Q&A for How to Calculate Wattage

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What is the advantage of going with more power than recommended, for example an extra 50-100 watts?

Community Answer

Assuming you mean the supply side (it is not recommended to go over on the consumption side), the advantage would be having more "room" in case you needed to add something to the circuit. Your power supply might be able to handle the addition without needing to be replaced.

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If I use a 600 W inverter from my 12 V cigarette socket in my van to power a mini fridge, small TV, and to charge mobile devices, will it damage the battery in my van?

Community Answer

No. It will not damage the battery, but the inverter can damage your devices. Most inverters do not put out Smooth AC sine wave; they put out Square Wave or Sawtooth Wave. Many devices do not like to operate using those waves. Also, many car cigarette lighters are only rated for 10 Amps. You can pop the car's fuse.

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How do I find the resistance of a 40 watt light bulb?

Community Answer

If the bulb is in a socket, take it out. Using an ohm meter, connect the probes to the contact points on the bulb (for most bulbs this will be the threads and a little spot in the middle of the bottom). Set the range for ohms to the highest setting, then work your way down to see if you get a reading.

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Question

How do I find the watts of something when I know the voltage?

Community Answer

You need to know either the resistance or the amps going to what you are powering. If you are trying to find one of these in a real life electrical application, you can find either of these using a multimeter.

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Question

How many watts does it take to run two motors?

Community Answer

It depends on the size of the motor you are trying to run. Your motors will probably have some sort of voltage, RPM, and/or amperage rating, and depending on those numbers, you can determine how many watts you need.

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Question

How much current does a 60watt light bulb use on a 120 ac line?

Sherlock Ohms

Community Answer

Amps x Volts = Watts. 120 V x .50 Amps = 60 Watts. 120V x 1 Amp = 120 Watts.

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Question

If I drove my Tesla for 1,778 miles and the power company charges 0.11068 KW, what will my total charge be?

Community Answer

Assuming the Tesla gives you a 125 MPGe rating with a KWh battery capacity of 50 KWh, that gives you 220-mile range (base model 3). You can calculate the amount due using the following method: 50 KWh x (1178 miles / 220 miles) = 267.73 KWh. 267.73 KWh x $0.110681 = $29.63.

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Question

How do I calculate the wattage of a soldering iron with a hot resistance 4K ohm and an operating voltage of 200V?

Community Answer

To find power (P) when voltage (V) and resistance (R) are given, as in this case, you need to find it by using P=(V*V)/R. So here, P = (200 V & 200 V)/(4K ohm).

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Question

How can I measure the startup and running watts for my 4 ton A/C? Need info to determine what size generator to buy.

Community Answer

Best would be to contact the manufacturer of your AC. To get running watts, you could use a clamp-on ammeter to get the current draw on each leg and then calculate the total watts. The startup power is going to be more difficult, as you need a meter that can measure and record instantaneous current. There are devices that will do this from companies like Dranetz, but they are expensive; you may be able to rent one.

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How do I calculate the amperage of bulbs?

Community Answer

Find the number of watts (sometimes it can be found on the base of the bulb in small print). Divide the watts by the voltage. The voltage will depend on where the bulb is being used. In the US, voltage is usually 120 for the wiring to a household light fixture. For example, if the bulb is 12 watts: 12 divided by 120 = .1 A (one tenth of an amp). If you want to figure out the amperage of a bulb in a car, almost all cars use a 12-volt battery. So, assuming the car is not rare or very unusual, you would divide by 12 volts. For example, if the bulb is 24 watts: 24 divided by 12 = 2 amps.

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Question

What is the difference between an amp and a volt amp?

Community Answer

An amp is current. Think of electrical wire as the path of a river. The current is like the water. If too much current flows through the path, it's like the wire is being flooded, which is dangerous. A volt amp is the same as wattage. V times A = W, so if an appliance tells you the volt amps, you don't have to multiply to get the wattage, and if you want to know the amps, you'll need to find out the voltage of the circuit being used to supply power. Then you can divide the volt amps by the volts to find out the amps. Example: 1100 volt amps divided by 110 volts = 10 amps.

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Question

How long does a 100 Ah battery last?

Community Answer

If you connected 10 amperes of load, it will last for 100Ah/10 hours. That's 10 hours.

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Question

If something is 3500 watts at 230 volts how many watts would it be at 208 volts?

Community Answer

That would be 3165. First you need to find out amps: 3500/230=15.21839 amp. Then using the formula: W=VxA plug-in numbers, 15.21839x208=3165.

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Question

Can I charge a DC battery directly from a solar panel?

Community Answer

Only if you monitor it. It can overcharge without monitoring, potentially causing an explosion or damaging your panel by backfeeding.

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Question

Bulb is 120v, 60Hz, 95mA. Does this mean the bulb is a 11.4 watt bulb?

Ricky B

Community Answer

The bulb uses 11.4 watts, if it is an incandescent light bulb then it is a dim bulb. If it is an LED, CFL or Halogen bulb then there are other factors involved it will be marked x number of lumens for the brightness.

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Question

I have a cordless screwdriver charger which says AC 100V 50/60Hz 3.75VA and I need to use it in India, and need to know how many watts converter do I need for this?

Ricky B

Community Answer

In India, the voltage at the outlet is 230 volts. Do an online search for "travel adapter for India" and you will find all the information you need for your trip.

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