Size a mobile-home generator from the appliances you intend to run together, then check motor starting demand and the required supply voltage. Floor area can guide an early estimate, but electric heat, a well pump, or central air conditioning can change the answer far more than another bedroom.
Open the Mobile Home Generator Size Calculator.
Start with the outage plan for your mobile home
A generator for food storage, lights, communications, and a furnace blower has a different job from one expected to run every installed appliance. Decide which job matters before comparing generator labels. Otherwise, a broad promise such as whole-home backup hides the loads that make the system expensive or impractical.
Write down what must continue automatically, what someone can switch on occasionally, and what can remain off. Water supply may belong in the first group if a well pump serves the home. An electric dryer usually fits the last group during a short outage. Cooking and water heating require a deliberate decision because both can overlap with climate control.
Also identify whether the home has propane heat, electric resistance heat, a heat pump, or another system. A gas furnace still needs electrical power for its blower and controls, but that demand is very different from supplying electric heating elements. The outdoor shape of the home does not reveal that difference.
Treat medical equipment and other essential loads according to their own backup requirements. A household generator estimate does not establish uninterrupted operation through generator startup, refueling, or a fault. If a device must never lose power, account for that separate requirement in the backup plan.
Build an appliance list with running and starting values
Use equipment labels and manuals rather than assigning the same wattage to every major appliance. Record voltage, running watts or amperes, and available starting information. Note whether the figure describes electrical input or useful output. A microwave cooking rating, for example, is not necessarily its supply demand.
For a single-phase load, apparent power is volts multiplied by amperes. Real power is volts multiplied by amperes and power factor. When you only know voltage and current, the product is a useful VA figure, but calling it measured watts can overstate your knowledge. A generator must meet both real-power demand and the applicable current limits.
Create a simple worksheet like this illustrative one. These figures demonstrate the method and are not specifications for a particular appliance.
| Planned load | Running demand | Starting allowance used here | Supply |
|---|---|---|---|
| Refrigerator | 180 W | 900 W total at start | 120 V |
| Furnace blower and controls | 600 W | 1,800 W total at start | 120 V |
| Lights, router, and charging | 220 W | No extra allowance assumed | 120 V |
| Well pump | 1,000 W | 3,000 W total at start | 240 V |
| Microwave input | 1,400 W | Evaluate separately | 120 V |
Keep evidence beside the entry: manual page, manufacturer support response, or a properly obtained measurement. That makes the worksheet much easier to revise when an appliance changes.
Work through a mobile-home backup example
Consider an illustrative home with fuel-fired heat, the listed well pump, and a plan to run the microwave only when convenient. The essential running load is 180 + 600 + 220 + 1,000 = 2,000 W. With the microwave operating, it becomes 3,400 W.
Now examine starts. If the well pump needs 3,000 W in total during its start, its additional demand above the included 1,000 W running load is 2,000 W. The simplified starting scenario is therefore 3,400 + 2,000 = 5,400 W while the other listed loads run.
Do not calculate 3,400 + 3,000 and call 6,400 W the same scenario. That would count the pump’s running demand twice. Likewise, do not assume every motor starts together unless the control sequence or automatic restart behavior makes that plausible.
The furnace blower in this example adds 1,200 W above its running figure during its own start. If blower and pump start together, the simplified scenario becomes 6,600 W. If the microwave is deliberately off during those starts, it becomes 5,200 W. This is why a realistic operating plan can matter more than a square-foot estimate.
These watt totals are screening calculations. Starting current, power factor, voltage dip, and the generator’s transient response still need checking. Cummins treats load starting and sequencing as separate sizing considerations in its generator application manual.
Why heating and cooling can dominate the answer
Electric resistance heating introduces a large steady load. If a furnace has 15 kW of energized elements and a 0.6 kW blower, that is already 15.6 kW before refrigeration, lights, or water supply. Running a single 1.5 kW room heater is a different scenario, but it also heats a different portion of the home.
A heat pump needs another worksheet. Its compressor has running and starting requirements, and auxiliary resistance elements may energize in cold conditions or during certain operating modes. Do not size for the compressor alone if backup strips can switch on automatically. Record which stages the installed controls permit under generator power.
For central air conditioning, use electrical specifications and manufacturer starting guidance. Cooling capacity in BTU/h describes heat removal, not electrical watts. Dividing that rating by a universal number cannot establish the generator requirement across different equipment designs.
Hot weather and cold weather may also create different coincident loads. In summer, a pump might start while the air conditioner runs. In winter, furnace demand may overlap with water heating. Build separate seasonal cases rather than piling every installed appliance into one impossible scenario.
Use the mobile-home calculator as an initial estimate
The calculator combines floor area, a climate-control category, an appliance count, a pump allowance, and a percentage margin. Its floor-area component is 3 W per square foot, and its appliance component assigns 800 W to each counted appliance. Those are planning assumptions built into this tool.
For an illustrative 1,000-square-foot home, minimal climate control, two major appliances, a pump, and the medium margin, it calculates (3,000 + 1,600 + 1,000) × 1.25 = 7,000 W. This tells you what the selected assumptions produce. It does not establish that your particular refrigerator, pump, or heating system has those demands.
Be careful about overlap. If the area allowance is already standing in for household usage, adding a detailed list of the same usage without understanding the formula can duplicate it. The calculator is most useful for trying broad scenarios, then replacing that estimate with a documented appliance schedule.
The climate-control menu is especially coarse. A category labeled for central air conditioning or an electric furnace cannot capture the full range of electric-furnace element ratings. If heat must run, use its actual electrical input rather than accepting a category allowance.
Check voltage, receptacles, and each supply leg
A total watt figure is only useful when the generator can deliver it at the right voltage and through the intended connection. A 120 V-only generator cannot directly supply a 240 V well pump simply because its watt rating looks adequate. Check the generator output configuration and the load nameplates together.
For a 120/240 V split-phase system, 120 V loads occupy individual legs. A heavy concentration on one leg can reach an output limit while the combined watts remain below the headline rating. Have the circuit schedule and proposed backup arrangement reviewed as part of selecting the equipment.
The inlet, cable, transfer equipment, and protective devices also limit delivery. An adapter changes a connection shape; it does not increase generator capacity or establish that the installation is suitable. Permanent household connection needs appropriate transfer protection that prevents unintended connection to utility power.
Use continuous or rated output for the sustained plan. A short-duration maximum figure may help describe temporary capability, but it is not interchangeable with the running rating. Confirm whether the selected fuel changes the available output as well.
Make a practical load-management schedule
If a smaller generator can carry the essential loads but not all discretionary appliances together, write an operating schedule that people can actually follow. For example, leave the water heater and dryer off, avoid microwave use during known heavy loads, and allow the pump to complete a cycle before adding another major appliance.
Manual sequencing is useful only when it remains credible during an outage. An automatic pump or thermostat can start without warning. A plan that depends on guessing those moments needs more reserve or a professionally designed load-management arrangement.
Check fuel endurance at the expected load using the selected generator’s data. A runtime quoted at a light load does not describe your heavier winter scenario. Allow for refueling and routine checks without assuming the unit will run unattended for the entire outage.
Portable engine generators belong outdoors, at least 20 feet from the home, with exhaust directed away, following CPSC guidance. The space beneath a mobile home, an attached enclosure, and an open garage are not appropriate substitutes for outdoor placement.
Questions to resolve before choosing a rating
Is a larger mobile home guaranteed to need a larger generator?
No. A smaller home with electric resistance heat, electric water heating, and a well pump may have a much larger electrical backup requirement than a larger home that uses fuel-fired heat and backs up only essentials. Area matters indirectly through equipment and use.
Can I leave electric heat out of the calculation?
Yes, if the outage plan reliably keeps it off and provides an acceptable alternative. A thermostat setting alone may not prevent every auxiliary stage from operating. Verify the actual control arrangement rather than assuming that an unwanted load cannot energize.
Is a percentage margin enough for motor starts?
A margin creates planning room, but it does not characterize a motor or a generator. Compare the documented starting requirement with the generator’s specified capability and acceptable voltage and frequency response. Revisit the calculation when a pump, air conditioner, or heating system changes.