365 × a 0.80 derate factor for real-world losses. A home using 1,000 kWh/month at 4.5 peak sun hours needs a 9.13 kW system — 23 panels at 400 W each, about 414 sq ft of roof, and $22,825–$34,238 installed before any incentive. Fewer peak sun hours or lower-wattage panels both push every downstream number up.
How this calculator sizes a system
The core formula is: system size (kW) = annual usage (kWh) ÷ (peak sun hours × 365 × 0.80). Annual usage comes from your monthly usage × 12. Peak sun hours is a regional average — the number of hours per day sunlight arrives at an intensity equivalent to full solar noon, not the number of daylight hours, which is why a place with long summer days can still have a modest peak-sun-hours figure. The 0.80 derate factor accounts for real-world losses: inverter conversion, wiring resistance, soiling, and temperature effects between the panels' rated output and what the system actually delivers.
Why peak sun hours is the input that moves everything else
Because peak sun hours sits in the denominator, a lower value inflates the recommended system size, and every downstream figure — panel count, roof area, cost — scales with it directly. Holding the same 12,000 kWh annual usage constant: at a lower 3.5 peak-sun-hours region the system grows to 11.74 kW, while at a higher 5.75 peak-sun-hours region it shrinks to 7.15 kW — a 64% swing in recommended size driven by location alone, before a single input about the house itself changes.
This is the calculation's most common failure point when someone reuses a system-size figure from a friend or a different region: the same electricity usage in a lower-sun-hours climate genuinely needs more panels, not the same count installed less efficiently. Getting an accurate peak-sun-hours number for your specific location, rather than a rough regional guess, is worth more to the accuracy of this result than precision on any other input.
Panel wattage: fewer panels, same roof, same output
Panel wattage changes panel count and roof area without changing the system's kW size or its cost — cost here is calculated per watt of system size, not per panel, so swapping panel wattage is purely a packing decision. Using the same 9,130 W system from above: at 400 W panels that's 23 panels needing 414 sq ft; at 450 W panels it drops to 21 panels needing 378 sq ft — two fewer panels and 36 fewer square feet of roof for the identical system size and identical price range.
| Panel wattage | Panel count (⌈watts ÷ wattage⌉) | Roof area (panels × 18 sq ft) |
|---|---|---|
| 350 W | 27 | 486 sq ft |
| 400 W | 23 | 414 sq ft |
| 450 W | 21 | 378 sq ft |
System size and installed cost range ($22,825–$34,238) are identical across all three rows — this calculator prices by system watts, not panel count.
Higher-wattage panels are worth prioritizing specifically when roof space, not budget, is the binding constraint — a smaller usable roof area may only fit a given system size with the higher-wattage option.
What a usage-based size doesn't account for
This calculator sizes a system to match your historical usage — it does not know about shading from trees or neighboring structures, roof orientation and tilt relative to true south, or future usage increases like an EV or a home addition, all of which a local installer's site assessment would catch and this form cannot. It also doesn't include any federal tax credit in its cost range, since Section 25D expired for property placed in service after December 31, 2025 — the $22,825–$34,238 figure above is a pre-incentive number for a 2026 installation.
Sizing to exactly 100% of current usage is also a choice, not a requirement. Many homeowners size somewhat below 100% to reduce upfront cost, or somewhat above it to leave headroom for an EV charger or an addition — your utility's net metering policy determines how much value, if any, production beyond your own usage actually returns.
Checking an installer's proposed size against this one
If an installer's proposal lands meaningfully above or below the size this calculator recommends, the gap usually traces back to one of the same three inputs. A larger proposed system often means the installer used your actual annual usage from utility bills rather than an estimated monthly figure, or priced in future usage you mentioned like an EV. A smaller one often means a site visit found better sun exposure — a higher effective peak-sun-hours figure — than the regional average this calculator defaults to, or a different derate assumption for a newer, more efficient inverter.
It is worth asking directly which of the three inputs — usage, sun hours, or derate factor — an installer's proposal is built on, since a size disagreement that traces to real site data (an actual bill, a shading study) is more trustworthy than one that traces to a different default assumption.
Methodology
System size, panel count, roof area, and the per-watt cost range are the literal formula and constants in this calculator's own module: system kW = annual kWh ÷ (sun hours × 365 × 0.80 derate); panel count = ⌈watts ÷ panel wattage⌉; roof area = panel count × 18 sq ft; cost = watts × $2.50 to watts × $3.75. The federal-credit exclusion follows the IRS residential credit expiry below. No number here is estimated beyond running the module's own arithmetic.
Sources
- IRS — Residential Clean Energy Credit (Section 25D) — accessed 2026-09-05