HOW TO DETERMINE THE RIGHT WATTAGE OF A HOME GENERATOR

I'll show you how to properly determine the required wattage, but first I need to explain the basic configuration of the wiring. Most residential houses in U.S. have single-phase three-wire service (see wiring diagram). In essence, you have two separate 120V lines connected to two busses in your main panel with common neutral. Large appliances such as a central a/c or a dryer run off 240V and therefore draw equal currents from both lines. However, all other devices draw current only from one of two lines. As the result, two busses in most cases are imbalanced. A home generator likewise has two 120V outputs with common neutral, each of which can supply not more than half of its total rated wattage. That's why it is not enough to know your lump power consumption- you need to know how it is split between L1 and L2. Let me illustrate it with the following example. Suppose your house consumes P=8kW. If this load splits equally between L1 and L2, in theory you could use an 8kW genset. However, if you have for example P1=6 kW and P2=2 kW, you would need a 12kW system to be able to supply 6kW on the first bus. Of course, it is always recommended to have at least 20% derating, but we will get to that later.

HOW TO MEASURE YOUR HOUSE POWER CONSUMPTION

To properly size a whole-house generator, the first thing to do is to measure electric current on each of two lines of your home. If you have a proper electrical training and know how to work safely with AC voltage you can do it by yourself. You will need special rubber gloves and a clamp meter preferably with peak reading function. The cover from the main distribution panel has be removed in order to reach L1 and L2. If you found a significant imbalance between them, ask your electrician to swap appropriate breakers in the circuit panel to better balance the loads. Once you know final I1 and I2, the minimum required wattage of your genset is the larger of these two currents times 240 plus 20% margin:

GENSET RATING(watts)= max(I1,I2)×240×1.2

For example, if I1=40A, I2=30A, you would need 40×240×1.2=11,520 watt system. If you have a central a/c or another high-power motor-driven appliance, you may have to increase the size of your system to handle high startup currents. To determine the requirement you would need to measure initial surge current of your device by using peak function of the clamp meter and choose a generator with corresponding surge capability (read about sizing a generator to start a/c or motors). Unfortunately, generator manufacturers rarely specify their surge characteristics for standby models. As a rule of thumb, most fixed models can briefly provide at least 50% more current compaired to their continuous rating. For more details and step by step sizing procedures for various setups, see my Home Generator Guide.