For that essential-circuit set, a mid-size portable generator is often enough — if the furnace is gas or oil with only a blower to power, and if the sump pump matches the Honda horsepower you actually have. 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 a 1/3 hp sump pump (800 W running, 1,300 W starting) total 1,692 running watts. The largest starting increment is the refrigerator (1,008 W), so the starting need is about 2,700 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 |
| Sump pump, 1/3 hp | 800 | 1,300 |
| Simultaneous running total | 1,692 | — |
| Plus largest starting increment (fridge, 1,008 W) | — | 2,700 |
| Same set with Honda’s 1/2 hp sump (1,050 W run, 2,150 W start) | 1,942 | 3,042 |
Look for a generator whose running watts are at least the running total and whose starting watts are at least the starting total, then leave practical margin because published wattages are approximate. Honda also lists a 1/2 hp furnace fan (875 W running, 2,350 W starting), which would change the surge. The sump may start while the furnace is already running — that is why the method adds only the single largest starting increment to the combined running load. Build the list in the calculator. A well pump is not in this table; see the well-pump pages and use the nameplate.
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.