Generator Size Calculator
Last updated: October 6, 2026 (CT)
Add your appliances. Get the wattage to buy, how long a tank lasts, and what fuel costs per day — running watts, startup surge, and headroom handled.
The form below is the first pass every site has. The Lab tools are the reason this domain is not just another wattage widget: staggered-start coincidence, altitude derate, a printable 72-hour runbook, 5-year TCO, and lot-line dBA.
Load sequencer
Always / wait / never-overlap. Buy the sequenced class.
Altitude derate
Usable watts at your elevation and inlet air.
Storm runbook
Print fuel, oil, circuit order, and CO distance.
5-year cost
Portable fuel vs standby install, in hours.
Lot-line noise
dBA at the fence, not the brochure.
Our math
Three worked houses + the coincidence method.
As an Amazon Associate I earn from qualifying purchases. The Amazon links below are affiliate links; they don't change the math.
Size your load first — we'll match generator classes to your recommended wattage below, plus the popular classes side by side.
Off-grid or battery setups need an inverter, not a generator. Size it by load type — motors and compressors pull multiples of their running draw at startup.
Running an inverter off a battery bank instead of fuel? Here's how long it lasts — after depth-of-discharge and inverter losses.
Sizing a whole bank instead of testing one battery? Use the battery bank sizer.
List the household circuits you want on backup power, one per line. When you know the watts from the nameplate, write them after a dash, as Circuit name - watts. The planner totals those watts and matches a switch class.
Transfer switches are licensed-electrician work — code prohibits back-feeding, and an ATS install needs permits. Need a pro to install it? This slot is reserved for a vetted local generator installer.
What the open form already adds
The generator-size form opens with two loads: Refrigerator (Energy Star, 20 cu ft) at 150 running watts and 600 starting watts, and LED lights (whole home) at 80 running watts and 80 starting watts. Headroom is 20%. Running total is 230 W. The refrigerator is the largest start, so extra surge is 600 − 150 = 450 W. Peak is 680 W. 680 × 1.20 = 816, and the button rounds that up to 1,000 watts recommended. Those watts are the calculator's presets and its own rounding rule.
Nameplate running watts, starting watts, tank size, and run time at a stated load are on the portable generator watts chart. To put this calculator on another site, use embed the calculator.
Need a pro to install it?
A hardwired connection to household circuits, and all commercial standby work, is licensed-electrician territory — permits, code, and the inspection log matter as much as the genset. This slot is reserved for a vetted local generator installation and service operator. Read the facility manager's vetting checklist →
Generatorsizer connects its readers — homeowners sizing backup and facility managers planning standby — with vetted installers and maintenance contractors. Operator inquiries: this space is reserved pending terms.
How does generator sizing work?
Three numbers decide what you buy. Running watts — the sum of everything switched on at the same time. Starting watts — the brief spike when a motor kicks on; only the largest one counts at any instant, because motors don't all start together. Headroom — the 20% margin on the form, so the generator is not pinned at its rated output, and so a load you forgot still fits.
Full method, three worked houses, and how our math differs: GeneratorSizer methodology. Amazon links on this site are affiliate links; they do not change the math.
How many watts do common appliances use?
Read the running and starting watts on the appliance nameplate, then add that list in the calculator. Honda and Champion rows for one-appliance questions are on the generator sizing chart.
Computers & devices in the picklist
These picklist items use the maker's own published number, labeled by type. A graphics card or CPU figure covers that part only, not the whole PC, so add the rest of the system as a custom row (the PC power supply label is the safe ceiling). A charger rating is the most that charger can draw while charging. Starting watts equal running watts for these items.
| Device | Watts | Wattage type | Source |
|---|---|---|---|
| NVIDIA DGX Spark | 240 | Power supply rating | www.nvidia.com |
| NVIDIA GeForce RTX 5090 card | 575 | Total graphics power (card only) | www.nvidia.com |
| NVIDIA GeForce RTX 5080 card | 360 | Total graphics power (card only) | www.nvidia.com |
| NVIDIA GeForce RTX 4090 card | 450 | Total graphics power (card only) | www.nvidia.com |
| NVIDIA GeForce RTX 4080 card | 320 | Total graphics power (card only) | www.nvidia.com |
| NVIDIA GeForce RTX 4070 card | 200 | Total graphics power (card only) | www.nvidia.com |
| AMD Radeon RX 9070 XT card | 304 | Typical board power (card only) | www.amd.com |
| AMD Radeon RX 9070 card | 220 | Typical board power (card only) | www.amd.com |
| AMD Radeon RX 9060 XT card | 160 | Typical board power (card only) | www.amd.com |
| AMD Radeon RX 7900 XTX card | 355 | Typical board power (card only) | www.amd.com |
| AMD Ryzen 9 9950X3D CPU | 170 | Default TDP (CPU only) | www.amd.com |
| AMD Ryzen 9 9950X CPU | 170 | Default TDP (CPU only) | www.amd.com |
| AMD Ryzen 7 9800X3D CPU | 120 | Default TDP (CPU only) | www.amd.com |
| AMD Ryzen 7 9700X CPU | 65 | Default TDP (CPU only) | www.amd.com |
| AMD Ryzen 5 9600X CPU | 65 | Default TDP (CPU only) | www.amd.com |
| MacBook Air (M5) | 60 | Charger rating (included 40W Dynamic Power Adapter, 60W max) | www.apple.com |
| MacBook Pro 14-inch (M5) | 70 | Charger rating (included 70W USB-C Power Adapter) | www.apple.com |
| MacBook Pro 14-inch (M5 Pro 18-core or M5 Max) | 96 | Charger rating (included 96W USB-C Power Adapter) | www.apple.com |
| MacBook Pro 16-inch (M5 Pro or M5 Max) | 140 | Charger rating (included 140W USB-C Power Adapter) | www.apple.com |
| iMac 24-inch (M4, four ports) | 105 | Max power consumption (Apple CPU Max figure) | support.apple.com |
| Mac mini (M6) | 70 | Max power consumption | support.apple.com |
| Mac mini (M5 Pro) | 145 | Max power consumption | support.apple.com |
| Mac Studio (M5 Max) | 200 | Max power consumption | support.apple.com |
| Mac Studio (M5 Ultra) | 385 | Max power consumption | support.apple.com |
| Mac Pro (2023, M2 Ultra) | 330 | Max power consumption | support.apple.com |
| iPhone 18 Pro or Pro Max (fast-charge adapter) | 60 | Charger rating (60W-capable adapter Apple lists for fast charge; sold separately) | www.apple.com |
| iPhone 17 (fast-charge adapter) | 40 | Charger rating (40W adapter Apple lists for fast charge; sold separately) | www.apple.com |
| iPad (included 20W adapter) | 20 | Charger rating (20W USB-C Power Adapter in the box) | www.apple.com |
Gas or propane: how do the fuels compare?
| Factor | Gasoline | Propane |
|---|---|---|
| Energy per gallon | EIA sample heat contents, in the note under this table | |
| Storage | A short-storage fuel; rotate what you keep | Does not share gasoline's storage limit |
EIA's sample heat contents list motor gasoline at 120,214 Btu per gallon and propane at 91,452 Btu per gallon, about 24% less energy per gallon of propane. A dual-fuel wattage drop is a separate manufacturer-spec figure, not this EIA ratio. The home calculator's propane volume factor is explained on how much propane a generator uses. For current fuel price data, see the 2026 Generator Cost Study.
Related calculators
- Battery Bank Sizer sizes an outage battery bank when the load is small enough to live on stored watt-hours instead of a fuel tank.
- SolarSizer sizes off-grid charging for that bank.
- TrailerTowCalc sizes RV towing capacity and tongue weight, separate from the roof air conditioner's watts.
Guides
What size generator runs a refrigerator and freezer?
Read the nameplate, then the sourced refrigerator-and-freezer page. The calculator adds the list.
How many watts does a well pump use?
Startup, not the running figure, is what stalls a generator. Size it from the nameplate and the sourced well-pump page.
Best 10 portable generators for home backup
Match a class to the load list from the calculator, then compare the manufacturer's current spec sheet.
Find a commercial generator service
The facility manager's vetting checklist: contract red flags, and the seven questions to ask before signing.
Battery bank sizer ↗
If your load is small and daily, a LiFePO4 battery bank can replace fuel entirely — that tool sizes it.
Sources & Standards
Method context (not a design stamp):