There is a number on the side of every generator, and most homeowners learn what it means during an outage. Whether a generator can carry the refrigerator, the well pump, or the air conditioner is a matter of adding up loads, and the adding is manageable once you know which figures to use. This post explains how generator capacity works, what a typical home here draws, and why heating and cooling equipment is the part of the math that trips people up.
Two numbers for every appliance
Generators are rated in watts or kilowatts, the total the unit can supply at one time. Each appliance has a running draw, the power it uses once going, and anything with a motor or compressor also has a starting draw, a short surge of two to three times the running figure. A refrigerator may run at 700 watts and start at 2,000. The generator must cover the running draw of everything already on it plus the starting surge of whatever comes on next, and when it cannot, it trips its breaker or stalls. Overloading also harms appliances, since a straining generator feeds unstable voltage to everything connected.
What a coastal home typically draws
Outage loads fall into three groups. The small ones, lights, chargers, a television, and the router, total a couple of thousand watts running, and any generator manages them. The medium ones are what families care about: a refrigerator and freezer at roughly 700 watts apiece running, a well pump at 1,000 to 2,000, a sump pump at 800 to 1,500, and a gas furnace or boiler, which needs only a few hundred watts. A portable unit of 5,000 to 7,500 watts can handle this group if loads are started one at a time.
The large loads are where a coastal home diverges from an inland one. Central air runs at 3,000 to 5,000 watts and needs two or three times that to start. An electric water heater draws 4,500 watts, an electric range 8,000 or more, and a dryer around 5,000. None of these belongs on a portable generator.
The heat pump complication
Most homes in Hampton Roads heat with a heat pump, which changes the sizing math in a way that is easy to overlook. Cooling, a heat pump draws roughly what an air conditioner does, but heating on a cold night it also switches on electric auxiliary strips to replace the heat it cannot pull from 30-degree air, and those strips alone draw 5,000 to 15,000 watts. A generator that carries the house through a July thunderstorm can be overwhelmed by a January nor’easter, which is the outage when heat matters most. There are two remedies: size the standby unit for the strips, which generally means a larger generator, or install a load management module that lets the heat pump run while locking out the strips and other big loads whenever the generator nears its limit. When our technicians size a whole-home generator, they read the strip rating off the equipment nameplate rather than estimating from the square footage of the house.
The limits of a portable unit
A portable generator falls short in three ways. It cannot start central air or a heat pump in most homes, because the compressor’s surge exceeds what a portable delivers. It cannot power anything hardwired into the house, such as a well pump, a furnace blower, or a wired sump pump, unless an electrician has installed a manual transfer switch, and plugging a generator into a wall outlet instead pushes electricity back into the utility lines, where it can kill a line worker. And it cannot run on its own, since it needs fuel every several hours and produces carbon monoxide that must stay far from the house. A portable unit will keep the food cold and the lights on, and it can keep a sump pump running through a storm if someone is home to tend it, but it will not keep a Hampton Roads house cool in August or warm in an ice storm.
What a standby unit does when it is sized correctly
Connected through an automatic transfer switch, a standby generator powers the circuits it was designed for without anyone starting it or plugging anything in. A smaller unit of 10 to 14 kilowatts usually covers the essentials: refrigeration, the well or sump pump, the furnace or part of the heating system, lighting, and outlets. A larger unit of 18 to 26 kilowatts covers a typical whole house including air conditioning, and with load management the heat strips too. Since Russell’s American Mechanical does the electrical work as well as the generator, the transfer switch, any added panel capacity, and the load management come from one visit.
Getting the most from any generator during an outage
Even a correctly sized generator does better with a little management, so bring large loads on one at a time and let each settle before adding the next. Leave the electric range, dryer, and dishwasher off unless the generator was sized to include them. Set the thermostat a few degrees warmer in summer and cooler in winter so the system cycles less. When the outage ends, write down what ran and what did not, since that list is the best guide to whether the generator you own is the one you need. Our generator service visits include a load review for that reason, and members of the Generator Care Agreement receive 20 percent off service and diagnostic fees.
Russell’s American Mechanical has served Hampton Roads since 1977 and installs and services standby generators across the region. If you are unsure what your current unit will handle, or you are deciding what size to buy, schedule a visit online, through the website chat, or by phone. A technician will total your loads from the equipment itself, explain what the figures mean, and give you a firm price at everyday pricing, with financing available.