Solar PV System Sizing: A Contractor's Bid-Day Guide
A solar bid that wins isn't the one with the largest array. It's the one with the right array — sized to the kWh profile, the roof plane, the local utility's net-metering rules, and the homeowner's actual electrification plan. Oversize and the system loses money on every kWh exported. Undersize and you lost the bid to a contractor who did the math.
What drives system size
Four inputs, in order:
- Trailing 12-month kWh consumption from the utility bill. Not the recent month. The full year, because August and February are not the same house.
- Roof orientation + tilt — south-facing at the latitude tilt is 100%. East/west is 82-86%. North is don't bother. Tilt matters less than people think; orientation matters more.
- Shading — a single chimney or a neighbor's tree can knock 15-25% off the production estimate. Walk the roof at noon and at 3 PM.
- Net metering rules — NEM 3.0 in California credits exports at 5-12¢/kWh instead of 25-30¢. That changes the optimal array size from "as big as the roof" to "as close to consumption as legal."
A homeowner using 12,000 kWh/year in a 1.45-sun-hour-equivalent climate (most of New England) needs roughly an 8.3 kW DC array. In Phoenix at 5.7 sun-hours, the same load needs 5.8 kW. Same house, different bid.
Inverter sizing — the trap
The inverter DC/AC ratio drives a quiet trade-off. A 1.20-1.30 DC/AC ratio is the residential sweet spot — enough clipping in peak summer to pay for itself with cheaper inverter $/W. Push to 1.40 and you'll hand back 6-8% of annual production as clipped energy. Stay under 1.10 and you bought too much inverter.
String inverters cost less per watt. Microinverters cost more per watt and unlock shading tolerance + per-module monitoring. Bid both. Let the homeowner choose with their eyes open.
NEC 690 — the line items that wreck margin
NEC 690.12 (Rapid Shutdown) requires module-level shutdown for any rooftop array. That's microinverters, MLPEs, or string with module-level shutdown devices. Bid it.
NEC 690.31 conductor sizing isn't a guess — it's a 1.25 × 1.25 × ambient correction. Undersize the home run and you'll fail inspection and re-pull wire. NEC 690.7 maximum voltage calculations limit your string length more than the inverter datasheet does at cold temperatures.
The contractors winning solar bids carry the NEC math inside the Estimate.Pro solar template — the bid prices the conductor at code-compliant size on the first pass, not after the EI markup.
The honest production estimate
PVWatts is the open-source NREL tool that gives a defensible kWh/year. Anything tighter than ±5% from PVWatts is overselling. Bid the PVWatts number, not the manufacturer's brochure number. The homeowner will check it. The reviewer at the financing company will check it. Bid honest production.
How Estimate.Pro handles it
The solar trade template in Estimate.Pro carries the orientation, tilt, sun-hour, and NEM regime by ZIP code. You enter the kWh consumption and the available roof planes. The bid engine returns an array size, an inverter spec, a string layout, a conductor schedule, and a PVWatts-anchored production estimate — eight minutes, not eight hours.