What Size Generator Do I Need?

The short answer: add up the running watts of everything you want on at the same time, then add the starting watts of your single biggest motor — on a rural property, that's almost always the well pump. Round up to the next standard generator size. That's the whole method. Everything below is about getting the two inputs right, because that's where people go wrong.

The most common sizing mistake is shopping by the big number on the box. That number is usually the generator's surge rating — a few seconds of burst power. What you can run continuously is the smaller running watts number, buried in the spec sheet. The number that actually determines your size is a third figure most buyers never look up: your well pump's starting surge, which dwarfs the generic estimates. Get those three numbers straight and the decision gets simple.

Our generator size calculator runs this exact arithmetic for you, with well-pump starting watts built in. This article is the decision guide around it: which loads to count, how to find your pump's real numbers, when bigger is smart, and when it's just burning money.

Two strategies: essentials-only vs. whole-house

Before any math, decide which game you're playing — it changes the answer by a factor of two or three.

	Essentials-only	Whole-house
Goal	Keep the critical stuff alive through an outage	Live normally, outage or not
Typical loads	Well pump, fridge/freezer, sump pump, furnace blower, a few lights, internet	Everything above plus central AC, electric water heater, full lighting, microwave, more
Generator class	Large portable or small standby	Standby (automatic)
Typical size	~12–15 kW	~20–30+ kW
Trade-off	You manage the load — no dryer, no oven during an outage	Costs much more, drinks much more fuel

There's no wrong answer here, only a mismatch. The regret we hear about most is buying a mid-size portable, loading it like it's whole-house, and watching it stall every time the well pump cycles. Pick the strategy first; the math just prices it.

The "well pump first" method

On a city lot, sizing starts with the fridge and the furnace. On a well, it starts with the pump — and the pump is a worse actor than most worksheets admit.

Here's why. Generic charts estimate a 1/2 HP well pump at roughly 1,000 running watts and ~3,000 starting watts. But the motor manufacturer's own spec sheet tells a different story. Franklin Electric's AIM manual — the book well professionals actually use — lists the locked-rotor amps for a 230 V single-phase submersible: 32.2 amps for a 1/2 HP motor. Locked-rotor is the current the motor tries to pull in the instant it starts, before it gets spinning. Multiply by your voltage:

32.2 A × 240 V = 7,728 volt-amps — call it 7,700 W of starting surge.

That's not a typo. A pump that sips 1,000 watts running can demand roughly 7,700 watts for the second or two it takes to start. The spec-sheet figures for the common sizes:

Pump size	Running watts	Starting surge (locked-rotor × 240 V)
1/2 HP	~1,000 W	~7,700 W
3/4 HP	~1,500 W	~9,800 W
1 HP	~2,000 W	~11,700 W

A note on honesty: those starting figures are on a VA basis — amps times volts, with no power-factor conversion to shrink them. Some worksheets convert locked-rotor amps to kilowatts using an assumed power factor, producing much smaller numbers (roughly 3–4 kW for a 1/2 HP pump). We deliberately skip that conversion: a generator's alternator is limited by current, and Generac's own sizing guide publishes surge capability in locked-rotor amps. Sizing in amps is the conservative basis — and it's what our calculator uses.

That's why the pump sets your floor. Almost nothing else in an essentials-only list comes close to a 7,700-watt surge — the fridge is ~2,400, the sump pump ~1,800. The practical method: size the surge to the pump, size the running watts to everything else. If the generator's surge rating clears the pump's locked-rotor draw on top of your running loads, everything smaller starts fine.

One exception: a constant-pressure (VFD) well system ramps the motor up softly instead of slamming it on. Franklin Electric's SubDrive documentation states "no inrush current" and "low motor startup current (soft-starting)" — which can drop you a full generator size. (See our VFD vs. conventional comparison.)

How to read your well pump's nameplate

Guessing your pump's horsepower is how people buy the wrong generator. The information exists — it's just scattered. Work through these in order:

1. The control box. If you have a 3-wire submersible pump, there's an above-ground control box (usually near the pressure tank). Its nameplate lists the horsepower and voltage, and the pump manual requires that rating to match the motor. No control box? You likely have a 2-wire pump — skip to step 2.
2. The paperwork. The installer's invoice, your home purchase documents, or the well completion report (well log) usually list the pump size. Well drillers tend to keep these records — calling the company that drilled or serviced the well is one of the most reliable moves.
3. The model number. Franklin Electric motor model prefixes encode the size: 244505 is a 1/2 HP, 244507 is a 3/4 HP, 244508 is a 1 HP. If you can find any label with the motor model, the manual's table decodes it.
4. The breaker. A 240 V pump breaker is only roughly sized to the pump — treat it as a hint, not an answer.

The actual motor nameplate is on the motor itself — hundreds of feet down the well. You're not pulling the pump to read it. The control box, the paperwork, and the driller are your realistic sources. If you truly can't determine the size, assume the larger of your candidates: sizing for a 3/4 HP pump when you have a 1/2 HP costs you one generator size; sizing for a 1/2 HP when you have a 3/4 HP buys you a generator that stalls. Our well pump troubleshooting guide has more on identifying what you've got.

Running watts vs. surge watts on the generator's own nameplate

Generators have the same two-number problem as motors, and marketing exploits it.

Running watts (sometimes called rated or continuous watts) is what the generator can sustain indefinitely. Starting watts (surge or peak watts) is the burst it can deliver for a few seconds. Both numbers matter: running watts must cover your total running load, and starting watts must cover your running load plus the pump's surge.

The trap: many portables are sold by the big number. A popular dual-fuel portable is marketed as a "9500 Peak Watt" unit — its rated, continuous output is 7,500 watts. Both numbers are in the spec sheet, so it's not fraud. But if you bought it thinking you had 9,500 watts to work with, your real budget is 7,500 — and per the math above, a 1/2 HP well pump's 7,700-watt surge alone can exceed a unit like that’s surge rating once anything else is running.

So when comparing units, ignore the box. Open the spec sheet and write down two numbers: rated/running watts and starting/surge watts. Size the first against your running total and the second against running-plus-pump-surge. If a listing only shows one number, assume it's the surge rating and keep looking.

Worked example 1: frugal essentials-only

A rural household that wants water, cold food, heat circulation, and a dry basement — nothing more. 1/2 HP well pump.

Load	Running	Starting
Well pump, 1/2 HP	1,000 W	7,700 W
Refrigerator	1,200 W	2,400 W
Sump pump	600 W	1,800 W
Furnace blower, 1/3 HP	1,200 W	2,000 W
Lights, internet, chargers	~500 W	~500 W
Total running	4,500 W

The method: 4,500 W running + the single largest starting surge (7,700 W, the well pump) = 12,200 W. Round up to the next standard size: 14 kW.

Notice what just happened. Your running load is a modest 4,500 watts — a 5,000-watt generator could sustain it all day. But the pump's surge pushes the requirement to 12,200, ruling out every portable in the popular 7,500-running-watt class: their ~9,500-watt surge ratings can't clear the ~11,200-watt momentary draw when the pump kicks in. This is the single most common generator regret — a unit that runs the house beautifully right up until the pressure tank calls for water. (The fridge figure comes from an older manufacturer chart and is generous for a modern fridge — conservative by design.)

Worked example 2: full comfort

Same house, but the outage shouldn't change how anyone lives: central AC, hot water, everything on. 3/4 HP well pump.

Load	Running	Starting
Well pump, 3/4 HP	1,500 W	9,800 W
Central AC, 24,000 BTU	3,800 W	11,400 W
Electric water heater	4,500 W	— (resistive, no surge)
Refrigerator	1,200 W	2,400 W
Freezer	500 W	500 W
Furnace/air-handler blower	1,200 W	2,000 W
Microwave	1,000 W	—
Sump pump	600 W	1,800 W
Lighting, electronics, misc.	~2,000 W	~2,000 W
Total running	16,300 W

Here the largest single surge is the central AC at 11,400 W — it beats the well pump's 9,800. Total: 16,300 + 11,400 = 27,700 W → round up to 30 kW. That's a large standby unit.

Now the useful part: the electric water heater costs you 4,500 running watts with zero surge — pure dead weight on the total. Take it off the generator (or put it on a load-shed module that drops it when big motors start): 27,700 − 4,500 = 23,200 W → 24 kW. One appliance decision moves you down a full generator size. Make these trade-offs before you buy, because the worksheet assumes everything runs at once — and an electrician will use load management to avoid paying for that assumption.

A caveat on the AC figure: 3,800/11,400 is a worksheet estimate for a 2-ton unit, and real starting draw varies widely with tonnage, SEER, and age. Verify against the condenser's nameplate. And as noted above, a soft-start kit (manufacturer-claimed 65–75% start-current reduction) or a VFD-driven system can change this number substantially.

When oversizing is smart

"Bigger" gets a bad rap in sizing guides. Sometimes it's the right call:

• Rounding up is the method, not a luxury. The worksheet says to round up to the next standard size — that headroom is what lets the freezer compressor kick on while the well pump is already running. Running a generator pinned at its absolute limit shortens its life and leaves zero margin.
• Two motors can start together. The method adds only the single largest surge, on the assumption that big motors don't all start in the same second. Usually true — but a well pump cycling while the AC compressor kicks in is a plausible bad minute. One size up is cheap insurance against it.
• Your loads will grow. A shop, an EV charger, a heat-pump conversion — if any of those are on your five-year horizon, buying the inlet and switch for the generator you'll need then is far cheaper than redoing the electrical work later.
• Altitude. Engines make less power in thin air. At real elevation, check the generator's altitude-derate spec sheet — don't assume sea-level numbers.

When oversizing is wasteful

But past a point, extra capacity is just money on fire:

• You pay for it twice — at purchase and every hour it runs. A Generac 22 kW standby burns about 2.5 gallons of propane per hour at half load — more than half its full-load consumption just to keep the engine spinning. On a 500-gallon tank (400 usable gallons at the standard 80% fill), that's the difference between a week of runtime and a long weekend. A right-sized unit sips; an oversized one drinks whether you're using the capacity or not.
• The hardware cascades. Generator size dictates the transfer switch, the inlet box, and the cord. A 30-amp setup handles generators up to 7,500 running watts; step up to a 50-amp inlet and switch and you're buying heavier, pricier hardware for up to 12,500 watts. Oversizing the generator drags the whole electrical bill up with it.
• Capacity you never use is the most expensive kind. A 26 kW standby running a 6 kW essentials load is a luxury car idling in the driveway. Size to the loads you'll actually carry, plus one band of headroom — not to the loads you imagine in a disaster movie.

How your answer changes the transfer switch and inlet

This is the part most sizing guides skip, and it's where the money goes. Your generator size doesn't just pick the unit — it picks the connection hardware, and the tiers are fixed:

• Up to ~7,500 running watts → 30-amp hardware. A 30-amp manual transfer switch kit (transfer switch, outdoor inlet box, 30-amp cord with L14-30 ends) is the standard package for generators up to 7,500 watts of continuous output. This is the sweet spot for essentials-only portable setups.
• Up to ~12,500 running watts → 50-amp hardware. Bigger portables need a 50-amp transfer switch, a 50-amp inlet box, and a heavier cord. Everything gets more expensive — the switch, the wire the electrician runs, the inlet.
• Above that → standby territory. Whole-house loads in the 20+ kW range mean an automatic transfer switch sized to your service, installed as part of a standby system.

Three rules for buying the hardware:

1. Match every link to the generator's running watts — inlet, cord, and switch all rated for the full continuous output. The weakest link caps the system, and an undersized inlet on a big generator is a fire risk, not a bargain.
2. The generator's outlet decides the plug. If the unit's 240 V outlet is a 30-amp L14-30, a 50-amp inlet doesn't give you more power — it gives you a cord that doesn't fit. Check the receptacle panel in the spec sheet before ordering anything.
3. An electrician installs it. The only safe ways to connect a generator to house wiring are a transfer switch or a listed interlock kit installed by a qualified electrician — our backup power guide covers why the alternatives can kill lineworkers. Extension cords are fine for cord-and-plug loads, but each cord's rating must exceed the total wattage plugged into it.

Size the generator first, then buy the switch and inlet to match — not the other way around. A 30-amp kit is a great purchase exactly once: when the math says 7,500 watts covers your loads with the pump's surge included.