# Florescent bulbs starting energy

1. Jul 14, 2006

### Pengwuino

Now Ive heard this so much that i just have to get a complete answer to it. Since they run on AC current, i wonder "Ok maybe this myth is right because i don't have a good enough understanding about alternatig current". The idea is that a flourescent bulb (sp?) uses more energy turning on then it will take to run for 30minutes to 1 hour. I figured i could try to guesstimate the OOM with DC current figuring "ok i dont think the estimation can be THAT far off using DC principles".

When i did the math.... ok yah you need a power cable capable of 'hooking up' directly to a nuclear reactor . So im skeptical! I'm calling on hte experts! Does it require more energy to keep a flourescent bulb on for 30 minutes then it does to start the thing?

2. Jul 14, 2006

### FredGarvin

3. Jul 14, 2006

### Pengwuino

Yah that's what i've wondered about. I had looked them up and figured there must be a large jump in current... but THAT big? Obviously not...

4. Jul 14, 2006

### SGT

The starting power is much greater than the average one, but it lasts for only a short time, so the energy (power x time) is not great.

5. Jul 14, 2006

### Staff: Mentor

I think this misconception may have come from the fact that HID lamps need to be heated for several minutes before lighting, and that does use a significant amount of power.

Last edited: Jul 14, 2006
6. Jul 14, 2006

### FredGarvin

I don't like being next to one of our metal hailide lamps when they turn on. It kinda scares me.

7. Jul 14, 2006

### Pengwuino

So this is all officially a 'myth'?

8. Jul 18, 2006

### Rach3

Basic estimation will get you far. Residential circuit-breakers typically max out at 15A; at 120V this means the instantaneous power consumption of all appliances on that circuit, can never exceed 1800W. Even a 100ms (~6 cycles) startup transient at the maximum possible current, which is clearly a strong upper bound, would consume 180J; a 20W fluorescent bulb would consume as much in 9 seconds of normal use. So there you go.

The OP mentions "more energy turning on then it will take to run for 30minutes to 1 hour." In an hour, a 20W bulb consumes 72,000 Joules; it would take 40 seconds for any appliance to consume this much without tripping the breakers. This of course is grossly unresonable for a transient.

Last edited: Jul 18, 2006