Generator sizing is where most first-time buyers either over-spend or under-buy. Go too small and the generator overloads the moment your fridge compressor kicks on alongside the microwave. Go too big and you've spent extra money on capacity you'll never use, while burning more fuel and dealing with more weight and noise. This guide walks through the math the right way, with realistic wattage figures for actual appliances.
Running watts vs starting watts (the 30-second version)
Every electrical load has two wattage numbers that matter for generator sizing. Running watts is the continuous power the appliance draws while operating normally. Starting watts (also called surge or peak) is the brief spike — typically lasting 1 to 5 seconds — that motor-driven appliances pull when the motor first engages.
A refrigerator might run at 150 watts continuously but pull 800 to 1,200 watts for a second when the compressor cycles on. A 1.5 HP well pump might run at 1,500 watts and surge to 4,500 watts at startup. This is why generators publish two numbers — running and starting — and you need to size against both. The detailed treatment is in running watts vs starting watts.
Step one: list what you actually want to run
The most useful generator-sizing exercise is also the simplest one most people skip. Sit down with a pen and write a list of every appliance you want to run at the same time. Not everything in the house. Just the things you need simultaneously during an outage or at a campsite.
A typical "essentials only" home backup list looks like this:
- Refrigerator (running and occasional compressor cycle)
- Chest freezer (similar pattern)
- Gas furnace blower fan (forced-air heating during winter outage)
- Sump pump (basement, intermittent cycling)
- A few LED lights
- Modem and Wi-Fi router
- Phone chargers
- Maybe a microwave or coffee maker, not run simultaneously with other heavy loads
A typical RV-camping list:
- Rooftop air conditioner (the big driver — usually 13,500 or 15,000 BTU)
- Refrigerator (if running on AC rather than propane)
- Microwave (briefly)
- TV and lights
- Phone and laptop chargers
Step two: look up real wattage figures
Every appliance has a nameplate (usually a sticker on the back or bottom) listing voltage, amps, and often watts. Multiply volts × amps for the running wattage if watts isn't listed directly. For motor-driven appliances, multiply running watts by 2 to estimate starting surge (some motors only need 1.5× — induction motors with high inertia can need 3 to 5× — but 2× is a safe planning figure).
Approximate running and starting watts for common household loads:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator (full-size) | 150–200 | 800–1,200 |
| Chest freezer | 100–150 | 500–900 |
| Gas furnace blower (1/2 HP) | 700–900 | 1,400–2,100 |
| Sump pump (1/3 HP) | 800 | 2,000 |
| Sump pump (1/2 HP) | 1,000–1,500 | 3,000+ |
| Well pump (1 HP submersible) | 1,000–2,000 | 3,000–6,000 |
| Microwave (1,000W cooking power) | 1,500 | 1,800 (negligible surge) |
| Coffee maker (drip) | 800–1,200 | resistive (no surge) |
| Toaster | 800–1,500 | resistive |
| Electric space heater (high) | 1,500 | resistive |
| Window AC (8,000 BTU) | 800–1,200 | 2,500 |
| Window AC (10,000 BTU) | 1,200–1,500 | 3,000 |
| Central AC (3-ton) | 3,500–4,500 | 7,000–10,000 |
| RV rooftop AC (13,500 BTU) | 1,400–1,700 | 2,800–3,500 |
| RV rooftop AC (15,000 BTU) | 1,500–2,000 | 3,500–4,500 |
| LED light (60W equivalent) | 9 | 9 |
| CFL light bulb | 13–20 | 13–20 |
| Laptop (charging) | 60–100 | negligible |
| Smartphone charger | 5–20 | negligible |
| Wi-Fi router | 10–20 | negligible |
| TV (55-inch LED) | 80–150 | negligible |
| CPAP machine | 30–60 | negligible |
| Electric range (one burner on high) | 1,500–2,500 | resistive |
| Electric clothes dryer | 5,400 | 6,800 |
| Electric water heater (40-gal) | 4,500 | resistive |
| Circular saw (7-1/4") | 1,400 | 2,300 |
| Air compressor (1 HP) | 1,600 | 4,500 |
These are mid-range estimates. Your specific appliance may draw more or less — always check the nameplate when you can.
Step three: do the math
Two calculations: total running watts and worst-case starting watts.
Total running watts is straightforward — just add up the running wattage of everything you want to run simultaneously.
Worst-case starting watts works like this: take the running-watts total, then add the difference between running and starting watts for whichever single appliance has the largest gap. The reason we only count one starting surge is that motor-driven appliances almost never start simultaneously — sump pumps cycle independently from fridge compressors, which cycle independently from well pumps.
Worked example — essentials-only home backup:
| Load | Running W | Starting W |
|---|---|---|
| Refrigerator | 180 | 1,000 |
| Chest freezer | 120 | 700 |
| Furnace blower | 800 | 1,800 |
| Sump pump (1/2 HP) | 1,200 | 3,000 |
| 5 LED bulbs | 45 | 45 |
| Modem + router | 30 | 30 |
| TV | 100 | 100 |
| Phone chargers (2) | 40 | 40 |
| Total running | 2,515 W | |
| Largest motor gap | Sump pump: +1,800 W | |
| Worst-case surge | ≈ 4,315 W |
This list calls for a generator with at least 2,515 running watts and 4,315 starting watts — a 4,500W or 5,000W class generator would handle it cleanly with margin. Going up to 7,500W gives room to add the microwave for short cooking sessions or to handle a larger sump pump if the basement gets aggressive water.
Step four: add a 20 percent buffer
You never want to run a generator at 100 percent of its rated capacity continuously. Generators are happiest at 50 to 75 percent of running watts. Adding a 20 percent buffer above your calculated total gives the engine breathing room, extends its operating life, and leaves headroom if you ever add a load you forgot to plan for.
Take the example above: 2,515 running watts ÷ 0.8 = 3,144 watts. So we'd want a generator rated for at least 3,144 running watts. Combined with the 4,315W starting surge requirement, a 5,000W starting / 4,000W running generator is the right size — and a 7,500W class unit is the comfortable size.
The four common sizing brackets
1,800–2,500 watts (small inverter class)
Laptops, phones, a small TV, lights, a few small fans, a CPAP machine. One full-size refrigerator if nothing else is running. Will not start most central AC units, well pumps, or larger sump pumps. Pair (parallel) two units to double the capacity. Examples: Honda EU2200i, Westinghouse iGen2500, Champion 2500W.
3,000–4,500 watts (medium inverter class)
One RV rooftop AC plus essentials. Fridge plus microwave plus a few lights (sequentially, not simultaneously). The most common size for camping and lightweight home backup.
5,000–7,500 watts (mid-size home backup class)
Essentials plus a window AC unit. Fridge plus furnace blower plus sump pump plus microwave for short cooking sessions. Many gas-only households can cover all critical loads in this range.
9,000–12,500 watts (full home backup class)
Central AC, well pump, electric range, all essentials simultaneously. The upper end of "portable" — units in this class typically weigh 200 to 280 pounds with a wheel kit. Above this, look at permanently installed standby generators instead.
Things that throw off your sizing math
Electric heat strips and resistive loads
If your house has electric baseboard heat or heat strips in a heat pump, you can't reasonably plan to run them on a portable generator — they pull 4,000 to 10,000+ watts of pure resistive load with no surge but huge sustained draw. Plan to use space heaters and a furnace blower instead, or accept that staying warm during a winter outage will mean a different strategy.
Soft-start kits for AC units
If you want to run a rooftop or central AC unit but the starting surge exceeds your generator's capacity, a soft-start kit (a capacitor-based device wired into the AC's compressor) reduces the inrush current by 50 to 70 percent. This is what lets a 2,200W inverter generator run a 13,500 BTU RV AC that would otherwise need 3,500W starting capacity.
Altitude derating
Gasoline generators lose roughly 3.5 percent of their rated output for every 1,000 feet above sea level. If you're at 5,000 feet of elevation, a "5,000W" generator delivers closer to 4,100W. Add altitude to your sizing buffer if you're at elevation.
The fridge-and-microwave conflict
If your generator is sized tight and the fridge compressor cycles on while you're microwaving lunch, the breaker might trip. The fix is either to size up by 20 to 30 percent or accept the discipline of not running heat-intensive resistive loads simultaneously with motor-driven cycling loads.
RV-specific sizing notes
The biggest decision for RV use is your air conditioner. A single 13,500 BTU rooftop AC needs roughly 2,800 to 3,500 starting watts. A 15,000 BTU unit needs 3,500 to 4,500. Two AC units (common in 50-amp RVs) double those numbers.
The classic two-paths setup: a single 3,000 to 4,000W inverter generator for solo AC operation, OR a pair of 2,200W inverters in parallel for the same effective capacity with more flexibility (you can deploy one for low-load situations). The RV-specific roundup is at best generators for RV use.
Don't oversize "just in case"
Buying double the wattage you need has real downsides. Bigger generators are heavier, louder, burn more fuel per hour even at light load (conventional generators) or run inefficiently at very light load (some inverter generators), and cost more upfront. A 4,000W generator running at 50 percent load is more efficient than a 12,000W generator running at 17 percent load.
The right size is "matches your real loads plus 20 percent." That's the formula.
Frequently Asked Questions
How many watts does a refrigerator need from a generator?
A typical full-size refrigerator runs at 150 to 200 watts continuously but pulls 800 to 1,200 watts for one to two seconds when the compressor cycles on. Plan generator capacity around the starting surge, not the continuous draw.
Can a 2,000 watt generator run a refrigerator and a microwave together?
Briefly, yes — but it's tight. A 200W fridge plus a 1,500W microwave is 1,700W continuous, which leaves only 300W of headroom. If the fridge compressor cycles on while the microwave is running, the generator will likely trip a breaker. Size up to 3,000W or alternate the loads.
Will a 7,500 watt generator power my whole house?
It depends entirely on what's in your house. A 7,500W unit easily handles essentials (fridge, furnace blower, lights, sump, modem, microwave for short bursts). It probably won't run central AC, an electric range, or an electric clothes dryer. For full whole-house coverage including those, look at 12,000W+ units or a permanently installed standby generator.
What size generator do I need for a 30-amp RV with one air conditioner?
A 3,000 to 4,000 running watt inverter generator is the standard recommendation. The RV's 30-amp main feed limits total simultaneous draw to about 3,600 watts, and the rooftop AC's starting surge is the main sizing driver. A 13,500 BTU AC typically needs about 3,000 starting watts.
Should I size my generator larger than I think I need?
Add a 20 percent buffer above your calculated total — that gives the engine healthy headroom and accommodates loads you may add later. Going larger than 20 to 30 percent over starts costing you in fuel consumption, weight, noise, and price without delivering benefit.