Whole-Home Surge Protection Sizing Calculator
Size a whole-home surge protective device (SPD): the surge-current rating and joule headroom to aim for based on your service and exposure.
How it works
kA rating ← exposure (low 40 / med 60 / high 80) + service bump · Joules ← exposure tier · Layers ← panel Type 2 (+ Type 3 point-of-use)
A whole-home SPD clamps voltage spikes at the panel before they reach your wiring. Its two headline numbers are the surge-current rating in kiloamps (how big a hit it can take) and the joule rating (how much energy it can absorb over its life) — both should scale with how exposed your home is to lightning and grid switching surges. The best protection is layered: a Type 2 device at the panel stops the big transients, and Type 3 point-of-use protectors mop up the residual let-through voltage right at sensitive electronics.
Worked example
A 200 A suburban home on overhead lines is medium exposure, so aim for a Type 2 panel SPD rated at least 60 kA per mode and around 40,000 J, with a let-through voltage of about 1 kV or less. Add Type 3 strips at the TV, computer and network gear for a two-stage system.
Frequently asked questions
What do kA and joules actually mean on an SPD?
The kA (surge current) rating is the peak momentary current the device can divert in a single large surge; the joule rating is the cumulative energy it can absorb across many smaller surges before it wears out. Higher numbers mean more headroom and a longer service life, especially in high-lightning areas.
Do I still need power strips if I have a panel SPD?
Yes — a panel SPD stops the largest transients, but a small residual voltage still passes through to your outlets. Type 3 point-of-use protectors at sensitive electronics catch that let-through, which is why a two-layer approach is recommended for computers, TVs and network equipment.
How long does a surge protector last?
SPDs degrade as they absorb surges; the joule rating is effectively a lifetime budget. Many have an indicator light that shows when protection is depleted — in a high-exposure area plan to check it periodically and replace after a major nearby strike.
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This calculator is for educational purposes. Double-check important results before acting on them.