What size generator runs a well pump?
The pump itself is not the problem, it's the restart. In our appliance dataset, a 1 HP well pump runs at 2,000 W but needs 4,000 W for the seconds it takes to start, and it restarts every time the pressure tank calls for water. That surge, landing on top of whatever the house is already doing, is why a well pump quietly sets the floor under so many generator purchases.
The pump numbers
| Pump | Running (W) | Starting (W) | Surge increment (W) | Load alone, with 20% headroom |
|---|---|---|---|---|
| Well pump (1/2 HP) | 1,050 | 2,100 | 1,050 | ≈ 2.5 kW |
| Well pump (1 HP) | 2,000 | 4,000 | 2,000 | ≈ 4.8 kW |
"Load alone" follows the sizing calculator's method, running watts plus the surge increment, with 20% headroom. Read it as the capacity the pump claims before a single light bulb: on its own, a 1 HP pump already wants a generator with roughly 4.8 kW of honest capacity, and the relevant spec on a candidate unit is its available starting (surge) capacity, not just its running rating. Horsepower alone doesn't determine this: well depth, flow, and the specific motor all move the real figure, which is why the dataset flags deeper and higher-flow pumps as drawing more. Our dataset doesn't publish a 3/4 HP entry, so don't interpolate one: work from your pump's nameplate or have it measured.
Why a 2,000 W pump dominates a small generator
Three things compound. First, the starting draw is double the running draw in our data, and a motor that can't get close to full voltage may not start at all, it sits there pulling surge current until something gives. Second, the pump cycles: it's not one start per outage but one every time the pressure tank empties, indefinitely, often while the fridge and furnace are mid-cycle. Third, the restart lands on top of the running house. Picture the calculator's essentials list, fridge, freezer, furnace blower, sump pump, lights, and the 1 HP well pump, running at 6,200 W total. When the pump restarts, the momentary load is 6,200 − 2,000 (pump already counted running) + 4,000 (pump starting) = 8,200 W. A generator sized to the 6,200 W running figure with no surge room fails that moment, every time the tank calls.
The worked examples
| Load list | Running (W) | Starting load (W) | With headroom | Size class |
|---|---|---|---|---|
| Essentials with 1/2 HP pump | 5,250 | 6,750 | ≈ 8.1 kW | 10 kW |
| Essentials with 1 HP pump | 6,200 | 8,200 | ≈ 9.8 kW | 10 kW |
| Essentials + 1 HP pump + 3-ton AC | 9,700 | 16,000 | ≈ 19.2 kW | 22 kW |
Two lessons hide in that table. The 1/2 HP and 1 HP houses land in the same class, because once headroom is applied both clear the 7.5 kW step, the pump's running load, not just its surge, keeps the smaller house from dropping a class. And the moment a 3-ton AC joins the list, its 6,300 W surge increment takes over as the largest and the pump stops being the sizing problem at all; the full AC math is on our 3-ton AC page. Reproduce all of this with your own list in the sizing calculator, use the "Outage essentials" preset, which already includes the 1 HP pump, then swap or add appliances, and match real models, rated on your fuel, in the model database.
Ways to keep the pump from upsizing you
- Sequence the big motors. Load management that delays the well pump while the AC starts (or vice versa) attacks the "largest surge" term directly. It's an installer conversation, and it's cheaper than a size class.
- Ask about your actual pump. A pump contractor or licensed electrician can read the nameplate or clamp the starting current. If your "1 HP" pump starts materially softer than 4,000 W, the calculation improves with real numbers. Don't buy capacity against a guess in either direction.
- Don't back the pump up halfway. A generator that runs the pump only when nothing else is on is a outage-day juggling act with your water supply. If the pump matters enough to include, size for it restarting under load.
This page is sizing guidance from typical figures, not electrical advice. Well pumps involve the pressure switch, control box, and motor circuit as a system; confirm the final generator size and any load-management controls with a licensed electrician or pump contractor working from your pump's nameplate.
Sources & notes
- Pump and essentials figures: TallyGauge appliance wattage dataset (Well pump (1/2 HP): 1,050 W running / 2,100 W starting; Well pump (1 HP): 2,000 / 4,000; dataset notes flag depth and flow as variables and the 1 HP startup surge as the sizing driver).
- Method and standard size classes: the sizing calculator, running watts plus the largest single surge increment, ~20% headroom, rounded up to the next class (7.5, 10, 14, 18, 22, 24, 26 kW). Worked-example totals were recomputed from the dataset for this page.
- Per-fuel model ratings for matching a size class to real machines: TallyGauge model database.