Sizing your solar: why the kW number matters less than you think
A field guide from 700+ NorCal installs on what is actually true about system sizing under NEM 3.0.
The kW number on your proposal is the headline figure, but it tells you almost nothing about whether your system will actually pay for itself. After seven hundred installs across Northern California, here is the field guide.
The old rule of thumb is dead
For a decade, "size for 100% of your annual kWh" was gospel. Under NEM 3.0 (since April 2023), exports pay 5–8¢/kWh versus the 30–50¢/kWh you pay to import. Oversizing for export is no longer a strategy — it is a write-down.
It depends on three things
Annual kWh usage, roof area and orientation, and whether you add battery. These three numbers determine the right kW and the right battery size. Anything else is hand-waving.
The math, simplified
Target 90–100% annual offset, not peak-monthly offset. A south-facing, unshaded 9 kW system in NorCal produces roughly 12,000 kWh. That covers a $300/mo bill comfortably.
Why bigger is not always better
Diminishing returns past 110–120% of annual offset. The extra kW costs more than the extra production is worth. We size to your load, not to your roof.
Battery changes the math
With Powerwall or Enphase IQ Battery, oversize the array 10–20% to charge the battery at off-peak. That arbitrage is the new Project finance lever.
Site by site, not rule of thumb
A 9 kW array in Concord at 180° south, 25° pitch, no shading is not the same as 9 kW in Walnut Creek at 90° east, 40° pitch, partial shade. Production can differ by 30%. We model your actual site.
The checklist that matters
Twelve monthly of usage history. Roof orientation and pitch. Shading study. Panel layout. Inverter with 10–20% headroom. Battery integration. Year-1 production estimate before you sign. That is the order.