There is a bitter irony in home backup power. The appliances you most need when the grid fails are the ones most likely to defeat the generator you bought. Water, heat and a dry basement all depend on electric motors, and motors are the hardest thing a small generator is ever asked to do.
Why motors are hard
A stopped motor has no back-EMF opposing the applied voltage, so at the instant of switch-on it looks nearly like a short circuit. The resulting inrush current can be three to seven times the running current, and for a motor starting against a load, such as a pump already full of water, it lands at the high end. It only lasts a fraction of a second, but a generator that cannot supply it will sag in voltage, the motor will fail to accelerate, and now you have locked rotor current flowing continuously into a motor that is heating rapidly. That is how a nuisance becomes a burned winding.
Well pumps
Submersible well pumps are usually 240 volts and typically 1/2 to 1-1/2 horsepower. A 1 hp pump might run at about 800 to 1,000 watts and surge to 3,000 or more. Three consequences follow.
- You need 240 volts. Many small portables are 120 volt only and simply cannot run a well pump at all, whatever their wattage.
- You need surge capacity. The pump is usually the largest single surge in the house and therefore sets your generator size.
- Cycling matters. Every time the pressure tank drops below its cut-in point, the pump restarts and surges again. A properly pressurised tank with a larger drawdown volume means fewer starts, which is easier on both the pump and the generator. Checking the tank's air charge is a ten-minute job worth doing before storm season.
If your generator is marginal, a soft starter on the pump is the fix. It ramps voltage up over a second or two, cutting inrush dramatically, and often lets a generator two sizes smaller do the job.
Furnaces and boilers
A gas furnace burns gas but is controlled electrically, and it will not run at all without power. The load is modest, usually a 1/3 to 1/2 hp blower drawing 500 to 900 watts running, plus the igniter and control board. What matters here is not size but quality of power.
Modern furnace control boards are microprocessor-based and check the incoming waveform. Many will not run on the rough output of a cheap conventional generator, and some detect reversed polarity or an improperly grounded neutral and lock out with a fault code. This is one of the strongest arguments for an inverter generator, or for feeding the furnace from a battery inverter with a true sine wave output.
Two details catch people: the furnace is usually hardwired, not plugged in, so powering it means a transfer switch or an interlock rather than a cord. And an ECM variable-speed blower is a motor with its own electronics; check the manufacturer's guidance before assuming it will tolerate generator power.
Sump pumps
The sump pump is the appliance whose failure costs the most money per minute. A 1/3 to 1/2 hp pump draws 600 to 1,000 watts running and surges to 2,000 or 3,000, and unlike the furnace it may cycle every couple of minutes during heavy rain.
Because the risk is so concentrated, this is where layered backup pays. A dedicated battery backup sump pump, with its own float switch and its own battery, keeps working when the generator will not start and while you are away. Treat it as insurance rather than as part of the generator plan, and test it by lifting the float twice a year.
Two more loads worth planning for
Central air conditioning is the largest surge in most homes: a 3 ton compressor can demand 5,000 watts or more at start. If air conditioning is a medical necessity in your climate, size for it or fit a soft starter, which is common practice in the RV world and increasingly on residential units.
Refrigerators and freezers are individually small but restart on their own schedule, and two of them can surge at the same moment. Keeping the doors closed matters more than powering them continuously: a full freezer holds temperature for a couple of days if left alone, so cycling power to it for a few hours at a time is a legitimate strategy on a small generator.
Practical strategies
- Stagger startups. Never bring everything online at once. Start the generator with no load, then add circuits one at a time, largest first.
- Fit soft starters on the well pump and the air conditioner. Cheaper than the next generator size up.
- Use load management on a standby install so the controller does the staggering for you.
- Prefer inverter output for electronics-heavy loads: furnace boards, ECM blowers, medical devices, computers.
- Layer the critical items. A battery unit that keeps the sump pump and the furnace board alive while the engine is being refuelled removes the worst gap in the whole system.
Design around the surge, not the average, and around the failure you can least afford. Ready.gov's outage guidance is a useful checklist for the non-electrical half of the same problem: water, food safety and heat.