18 kW generator
Take the Generac Guardian 18 as one example of the 18 kW standby class: it puts out 18,000 W on propane, while natural gas output is about 16,020 W. Other models rate a little differently. With natural gas, one unit covers a house of up to 1,500 sq ft plus 3-ton central air.
For natural gas installs, the 18 kW price begins at $9,669 in Arkansas and ends at $11,969 in Oregon. Labor costs per state come from BLS wages, May 2025.
Does 18 kW fit your house? Size it with three questions, including the AC or the well pump start.
What a 18 kW generator runs
| Loads running together | Running | Peak at a motor start | Natural gas | Propane |
|---|---|---|---|---|
| Essentials | 4.5 kW | 5.6 kW | runs | runs |
| Essentials and a 1 hp well pump | 6.5 kW | 8.5 kW | runs | runs |
| Essentials and a 2-ton central AC | 7.0 kW | 9.0 kW | runs | runs |
| Essentials and a 3-ton central AC | 8.0 kW | 10.5 kW | runs | runs |
| Essentials and a 4-ton central AC | 9.5 kW | 12.5 kW | runs | runs |
Listed as essentials: a fridge, a freezer, a microwave, lights, TV, a router, a 1/2 hp sump pump and the furnace blower. In the "runs" figure, your running load is held under 80% of the rating, and on top of that comes the largest motor start.
| Whole house load | Natural gas | Propane |
|---|---|---|
| No central air | up to 4,000 sq ft | up to 5,000 sq ft |
| Central AC, 3 ton | up to 1,500 sq ft | up to 2,500 sq ft |
| Central AC, 4 ton | essential circuits only | up to 1,500 sq ft |
NEC 220.83 is the basis for the whole house rows. Lights and outlets count at 3 VA per sq ft, gas heat comes in along with the kitchen and laundry circuits, and the rows assume no electric range, no dryer and no water heater. Check your own house.
Fuel use per hour
| Load | Natural gas | Gas cost | Propane | Propane cost |
|---|---|---|---|---|
| 25% | 130 cu ft | $2.85 | 1.04 gal | $2.84 |
| 50% | 169 cu ft | $3.70 | 1.70 gal | $4.64 |
| 75% | 208 cu ft | $4.56 | 2.36 gal | $6.44 |
| 100% | 247 cu ft | $5.41 | 3.02 gal | $8.24 |
Generac's fuel table lists half load and full load for the Guardian 18 kW, and the 25% and 75% figures are read in between those two. These costs are calculated with EIA US average prices.
Hours per propane tank
| Tank | 25% load | 50% load | Full load |
|---|---|---|---|
| 120 gal | 92 h | 56 h | 32 h |
| 250 gal | 192 h | 118 h | 66 h |
| 500 gal | 385 h | 235 h | 132 h |
| 1,000 gal | 769 h | 471 h | 265 h |
Tanks fill to 80%.
Installed price
| Line, 18 kW | Natural gas | Propane | Basis |
|---|---|---|---|
| Generator | $5,709 | $5,709 | 18 kW bundle list price from Generac, without the switch |
| Transfer switch | $1,060 | $1,060 | difference between Generac's two listings, one with a switch and one without |
| Electrician: switch, wiring, start-up | $3,000 to $5,500 | $3,000 to $5,500 | published install ranges, adjusted to the US average hourly wage of $34.37 (BLS) |
| Fuel line | $500 to $2,000 | $800 to $2,500 | the gas line comes from the meter, the propane line from the tank |
| Pad and permit | $300 to $1,200 | $300 to $1,200 | composite pad or slab, local fees |
| Installed | $10,569 to $15,469 | $10,869 to $15,969 | propane leaves out the tank |
See whole house generator cost for the numbers in each state.
18 kW and other brands
| Brand | Model at 18 kW or next up | List price | Installed, natural gas | Page |
|---|---|---|---|---|
 Generac | Guardian 18 kW, with switch | $6,769 | $10,569 to $15,469 | Generac cost |
 Kohler | 20 kW 20RCA, plus a switch | $6,016 | $10,876 to $15,776 | Kohler cost |
Each unit's installed price covers the same four charges: the gas line, the permit, the pad and the electrician. Units sold without a switch also get Generac's price for the nearest size switch added. List prices come straight from Generac and Kohler's own web pages. Brands like Cummins, Briggs & Stratton and Champion reach buyers through dealers, and those dealers quote the unit: Cummins cost, Briggs & Stratton cost, Champion cost.