Solar System Size Calculator

The Planning Formula

A grid-tied array is sized from energy, not from a guess at “typical Ohio homes.”

System size (kW) ≈ (annual kWh you want to offset) ÷ (production factor in kWh per kW per year)

Ohio State University Extension’s Ohioline fact sheet CDFS-4102 (adapted from a U.S. Department of Energy sizing fact sheet) uses about 1,100 kWh per installed kW per year as an Ohio planning estimate. That is a statewide starting point. Your roof will not match it exactly.

This is a planning estimate, not an engineered solar design. Shade, tilt, azimuth, snow, soiling, inverter clipping, and Ohio insolation all move the production factor. Confirm with NREL PVWatts for your address, then with a site visit. Dollar savings belong on a bill analysis, not here — see the Ohio solar savings calculator for a conservative savings estimate.

Estimate Size From Your kWh

This calculator estimates system size from electricity usage and production assumptions. Financial savings are calculated separately because electricity rates and export-credit rules vary by utility. For bill-reduction math, use the Ohio solar savings calculator.

Why the Production Factor Moves in Ohio

• Tilt and azimuth. South-facing at a reasonable roof pitch usually outperforms a shallow east-west plane. PVWatts lets you test both.
• Shade. Trees, chimneys, and neighboring roofs can drop annual kWh more than a larger nameplate can recover. Count usable sun hours, not just panel count.
• Ohio insolation. Summer production is typically stronger than winter. A year of kWh still includes those winter months; do not size from July alone.
• Losses. PVWatts includes a default system-losses input (soiling, wiring, mismatch, and similar). Change it only with a reason.
• Utility caps. Even if the math suggests a large array, interconnection and net metering rules (including the 120% screen) and available roof can cap size. Statewide context: Ohio net metering.

Hardware cost is a separate question: solar panel cost in Ohio. Payment path (cash, loan, PPA, or Propel only) is on solar financing in Ohio and loan vs PPA. Utility map: Ohio solar utilities. Programs: Ohio solar incentives. Residential overview: Ohio solar panels.

How to Use NREL PVWatts With This Formula

1. Open pvwatts.nrel.gov and enter your address or ZIP.
2. Set DC system size to 1 kW (or any trial size), plus your best-guess tilt, azimuth, array type, and losses.
3. Read estimated annual AC kWh. Divide that by the DC kW you entered. That quotient is your production factor for those assumptions.
4. Plug that factor into the worksheet above with your real annual kWh.

PVWatts itself warns that predictions include assumptions and uncertainties and are not a substitute for a site-specific design. We agree. Send a bill if you want us to size against usage and your utility: electric bill solar analysis.

Solar System Size — FAQs

Is 1,100 kWh per kW guaranteed in Ohio?

No. It is an OSU Extension planning estimate. Use PVWatts for the roof you have.

Should I oversize to maximize credits?

Net metering is meant to offset your own use. Ohio’s rule sizes facilities at interconnection to no more than 120% of requirements. Oversizing to “farm” export credits is usually the wrong goal — export treatment is not full retail statewide.

Does this include a battery?

No. Storage is a separate kWh/kW design. See solar batteries in Ohio.

How do I turn kW into a panel count?

How many solar panels do I need in Ohio — panels ≈ system watts ÷ panel wattage.

Sources

• OSU Extension Ohioline CDFS-4102 — Estimating the Size of Your Solar Electric System (Ohio ~1,100 kWh/kW/yr planning figure; DOE sizing steps)
• NREL PVWatts Calculator
• Ohio Administrative Code 4901:1-10-28 (net metering sizing)
• IRS — Residential Clean Energy Credit (credit not for §25D expenditures after Dec 31, 2025)

Last reviewed: September 2026.