Mounting
- Out of direct sun. Inverters derate when hot. A south-facing stucco wall in July costs you real production. Shade it or move it.
- Clearance. Follow the manual — typically 12" above, 8–12" below, 4–6" sides. It is for convection, not just service access.
- Never in the sealed attic. Ambient temp, no service access, and the fan noise transmits into the bedrooms.
- Wall type. Studs. On masonry, use the right anchor for the load. Hybrid inverters run 60–110 lb.
Interconnection: the 120% rule (705.12)
Load-side backfeed into a panelboard is the common residential method. The sum of the main breaker plus the solar backfeed breaker cannot exceed 120% of the busbar rating, and the backfeed breaker must be at the opposite end of the busbar from the main.
| Busbar | Main | 120% allows | Max solar breaker |
|---|---|---|---|
| 200 A | 200 A | 240 A | 40 A |
| 200 A | 175 A | 240 A | 65 A |
| 225 A | 200 A | 270 A | 70 A |
| 100 A | 100 A | 120 A | 20 A |
Out of room? Options in order of cost: derate the main breaker, use a line-side tap ahead of the main (supply-side, 705.11), or upgrade the service. A supply-side tap avoids the 120% rule entirely because it is not a busbar at all — but it requires a utility disconnect and the utility's blessing.
Do not do thisA backfeed breaker in the middle of the busbar. Current from both ends converges and the bar carries more than its rating at the center. It looks fine, it passes a resistance check, and it is a fire. Opposite end, always, and label it.
Sequence
- Set the box and conduitConduit runs planned before the first hole. Orange or marked conduit for PV DC where required, and keep DC and AC in separate raceways.
- Land the EGC firstEvery time, in every box. It is the last thing to disconnect and the first thing to land.
- AC output before DC inputWire the AC side and the disconnect, then the battery, then PV last. PV is a current source that turns itself on the moment the sun hits it.
- Cover the modules if you must work liveAn opaque tarp. Cardboard blows off.
- Program before you energizeGrid profile / UL 1741-SA or SB setting, export limit, battery model, backup priority. Wrong grid profile means the utility fails your PTO inspection.
- Commission and documentVoc of every string measured and written down. Insulation resistance test. Firmware version. Photos of every label. That folder saves you on the callback.
Commissioning checks
- Measured Voc within 5% of your calculated cold-corrected value at that temperature
- Polarity on every string, verified with a meter, before it lands on the MPPT
- Rapid shutdown initiator actually drops the array — test it, do not assume it
- Backup transfer: kill the grid at the disconnect and watch it island cleanly
- CT direction and export/import sign correct — a backwards CT will charge the battery from the grid all night