Electricity usage question.

[BLACKPANTHER]

I have just started using a free energy monitor I received when I switched to N-Power. It's a really handy gadget, and shows instantly exactly how much juice each appliance uses, (I'm glad we don't drink tea....the kettle sends it way up!)

So here's a puzzler. My 6x60w spots register as using 290w on full, 50w on lowest dim and interestingly, 330w when 1/2 dimmed! The First Law of Thermodynamics states that energy is always conserved, it cannot be created or destroyed.

I can only assume that maybe the bulbs heat up more when 1/2 dimmed?


Oh, here's another question. As this energy meter works by clipping around the live cable in the meter box, couldn't something similar be used to tell how much your bike battery is putting out whilst in use. This could be a really stupid question....maybe there's something like that already for sale?

 

[Synthman]

Thats funny because the other day I was playing with the dimmer switch and the power used didn't correspond to the 3x 60w bulbs. Most items use the same as the rating plate, e.g. 115w television shows up as 112w on the meter, 1500w heater using slightly less according to the meter. New vacuum cleaner rated at 2000w uses just 1000w according to the meter.

 

[NRG]

Incandescent tungsten light bulbs exhibit lower resistance at lower temperatures so will draw more current when cold than when hot. It may explain the difference....

Actually that's a load of rubbish but it's interesting all the same.

 

[flecc]

Thats funny because the other day I was playing with the dimmer switch and the power used didn't correspond to the 3x 60w bulbs. Most items use the same as the rating plate, (but) New vacuum cleaner rated at 2000w uses just 1000w according to the meter.

There are different kinds of watts:

First there's the actual watts that cost you money to your electrical supplier.

Then there are Marketing Department sales watts. For example, your company has a 1000 watt vacuum cleaner, a rival announces a 1500 watt one so you change the label on yours to read 2000 watts. These numbers are invariably higher than the actual watts, though see below.

Vacuum cleaners also have "air" watts, the stated rating according to suction power. These are used as part of the rating war between vacuum cleaner producers, knowing that consumers have no way of measuring these.

There's also legal watts, the ones used by e-bike manufacturers. Regardless of the actual motor power, it's stated to be the legal wattage of the country it's being supplied to, for example, 250 for the EU, 200 for Australia or 300 for New Zealand. These numbers are almost always lower than the actual watts.

And finally there's the "green" watts, used when the environmental objectives of a product are being extolled. These numbers of course are also lower then the actual watts. These commonly appear as the stated consumption for something in "Standby" mode.

Cynical? Not a bit of it. Heinzmann through a dealer have admitted that they have labelled the same motor as 400 watts or 700 watts, and not a single e-bike manufacturer correctly states their motor's continuous power capability. I also once had a Freeview box which claimed 8 watts On, 2 watts Standby, but was actually 8 watts On, 7 watts Standby.
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[clckn2000]

Dimmers

I have just started using a free energy monitor I received when I switched to N-Power. It's a really handy gadget, and shows instantly exactly how much juice each appliance uses, (I'm glad we don't drink tea....the kettle sends it way up!)

So here's a puzzler. My 6x60w spots register as using 290w on full, 50w on lowest dim and interestingly, 330w when 1/2 dimmed! The First Law of Thermodynamics states that energy is always conserved, it cannot be created or destroyed.

I can only assume that maybe the bulbs heat up more when 1/2 dimmed?


Oh, here's another question. As this energy meter works by clipping around the live cable in the meter box, couldn't something similar be used to tell how much your bike battery is putting out whilst in use. This could be a really stupid question....maybe there's something like that already for sale?

I sometimes wonder if dimmers don't just dump the unwanted power somewhere else rather than the light bulb.

 

[flecc]

I sometimes wonder if dimmers don't just dump the unwanted power somewhere else rather than the light bulb.

Only a resistive dimmer would do this, losing energy as heat.

Household dimmers aren't like that though, instead they curtail the AC waves, only using part of the supplied waveforms of the mains. In fact in the early days of dimmers they caused a huge problem for energy generators. They all operated on only one half of the waveform and were all on the same half. As a result the main waveforms were severely distorted by the sudden popularity of dimmers and power station generators suffered all manner of problems as a result. These half-wave dimmers were banned in consequence and the present ones cause no problems.
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[Deleted member 4366]

Oh, here's another question. As this energy meter works by clipping around the live cable in the meter box, couldn't something similar be used to tell how much your bike battery is putting out whilst in use. This could be a really stupid question....maybe there's something like that already for sale?

Only works with ac. When current flows through a wire, it makes a magnetic field around it. If you put a coil around (or near) it and change the current in the wire, there's a corresponding change in the magnetic field, which, by weird science induces voltage into the coil proportional to the change in magnetic field. The faster the current in the wire (and its magnetic field) changes, the bigger the induced voltage. That's how your device works - it's simply showing the induced voltage. Unfortunately, with DC, the current doesn't change enough to induce a meaningful voltage compared with ac, where it's shooting back and forth 50 times a second.

 

[rog_london]

There's also the business of 'power factor'. As mentioned, incandescent lamps are more or less purely resistive - so exhibit a power factor of 1 and generally all watt meters agree on what is being consumed.

Almost everything else - and that tends to include incandescent lamps on dimmers - don't manage that as they don't present a purely resistive load. Trying to keep the explanation as simple as possible, the AC current lags or leads the applied voltage depending on the type of load, and this produces what can be quite a large difference in the true watts being consumed compared with the VA (volts x amps) figure, which can be a lot higher.

Some watt meters allow you to display power factor as well as either true watts or VA. Most loads are partly resistive, partly inductive, and it used to be common practice in industrial electrical installations to install power factor correction capacitors which were automatically switched in and out of circuit to improve the overall power factor. The generating companies don't like 'poor' power factors because the VA figure relates to heat generated in the delivery system and therefore wasted. It used to be that they would penalize commercial users for poor power factor (the electricity meter would incorporate a PF recorder) - perhaps they still do, I'm not up to date with present conventions.

Fluorescent lamps which use a magnetic (i.e. non-electronic) ballast generally incorporate a power factor correction capacitor for this reason.

If you have used a mains inverter (mentioned on here from time to time) - an electronic box which connects to a 12 volt car battery and magically converts that to a 'mains' supply' - you'll probably have discovered that one of the limitations is that they don't like certain loads and will refuse to operate with them. Sometimes that's because the start-up surge is too much for the inverter, but more often it's because whatever you're trying to power has a poor power factor - some 'cheap' energy-saving lamps, TVs, computer power supplies, all can be a problem.

Rog.