Inverter capacity is the amount of power which can be supplied continuously by the unit. The inverter rating must be at least 25% more than the total power required by all connected appliances operating simultaneously. Multiple inverters can be series-wired or parallel-wired for the required capacity.

Power, measured in watts (W), is the product of voltage and current. Your home probably has multiple electrical loads at any time. To compute the inverter's continuous capacity, you will need to add up all the loads. For example, a 500W washing machine together with a 400W television and stereo, a 400W refrigerator and 100W of CF lamps operating simultaneously would require a total power of:

500 + 400 + 400 + 100 = 1,400W

There are also intermittent loads in operation. The above example should assume a 1,100W water pump and 250W gas-heated clothes dryer as well. Add on the safety factor of at least 25% over the estimated value, as electrical loads tend to consume more power over time.

(1,400 + 1,100 + 250) + 25% = 3,440W

You should calculate the essential load value when you fill out your Electrical Energy Consumption Worksheet. Choose inverters that come in "building block" sizes. Many small models under 1,000W are available but the more common ones are 2,500W and 4,000W. Remember that home electrical loads exceeding the inverter rating will cause frequent overload tripping, which shortens the inverter's lifespan.

Some inverters' continuous rating may be in VA rather than watts. This refers to the voltage multiplied by the current - the wattage in a DC circuit. But in an AC circuit, wattage is computed by multiplying VA and a power factor.

To avoid confusion, it helps to understand that AC motor operation creates an effect called the power factor which must be included in any calculations. The average home can use VA interchangeably with watts as they are close to each other but you should reduce the inverter rating by about 20% if you have air conditioning, pool pumps, spa pumps or any high-wattage induction motor loads which have power factor effects.

For a large woodworking tool shop with many motor loads in simultaneous operation, the best solution is a de-rated inverter of at least 25% extra capacity to cater to the power factor effect. In reality, there is not much to do except be aware of the power factor while reducing simultaneous operation of motor-driven appliances - or buy a bigger inverter. Note that universal motors with brushes (when the motor is on, sparks can be seen) can operate safely with no de-rating issues as there are no power factor effects.

Universal motor examples include:

• Vacuum cleaners – regular and central
• Food mixers and processors
• Drills, shop vacs, routers, radial arm and circular saws
• Electric chain saws
• Hedge trimmers
• Electric lawn mowers

When in doubt, ask the inverter dealer about off-grid large loads. And if you operate a business like welding or woodworking and prefer off-grid living, it is better to segregate home usage from shop operations with a generator for the big loads.