A homeowner in Seattle gets a proposal for an 8 kW system promising 14,000 kWh per year. It looks clean, professional, confident. There is just one problem: the sun does not cooperate. Seattle tops out at 3.6 peak sun hours per day. That system will produce roughly 8,400 kWh — 40% less than the proposal claims. The savings projections, the payback period, the 25-year ROI? All built on a number that was wrong before the first panel left the warehouse.

Sizing errors — whether the system is too large or too small — cost you money every year for the entire 25+ year life of the equipment. Verifying that a proposed system matches your actual usage takes about 15 minutes, a calculator, and your electric bills.

Step 1: Pull your actual electricity usage

Grab 12 months of electricity bills, either paper statements or from your utility's online portal. Add up total kWh consumed across all 12 months. That annual consumption baseline is the single most important number in solar system sizing — and the one most often fudged.

Usage benchmarks: The national average for a US household is approximately 10,500 kWh/yr. A 2,500 sq ft home in the Sun Belt (Texas, Arizona, Florida) with central air conditioning typically consumes 14,000-18,000 kWh/yr. A 1,200 sq ft home in the Pacific Northwest (Oregon, Washington) without electric heating might use 8,000-10,000 kWh/yr. A home with an electric vehicle adds 3,000-4,500 kWh/yr depending on driving habits. If the installer's proposal uses a number significantly different from your actual bills, ask why.

Seasonal variation matters more than people expect. In hot climates, summer bills can run 2-3x winter bills from air conditioning alone. In cold climates with electric heat, winter dominates. The full 12-month total captures these swings. Do not extrapolate from a single month — a July bill in Phoenix and a July bill in Portland tell completely different stories about annual usage.

Step 2: Check what the installer used for consumption data

Open the proposal and find the annual kWh consumption figure used for sizing. You want to see language like "based on 12 months of utility data" or "annual consumption: 13,200 kWh" — a specific number tied to your actual home.

Red flag: If the proposal says "based on average home in your area" or "estimated consumption for your home size" instead of citing your actual utility data, the system size is a guess. Average consumption figures can be off by 30-50% for individual homes. A 2,500 sq ft house with a pool pump and older HVAC might use 20,000 kWh/yr while a neighbor's identical house with efficient appliances uses 12,000 kWh. "Average" helps neither homeowner.

Here is the tell: if the installer never asked for your utility bills or login credentials to pull your Green Button data, they sized the system on assumptions. Full stop. Ask them to re-run the design with your actual 12-month consumption before you evaluate anything else on the proposal.

Step 3: Verify the production estimate

Every proposal includes an estimated annual kWh production. You can check it with one formula: system size (kW) × peak sun hours per day × 365 days × system efficiency factor. That is it.

The efficiency factor accounts for real-world losses — inverter conversion, wiring, temperature derating, soiling, shading. A typical value is 0.75-0.82 (75-82% of theoretical maximum). Use 0.80 as a reasonable default unless the proposal specifies a different derate factor.

Production verification — Phoenix, AZ: System size: 8 kW. Peak sun hours: 5.7 hours/day (NREL data for Phoenix). Calculation: 8 kW × 5.7 × 365 × 0.80 = 13,315 kWh/yr. If the proposal claims 13,000-13,500 kWh/yr from an 8 kW system in Phoenix, the estimate is reasonable.
Production verification — Seattle, WA: System size: 8 kW. Peak sun hours: 3.6 hours/day (NREL data for Seattle). Calculation: 8 kW × 3.6 × 365 × 0.80 = 8,424 kWh/yr. If the proposal claims 14,000 kWh/yr from an 8 kW system in Seattle, the math does not work. That production figure requires 6.0 peak sun hours — a number Seattle never reaches. The estimate is inflated by at least 66%.
Peak sun hours by region (annual average): Southwest (Phoenix, Las Vegas): 5.5-6.2 hrs/day. Southeast (Atlanta, Miami): 4.5-5.2 hrs/day. Midwest (Chicago, St. Louis): 3.8-4.5 hrs/day. Northeast (Boston, New York): 3.8-4.3 hrs/day. Pacific Northwest (Seattle, Portland): 3.4-3.8 hrs/day. California (LA, Sacramento): 5.0-5.8 hrs/day. You can look up your specific location on the NREL PVWatts Calculator for precise data.

Roof orientation and tilt add another layer. South-facing roofs (in the Northern Hemisphere) at a tilt angle equal to latitude produce the most energy. West-facing and east-facing roofs each lose 10-15%. North-facing panels can drop 25-40% and are rarely worth installing. If your proposal splits panels across multiple roof faces, the production estimate should account for each orientation separately — not assume a single average.

Step 4: Size to offset, not to over-produce

How much system you need depends not just on how much electricity you use, but on what your utility pays for the excess. Net metering policy changes the math dramatically.

  • 1:1 net metering (full retail credit for exports): Slight oversizing to 105-110% of annual consumption is reasonable. The extra 5-10% offsets panel degradation over the first few years, and any excess kWh earn full retail credit. On a 13,000 kWh/yr home, this means sizing to produce 13,650-14,300 kWh/yr.
  • Avoided-cost / wholesale net metering: Size to 95-100% of consumption. Every kWh you export beyond your consumption earns $0.03-$0.05/kWh instead of the $0.15-$0.30/kWh you would save by offsetting your own usage. Oversizing is money lost. On a 13,000 kWh/yr home, size for 12,350-13,000 kWh/yr production.
  • No net metering / net billing at wholesale: Size for maximum self-consumption only. This typically means sizing smaller (70-85% of total consumption) and pairing with a battery for time-shifting, or sizing to match daytime-only consumption if no battery is included.
Red flag: An installer proposing a 12 kW system for a home using 10,500 kWh/yr in a market with avoided-cost net metering is oversizing by roughly 30-40%. That extra capacity costs $6,000-$9,000 upfront and produces kWh that earn $0.04 each. The payback on those extra panels alone could exceed 30 years. Ask the installer to justify the system size relative to your net metering policy.

Step 5: Account for planned future changes

Sometimes the right system size is bigger than today's usage warrants. The key word is "planned" — not "imagined."

Common additions and their kWh impact: Electric vehicle (EV): 3,000-4,500 kWh/yr for an average US driver (12,000-15,000 miles/yr at 3-4 miles per kWh). Heat pump replacing gas furnace: varies widely by climate, but typically adds 3,000-8,000 kWh/yr while eliminating gas usage. Pool pump: 2,500-4,000 kWh/yr depending on pump size and run time. Hot tub: 2,000-3,500 kWh/yr.

Buying an EV in the next 1-2 years? Adding 3,500 kWh to your sizing baseline is smart. Harboring a vague notion of "maybe someday" getting one? That is not a reason to let the installer pad the system by 30% for a hypothetical that may never arrive.

The right conversation with the installer: "Here are my last 12 months of electric bills totaling X kWh. I plan to add an EV next year, which will add approximately 3,500 kWh. My target system production is X + 3,500 kWh, with a 5% buffer for degradation. Can you design to that number?" This gives the installer a specific, justified target instead of a blank check to upsell.

Sizing verification checklist

  • Your 12-month actual consumption (kWh): pulled from utility bills or portal, not estimated by the installer.
  • The proposal's consumption assumption (kWh): should match your actual data within 5%.
  • The proposal's annual production estimate (kWh): verify using system kW × peak sun hours × 365 × 0.80.
  • Target offset percentage: 105-110% for 1:1 net metering, 95-100% for avoided-cost, 70-85% for no net metering.
  • Future load additions: only include specific, planned changes (EV purchase date confirmed, heat pump installation scheduled) — not hypotheticals.
  • Production-to-consumption ratio: if the system produces more than 115% of your current + planned usage, it is likely oversized and the excess capacity has a poor financial return.

Every number on a solar proposal — cost per watt, monthly savings, 25-year ROI — grows from a single root: whether the system produces the right amount of electricity for your specific home. If the sizing is off, every projection built on top of it is fiction. Check the foundation first.

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