What size solar system do I actually need? The 5-minute sizing method.
The installer salesperson will quote you a system based on roof space, not your actual energy needs. Here is the honest sizing math, derived from 12 months of your utility bills and the local PVWatts production figure. It takes five minutes and saves you from the two most expensive mistakes: oversizing for export-poor net metering, and undersizing right before you buy an EV.
The one-line sizing formula
system_kW = (annual_kWh × coverage_target) ÷ state_kWh_per_kW
Three inputs, one number out. Let's break each one down.
Input 1 — annual kWh
Pull 12 months of bills from your utility account. Most US utilities offer Green Button CSV download. Add the kWh column. Don't trust a single month — summer AC or winter heat skews wildly.
Sanity benchmarks (US 2026 averages from EIA):
- Apartment / condo, no AC: 3,500-5,500 kWh/yr
- Townhouse, gas heat, moderate AC: 7,000-9,500 kWh/yr
- Single-family, gas heat, central AC: 10,000-13,000 kWh/yr
- Single-family, all-electric (heat pump): 15,000-22,000 kWh/yr
- Single-family with EV (12,500 mi/yr): add ~3,500 kWh/yr
- Single-family with pool pump: add ~2,000 kWh/yr
Input 2 — coverage target
How much of your usage you want solar to cover annually. The breakdown for 2026:
| Scenario | Recommended target |
|---|---|
| Net-metered state (most US), no EV/heat pump plans | 100% of current usage |
| Net-metered state, planning EV in <5 yr | 115-125% of current usage |
| Net-metered, planning EV + heat pump | 130-150% of current usage |
| California NEM 3.0 (post-April 2023) | 95-105% + battery |
| Hawaii, parts of TX (no net metering) | 100% self-consumption + battery |
Input 3 — kWh per kW per year (state factor)
This is the NREL PVWatts state average. The most-referenced numbers:
- Arizona, New Mexico, Nevada: 1,670-1,680 kWh/kW
- California, Texas, Colorado: 1,490-1,530 kWh/kW
- Florida, North Carolina, Alabama: 1,370-1,430 kWh/kW
- Massachusetts, Illinois, New Jersey: 1,280 kWh/kW
- Maine, Vermont, Michigan: 1,230-1,240 kWh/kW
- Washington, Oregon (cloudy PNW): 1,140-1,180 kWh/kW
For your exact ZIP code, use the NREL PVWatts calculator — it accounts for roof azimuth, tilt and weather. Default tilt = latitude.
Worked examples
Example 1 — typical Sacramento household, no upgrades planned. 10,200 kWh/yr × 100% coverage ÷ 1,400 = 7.3 kW. Round to 7.5 kW. 17 panels at 440 W.
Example 2 — Atlanta household, will buy an EV next year. 9,800 kWh/yr current + 3,500 future EV = 13,300 kWh effective × 115% buffer ÷ 1,380 = 11.1 kW. Round to 11 kW. 25-26 panels at 440 W.
Example 3 — Boston household considering full electrification. 9,500 kWh/yr current + 3,500 EV + 7,000 heat pump (offsetting gas) = 20,000 kWh × 105% ÷ 1,280 = 16.4 kW. That's at the edge of roof space — most likely 13-14 kW + 10 kWh battery, accepting some grid backup.
Example 4 — San Diego SDG&E household, NEM 3.0. 8,800 kWh × 100% ÷ 1,490 = 5.9 kW. With NEM 3.0 the export is worthless, so resist the installer's pitch for 9 kW "future-proofing". Right-size at 6 kW + a 10 kWh Powerwall 3.
From kW to panel count
Residential panels in 2026 are 400 W to 450 W per panel. Tier-1 standard (Q-Cells, REC, Silfab): 415-435 W. Premium (Maxeon, REC Alpha Pure-R): 440-460 W.
panel_count = ceil(system_kW × 1000 ÷ panel_watts)
A 9 kW system at 440 W panels = 9000 ÷ 440 = 20.45, round up to 21 panels. Each panel needs ~18-20 sq ft of unshaded roof at correct azimuth.
Inverter sizing (DC/AC ratio)
Most installers ship a system with a DC/AC ratio of 1.15-1.25, meaning the inverter is undersized vs panel nameplate. This is fine — peak DC output rarely happens (clouds, temperature, angle) so the inverter clips a few hours per year. Don't pay extra for a larger inverter unless you live somewhere clear-sky like Arizona; the math doesn't pencil.
When NOT to oversize
- NEM 3.0 California, no battery. Surplus exported gets ~$0.05/kWh while imported is $0.32. Don't make extra you can't use.
- Texas / Hawaii / state without net metering. Same logic — surplus is worthless without an export credit.
- Restrictive utility size caps. Some utilities cap residential at 105-115% of historical usage. Oversize and they'll deny interconnection.
- Roof can't hold it. Check shading + azimuth — don't add panels on the north-facing slope just to hit a number.
Frequently asked questions
How many kW of solar do I need for my home?
Divide annual kWh usage (12-month bills) by state's kWh/kW. For 10,800 kWh/yr in a 1,400 kWh/kW state: 7.7 kW.
Should I oversize my solar array?
In net-metered states: oversize 15-25% if planning future EV/heat pump load. In NEM 3.0 California or non-net-metered states: right-size + battery — oversizing makes no sense without proper export credit.
How many panels is that?
Modern residential panels: 400-450 W. Divide system kW by panel watts. A 7.7 kW system at 430 W = 18-19 panels. Typical US roof fits 30-40 max.
Sources: NREL PVWatts v8, EIA "Average Monthly Bill" Form EIA-861 (Feb 2026), EnergySage Marketplace 2026 panel pricing, ENERGY STAR residential panel database. Last reviewed May 12, 2026.