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Generator size calculator
A generator has to carry the running watts of everything that is on, plus the surge of the motor that starts hardest. Generac's worksheet does it in four steps, and its own example of a refrigerator, a sump pump, a belt sander and a circular saw comes to 4,350 W running and 6,750 W with the largest start. A central air conditioner is where the surge sets the size. A 3 ton unit that runs at 4,068 W reaches 22,284 W at start when its full locked rotor amps are counted as watts. Hold that peak against the generator's starting rating; the maker's motor starting data for a given set settles a close call. A soft start can bring that to 7,948 W at the 65 percent cut Micro-Air claims. This page works both, gives the amps at 120 and 240 V, and checks the result against a generator's running and starting ratings. It does not size the transfer switch. The AC amps and watts calculator gives the running current of one air conditioner, and the home electrical service load calculator works the panel.
- Enter how many of each household or job-site load will be on at the same time, from Generac's table. Add any other load by its nameplate watts, and choose whether motors start one at a time or all at once.
- For a central air conditioner or a heat pump, enter the compressor RLA and LRA and the fan amps from the data plate. If a soft start is fitted, enter the cut its maker publishes.
- Enter the running and starting watts of the generator. The result shows the running and peak watts, the current at 120 and 240 V, and whether each rating covers the load.
Example result for the default inputs
4 loads, motors start one at a time. Running watts plus the largest start, single phase 120/240 V.
Running load
4.35 kW
Peak with the start
6.75 kW
Peak current at 240 V
28.1 A
| Refrigerator or freezer | 1 x 700 W = 700 W running, 2,200 W more to start |
|---|---|
| Sump pump, 1/2 HP | 1 x 1,050 W = 1,050 W running, 2,150 W more to start |
| Belt sander, 3 in. | 1 x 1,200 W = 1,200 W running, 2,400 W more to start |
| Circular saw, 7-1/4 in. | 1 x 1,400 W = 1,400 W running, 2,300 W more to start |
| Total running watts | 4,350 W |
| Highest start added | 2,400 W for the belt sander, 3 in., the one that asks for the most. |
| Peak watts | 4,350 W + 2,400 W = 6,750 W |
| Current at 240 V | Running 18.1 A, peak 28.1 A, as watts / 240 V. |
| Current at 120 V | Running 36.3 A, peak 56.3 A, as watts / 120 V. This is the figure if the whole load were 120 V on one leg. |
| Running rating check | 4,350 W is 79.1 percent of the 5,500 W rating, 1,150 W to spare. |
| Starting rating check | 6,750 W fits under the 6,875 W starting rating, 125 W to spare. |
- Appliance watts in the list are Generac's, and the table says they are approximate. Check the nameplate of the actual appliance.
- Stepped starts add only the largest start. If two motors can start in the same second, such as two air conditioners on one outage, choose all together or stagger them with a delay relay or a load-management module.
- This is a preliminary size. The generator maker's manual and sizing program, the transfer switch and the local authority having jurisdiction decide the final selection.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the running load, the start and the current are worked
A generator carries two things. One is everything that runs at the same moment. The other is the surge when a motor starts, which can run to several times that motor's running load. Generac's worksheet adds the running watts of every device you want to power, then adds the single largest additional starting figure on top. A central air conditioner is the case where the surge usually sets the size, so the page also works its start from the locked rotor amps on the nameplate, with and without a soft start.
- Running watts
W_run = sum of (count x running watts)Add the running watts of every load that will be on together. Generac's own four-item example totals 4,350 W. A refrigerator and a sump pump cycle on and off, and they still count.- Extra watts to start a motor
table value, or (I_start - RLA) x V for a nameplate compressorGenerac's table lists additional starting watts on top of running watts, 2,400 W for a belt sander and 2,200 W for a refrigerator. For a compressor the page takes the locked rotor amps from the nameplate and subtracts the running amps, because those are already in the running total. The 3 ton Goodman GSXN403610A (RLA 16 A, LRA 91.9 A) adds 18,216 W at 240 V.- Highest start
W_start = largest extra start, or the sum when motors start togetherGenerac adds only the one device with the highest starting watts, and its table assumes intermittent starting. For the example that is 2,400 W. If every motor can start at once, the sum is 9,050 W and the peak is 13,400 W.- Peak watts
W_peak = W_run + W_start4,350 + 2,400 = 6,750 W. Generac prints this as the minimum starting watts for the job, and it is compared with the starting rating of the generator.- Reserve
required = W x (1 + reserve / 100)Generac's worksheet applies none, and no maker guide read for this page names a percentage, so it defaults to 0. A 20 percent allowance turns the example's 6,750 W into 8,100 W.- Current
I = W / 240 and I = W / 120Watts divided by volts, power factor taken as 1, which is how the Generac table treats watts. The example's 6,750 W peak is 28.1 A at 240 V and 56.3 A at 120 V.- Soft start
I_start = max(RLA, LRA x (1 - cut / 100))The cut is the figure the soft starter's maker publishes. Micro-Air claims 65 to 75 percent against the LRA. At 65 percent the 3 ton unit starts at 32.2 A against 91.9 A with no soft start, and its peak falls from 22,284 W to 7,948 W. The running amps do not change.- Starting kVA and voltage dip
kVA = V x LRA / 1000, and kVA at a dip d = kVA x (1 - d)^2Kohler describes starting kVA as the motor's locked-rotor kVA, and says a 30 percent voltage dip cuts it by about half (0.7 x 0.7 = 0.49). The 3 ton unit is 22.06 kVA at full voltage and 10.81 kVA at that dip. This page shows the dip and does not use it, since the dip belongs to a particular generator.- Generator check
running rating >= W_run and starting rating >= W_peakThe Generac RS5500 is rated 5,500 W running and 6,875 W starting. Generac's example needs 4,350 W and 6,750 W, which fits with 125 W to spare on the start.
Eight cases through the same formula
The first row is Generac's worksheet example, and the page gives its 6,750 W. The third row is Generac's own figure for a 24,000 BTU/h central air conditioner, 8,750 W at start. Rows four and five work Goodman's 2 ton and 3 ton condensers from the nameplate and land at 10,056 W and 22,284 W. A table is approximate by its own footnote, and the nameplate belongs to your unit, so use the nameplate when you have it. Rows six and seven apply a 65 and a 75 percent soft start cut to the 3 ton.
| Case | Running, W | Extra to start, W | Peak, W | Peak at 240 V, A |
|---|---|---|---|---|
| Generac worksheet example, four tools and appliances | 4,350 | 2,400 | 6,750 | 28.1 |
| Refrigerator, sump pump, furnace blower and well pump | 3,625 | 2,300 | 5,925 | 24.7 |
| Generac table, central air conditioner 24,000 BTU/h | 3,800 | 4,950 | 8,750 | 36.5 |
| Goodman 2 ton, GSXN402410A, from the nameplate | 2,184 | 7,872 | 10,056 | 41.9 |
| Goodman 3 ton, GSXN403610A, from the nameplate | 4,068 | 18,216 | 22,284 | 92.9 |
| Goodman 3 ton with a soft start cutting the LRA 65 percent | 4,068 | 3,880 | 7,948 | 33.1 |
| Goodman 3 ton with a soft start cutting the LRA 75 percent | 4,068 | 1,674 | 5,742 | 23.9 |
| Goodman 3 ton at 65 percent soft start, plus the refrigerator, sump pump and furnace blower | 6,693 | 3,880 | 10,573 | 44.1 |
Amps are watts divided by 240 V. The nameplate rows treat volts times amps as watts, which runs a little high for a motor's true power and errs toward a larger generator. In the last row the refrigerator, the furnace blower and the sump pump are in the running total, and the largest start is still the air conditioner's, so the peak is 10,573 W.
Generac table used by the form
Running watts and additional starting watts as Generac printed them in its RS Series brochure of October 2016. A dash in the table is zero here. Only the largest additional start among the loads that run together is added to the total.
| Load | Running, W | Additional start, W |
|---|---|---|
| Coffee maker | 1,500 | 0 |
| Computer with a 17 in. monitor | 800 | 0 |
| Electric range, one 8 in. element | 2,100 | 0 |
| Garage door opener, 1/2 HP | 875 | 2,350 |
| Microwave, 1,000 W | 1,000 | 0 |
| Refrigerator or freezer | 700 | 2,200 |
| Sump pump, 1/2 HP | 1,050 | 2,150 |
| Television, 27 in. | 500 | 0 |
| Washing machine | 1,150 | 2,300 |
| Well pump, 1/2 HP | 1,000 | 2,100 |
| Central air conditioner, 24,000 BTU/h | 3,800 | 4,950 |
| Furnace fan blower, 1/2 HP | 875 | 2,300 |
| Air compressor, 1/2 HP | 1,000 | 2,000 |
| Airless paint sprayer, 1/3 HP | 600 | 1,200 |
| Belt sander, 3 in. | 1,200 | 2,400 |
| Circular saw, 7-1/4 in. | 1,400 | 2,300 |
| Hand drill, 1/2 in. | 600 | 900 |
| Reciprocating saw | 960 | 0 |
| Quartz halogen work light | 1,000 | 0 |
| Table saw, 10 in. | 2,000 | 2,000 |
Standards and data sources
- Generac, RS Series 5500 and 7000 Watts Portable Generators brochure (part 0K9559, October 2016), How much power do you need - the four-step worksheet, the table of running and additional starting watts for household and job-site loads, the worked example (4,350 W running plus 2,400 W is 6,750 W), and the RS5500 and RS7000 ratings. Its table is marked approximate.
- Kohler Power Systems, Sizing Generators for Motor Starting (Power Systems Topics 103, April 2024) - starting current of about six times full-load current, starting kVA from the nameplate, the voltage dip rules (30 to 35 percent contactor limit, 20 percent preferred by many engineers, kVA falling with the square of voltage), starters that reduce the start, and the advice to use the maker's sizing program.
- Goodman, GSXN4 split system air conditioner product specifications (SS-GSXN4, May 2022) - the compressor RLA and LRA and the condenser fan amps for the 2 ton and 3 ton examples, 8.4 A and 41.2 A, 16 A and 91.9 A. The sheet was read in a copy hosted by a retailer, and it says to check the data plate on the unit being installed.
- ACHR News, Micro-Air soft starter (December 2, 2021) - the maker's claim of a 65 to 75 percent start current reduction compared with the compressor's LRA, which the soft start input is checked against. It is a manufacturer statement, not a test.
Limitations of this calculator
- This page gives preliminary numbers for planning and for checking a quote. The generator maker's manual and sizing program, the transfer switch or interlock, and the local authority having jurisdiction (AHJ) decide the final selection. A licensed electrician installs and connects the set.
- Appliance watts come from the Generac table of October 2016 and are approximate by its own footnote. Appliances differ by model and age, so the nameplate of the unit on the job beats the table. Window air conditioners, electric water heaters, heat pumps and recreation items are not in the list. Enter them under other load or, for a heat pump compressor, from its nameplate.
- The method is Generac's portable-generator worksheet. It assumes devices start intermittently, and the page offers an all-at-once choice for the case where they do not. It does not model how long a start lasts, how far the voltage sags or how the engine slows, which Kohler says depend on the particular engine, alternator and regulator.
- The voltage dip is shown for reference and never used to shrink the requirement. Kohler's own advice is to run the maker's sizing program, which knows the dip of a given set.
- The locked rotor amps are the nameplate figure. When no LRA is entered the page uses 6 times the RLA, a typical figure from Kohler, and says so. A variable-speed or inverter-driven compressor can start with less than its plate suggests, so use the maker's data for it.
- Soft start reductions are the figure you type, and the 65 to 75 percent quoted here is a manufacturer claim. The reduction can change with the setting and the compressor. The start is never taken below the running current, and the soft start does nothing for the running watts.
- Amps and watts are single phase, volts times amps with no power factor. That is the Generac table's convention and slightly overstates a motor's true power. Three-phase sets, commercial standby generators and engine or alternator sizing are not covered.
- The page works peaks up to 150,000 W. It does not derate for altitude, ambient temperature or fuel, and it does not size the cable from the generator, the transfer switch, or the feeder (NEC articles 445 and 702 were not read for this page).
- A 240 V generator gives each 120 V leg about half its rated watts. The leg check divides the running rating by 240 V and treats the legs as equal, which the manual of a given generator may refine. It does not know which receptacle or breaker a load sits on.
Common mistakes when sizing a generator
- Sizing the air conditioner to its running watts. The 3 ton Goodman GSXN403610A draws about 4,068 W while it runs at 240 V, and starting it asks for 22.1 kVA from its 91.9 A locked rotor current. With nothing else on, the full locked rotor start puts the peak at 22,284 W. A generator picked on the 4,068 W of running load alone stalls or trips on the first start.
- Leaving out loads that cycle. A refrigerator and a sump pump are not always running, and they still belong in the total. Generac's own example counts both. If either one starts while the air conditioner is running, the generator has to carry it, so the page adds them even though they are idle at the moment you look.
- Counting a soft start as a guarantee. A 65 percent cut takes the 3 ton unit from 22,284 W to 7,948 W, and 75 percent takes it to 5,742 W. Those cuts are Micro-Air's published range against the LRA. Kohler warns that a prediction made at one setting changes when the settings change in the field. Use the maker's figure for your compressor and check it against the generator maker.
- Assuming motors start one at a time when they will not. Generac adds only the largest start, 2,400 W in its example, because devices start one at a time. After an outage two thermostats can call together. Two 3 ton units start at 26,352 W when staggered and at 44,568 W when they hit at once. Pick the reading that matches the job, and use a delay relay or load management to hold the staggered case.
- Putting every 120 V load on one leg. A 120/240 V generator gives each leg about half its watts. With the example's 4,350 W all on 120 V and on one leg, that leg carries 36.3 A, against about 22.9 A for the RS5500. Spread over both legs it is 18.1 A. Balance the loads at the panel or the transfer switch.
- Treating the locked-rotor kVA as the generator size. Kohler says that, except perhaps for small motors, matching a three-phase set's continuous kVA rating to the motor's starting kVA would oversize it, since the starting power factor is low and the set briefly gives more than its rating. A portable generator's starting (surge) rating already counts that short overload, and an air conditioner compressor is a small motor, so the page holds the full locked rotor start (22.1 kVA for the 3 ton) plus the running load against the starting rating. The maker's motor starting data for a given set decides a close call.
Common questions about generator size
What size generator do I need to run a central air conditioner?
Read the nameplate first. A 3 ton Goodman GSXN403610A has a 16 A compressor, 91.9 A locked rotor amps and a 0.95 A fan, so it runs at about 4,068 W at 240 V. Its start asks for 22.1 kVA, and adding that full locked rotor start to the running load gives a peak of about 22,284 W with no other load, which is the figure to hold against the generator's starting rating. The generator maker's motor starting data for a given set settles a close call. A soft start that cuts the start current 65 percent brings the peak to 7,948 W if the maker's claim holds on your unit. Generac's table puts a 24,000 BTU/h central unit at 3,800 W running plus 4,950 W to start, 8,750 W in all. Then add every other load that will be on.
How do I size a generator for a house?
Follow Generac's worksheet. List the devices that will run at the same time, add their running watts, find the one with the highest additional starting watts, and add that figure to the total. Generac's example of a refrigerator, a sump pump, a belt sander and a circular saw is 4,350 W running plus 2,400 W, so 6,750 W of starting watts. The generator's starting rating has to be at or above that number, and its running rating at or above the running total.
What is the difference between running watts and starting watts?
Running watts are what a load draws while it operates. Starting watts, also called surge watts, are what a motor draws for the first moments to get turning. In Generac's table the starting column is the additional amount on top of running watts, 2,200 W more for a refrigerator that runs at 700 W. A generator carries its starting rating only briefly, so a long load has to fit under the running rating.
How many amps does the generator need to supply?
Divide watts by volts. The example's 6,750 W peak is 28.1 A at 240 V and 56.3 A at 120 V, and its 4,350 W running load is 18.1 A at 240 V. A 240 V load draws that current from both legs together. A 120 V load draws it from the leg it is plugged into, so spread them across both legs.
Does a soft start let me use a smaller generator?
Often, because it lowers the start surge. Kohler says starters that reduce the start current can allow a smaller generator, and Micro-Air claims 65 to 75 percent less start current than the compressor's LRA. For the 3 ton unit the peak falls from 22,284 W to 7,948 W at 65 percent and 5,742 W at 75 percent. The running watts, 4,068 W, stay the same, and the real reduction depends on the compressor and the settings, so confirm it with the maker.
Where do I find the LRA and RLA of an air conditioner?
They are printed on the condenser's data plate for the compressor, along with the fan motor amps. Spec sheets list them too. Goodman's GSXN4 sheet shows a 3 ton compressor at 16 A RLA and 91.9 A LRA, and a 2 ton at 8.4 A and 41.2 A, with LRA running 4.9 to 6.2 times the RLA across the line. If the LRA is not available, Kohler gives six times full-load current as typical, and the page uses that when the LRA box is blank.
Should I add a reserve to the generator size?
The Generac worksheet adds none, and none of the maker guides read for this page gives a percentage, so the reserve box starts at 0. Use it for growth or for uncertain loads. At 20 percent the example's 6,750 W peak becomes 8,100 W, which no longer fits the 6,875 W starting rating of the RS5500 and needs the RS7000 at 8,750 W.
Is a 120 volt load a problem on a 240 volt generator?
It can be. A 120/240 V generator gives each 120 V leg about half its rating, so the RS5500 carries roughly 22.9 A a leg. Put all of the example's 4,350 W on one 120 V leg and it would see 36.3 A. Enter the share of 120 V load in the last box to see both the balanced and the worst case.
Does this size a standby generator or a transfer switch?
No. The page follows the portable-generator worksheet and the nameplate start of a motor, for a house or a small site on single phase. A permanent standby set, a three-phase set and the transfer switch or interlock are sized from the maker's program and the electrician's load study. Peaks above 150,000 W are refused.
Does this replace the generator maker's sizing program?
No. Kohler's guide makes the point that makers differ in how they treat motor starting and that the engine, alternator, voltage regulator and starter all play in. Use this page to get close and to check a quote, then confirm the model with the manufacturer's sizing program or an electrician.