For that essential-circuit set, a small portable generator is often enough — if the furnace is gas or oil with only a blower to power, not an electric furnace or a heat pump. Using Honda’s Home Applications table (retrieved 2026-11-11), a conservative Energy Star refrigerator (192 W running, 1,200 W starting), a 1/3 hp furnace fan (700 W running, 1,400 W starting), and 400 W of lights (use the bulb ratings, as Honda says) total 1,292 running watts. The largest starting increment is the refrigerator (1,008 W), so the starting need is about 2,300 W.
| Load | Running W | Starting W |
|---|---|---|
| Refrigerator or freezer (Energy Star) | 132–192 (192 used) | 1,200 |
| Furnace fan, 1/3 hp | 700 | 1,400 |
| Lights (example; use bulb watts) | 400 | 400 |
| Simultaneous running total | 1,292 | — |
| Plus largest starting increment (fridge, 1,008 W) | — | 2,300 |
Look for a generator whose running watts are at least 1,292 W and whose starting watts are at least 2,300 W, then leave some practical margin because published wattages are approximate. Honda notes you can also manage loads to use a smaller unit. A 1/2 hp furnace fan (875 W running, 2,350 W starting in the same table) would change the starting total. Build the list in the calculator. This does not cover well pumps, air conditioning, or electric heat.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.