SPD: surge protective device for your solar install
A lightning strike 600 ft away dumps tens of kV into your system and fries the inverter. SPDs cost $60 and divert it to ground. Types, placement and replacement schedule.
The SPD (Surge Protective Device) is your solar install's 'electronic lightning rod'. Costs $60 per unit and saves the inverter ($1700-3500) and controller ($350-600) every time a strike lands nearby.
How it works
Component with infinite resistance under normal conditions (no current passes). On sudden overvoltage (>1 kV), drops to nearly 0 Ω in microseconds, diverting all energy to ground. Returns to normal — or sacrifices itself if the surge was huge.
Types by protection level
Type I: protects against direct strike. Highly exposed areas, mountain, tall buildings. $230-470. Type II: protects against induction (nearby strike). Most common and enough for 95% of residential. $35-95. Type III: fine protection in panel of sensitive equipment.
Where to install
Minimum 2 Type II SPDs: 1) between PV array and controller (DC line). 2) between inverter and AC panel. With grid: add Type II at main panel. Distance <20 in from protected equipment.
How to know it's active
Visual front indicator: green window = OK, red window = sacrificed, must be replaced. Some models have aux contact to send alarm to inverter. Annual visual inspection.
When to replace
When red indicator appears (sacrificed absorbing a strike) or preventively every 3-5 years (natural degradation). Replacement cost: $35-95. Negligible vs a new inverter.
Where mandatory
USA: Florida (lightning capital of the world), Tornado Alley, Rockies, Gulf Coast. EU: Atlantic coast, mountain ranges. Tropical and coastal zones generally. Dry Mediterranean: optional but recommended.
Combine with good grounding.
Want to know how much energy your appliances use? Calculate it here.
Open calculator