Calculating lighting load relies on a single equation, watts = volts x amps, described mathematically with the following relationship:

where ‌P‌ is power (measured in watts), ‌V‌ is voltage (measured in volts), and ‌I‌ is current (measured in amps). In most lighting situations, you will know two of the three elements. From there, you can use the formula to find out the third. Household circuits in the U.S. are 120 volts, although a few circuits may have 240-volt capacity, and most allow 15 to 20 amps. If you are planning a special event with lighting or wanting to add specialty lighting to your home, you will need to calculate the lighting load to ensure you do not draw too much power and trip a circuit breaker.

The lighting load is a specific name for the electrical load of an electrical circuit. Simple circuits (like those found in lighting systems and other similar electrical systems at home) often have three main measurements/properties: voltage, current, and resistance. The voltage basically pushes energy throughout a system (think of a 9V battery that pushes energy through a circuit), and the resistance provides a regulation on this flow of energy so we can actually draw useful power from a circuit. The current is a measurement of this relationship between voltage and current, by the relationship:

where ‌I‌ is current, ‌V‌ is voltage, and ‌R‌ is resistance. In the case of a lighting system, the voltage is provided by the power grid, the resistance is a property of the lightbulb as it draws energy from the circuit, and the current is a measurement of this relationship. The power (wattage) of a system is a more compact description of this relationship. The electrical load could also apply to a wide variety of demand factors outside of just light fixtures; air conditioning, refrigerators, electrical water heaters, and many other appliances are receptacles for electrical power and carry their own electric load factors.

Verify circuit amperage by locating the breaker panel or circuit box. The breaker for each circuit should be labeled with the number of amps, often on the switch.

Multiply the amp number by the voltage. In the case of a 15-amp circuit at 120 volts, the formula would look like this:

The result, P = 1,800, means 1,800 watts is your maximum power load.

Subtract the wattage of any other loads on the circuit, such as appliances, portable heating units or office equipment. The number of watts left is your maximum load available for lighting. When we look at load calculations it can also be important to look at the amount of time appliances might be drawing power. According to the National Electrical Code (NEC) If something might be drawing power for three or more hours, it is considered a continuous load. Because we are looking at general lighting loads, we can assume all of these to be continuous loads.

Calculate the load your lights will place on your circuit by adding together the wattage of each lamp. For example, if you are calculating the lighting load for a special event that uses four 500-watt lights, multiply:

which is your load. Plug the load into your formula. In this case, you have 2,000 = V x A. Since standard household power is 120 volts, you can add that to your formula too, resulting in 2,000 = 120 x A. In math terms, 120A is the same as 120 x A. We can use 240 instead of 120 if you are on a 240-volt circuit.

Divide the wattage by 120 to even out the equation.

This means you need a circuit with a 16.67 or higher amp capacity to place the load of all four lights on that circuit. If you find that your circuits are only 15 amps, you must find a separate circuit for your fourth light.

The pre-existing load and types of power demand on an electrical system can vary greatly. Often, the occupancy, floor area, and lighting design plays a major role in how many lights, heating sources, and even distinct electrical systems there are. In a single family dwelling there might only be one electrical meter to connect to the power grid, and they might have a very low power draw compared to a multifamily dwelling with multiple meters and units.

The type of appliances used also has a huge effect. More efficient LED light bulbs draw much less power (and exert less of a load) than older incandescent bulbs. Newer water heaters might also exert less of a heating load on a system because they are now engineered to use power more effectively.