Watts, volts and amps. I still don't get it

I noted in my reply here, that a battery may have true specific specs. But the tool may be a poorly designed energy wasting piece of crap. Quick to the landfill too.

Actually, it doesn’t. It tells you how much power it consumes. Marketing people have realized you can sell an inefficient appliance as “better” than an efficient one. Blenders are a big area where they push wattage over performance.

Over here it was vacuum cleaners, all leapfrogging one another with how many watts they used.

The EU finally stepped in:

From 1 September 2014, a new EU energy label for vacuum cleaners means manufacturers will not be able to make or import vacuums with a motor that exceeds 1,600 watts.

~~ a large motor size does not guarantee impressive suction. Our independent testing has found many Best Buy vacuums under 1600w, proving that clever engineering and a well-designed floorhead are equally, if not more important, than a powerful motor.

Read more: EU rules cut vacuum cleaner power – Which? News - Which?

Note that BEST BUY here does not refer to a store of the same name.

True; one should distinguish between input power and output power. High output power means the device actually is doing more for you. High input power just means you’re paying more for it. It’s still relevant, though, in that output power can’t be any higher than input power, and for a well-designed device, will usually be only a little lower.

As I and others noted, the watts, voltage, amps specifications do not convey the overall effectiveness of the thing you are buying.
Generally, if the item is a battery, you can get a good idea of its power. You need a compatible voltage. Then you consider the Amp Hour or Milli-amp Hour numbers. More is better.
The tool that is being powered by the battery is an all too fuzzy component to decode. I noticed a post regarding vacuum cleaners. That is a good example. Vacuum is an easily measured quantity. Vacuum cleaners that I have seen have only on/off. But I do not see what the vacuum is in PSI.
A vacuum cleaner that produces X negative pressure at 12 volts while consuming 2 Amps is burning 24 Watts an hour.
But a better designed 12 Volt vacuum cleaner may produce X negative pressure while consuming 1 Amp. So burning only 12 Watts an hour.
Same battery will operate the second vacuum cleaner twice as long.
But as noted by others. Actual use effectiveness versus power consumed is usually absent in the specifications. Though it is easy to test and report.

Vacuum cleaners are a mess. They are not usefully characterised by the pressure drop they create. They can be characterised by efficiency, but even then there is no one figure of merit. The efficiency for maximum suction is zero, since there is no air movement. The efficiency at maximum flow rate is not useful, as it isn’t generally sucking up dirt then. So you need an efficiency measure when the vacuum is operating in a sensible fashion, and with dirt bag neither full nor empty, but some sort of representative level.
Properly designed vacuum cleaners have a motor and fan design that maximises efficiency at the right point. You get maximum flow with just the right amount of suck and maximum motor efficiency. In reality it shouldn’t be hard for even the cheapest to get this right. The difference between the cheapest and the best is not massive, but you pay for build quality and reliability.

Pretty much agree there.
The basic PSI at the open throat with a new bag should be fair easy to max out with basic design. I do wonder how much power is going into creating the horrendous extra whine that some of them make :slight_smile:

Sound itself carries very little energy, but a lot of noise is often an indicator of inefficiency in general, even though almost all of the wasted energy is actually going to heat, not sound.

The actual effectiveness of a vacuum cleaner is going to be in how much dirt it sucks up, which is itself a many-dimensioned quantity. I might have one vacuum cleaner, for instance, that’s so effective that it even manages to pull pet hair off of upholstery, but that vacuum might take a lot longer to pull ordinary dust off of the floor of a large room (while the one that does the floor quickly might not remove any pet hair at all) And which one is most effective might be different on shag carpet, packed carpet, or hardwood. Further, you can have two different vacuums with identical motors and fans, but where one is much more effective than another due to a different design of nozzle, or something of that sort.

Thank you, one and all, for … clarifying my original question. With your accumulated wisdom at hand, I recently purchased the following items from Harbor Freight:

  • A lightbulb rated at 300 horsepower

  • A new kitchen range that consumes just 13 milliwatts

  • A lawn edger that operates at 23,000 hertz

  • A 12-cylinder snowblower–with side-mounted phased plasma rifle in the 40-picowatt range

I’m still on the fence about the electric car that’s advertised to get 40 miles per gallon.

It may distress you to know that the EPA actually does rate electric cars in terms of miles per gallon equivalent (MPGe): :wink:

Forgot to mention that bigger batteries do have more gallons of Watts in them :slight_smile:

No free lunch, but we can reduce energy waste by using higher voltage. This is, after all, why we send electricity across the countryside at tens of thousands of volts instead of just 120 volts.

Yes, if you just swap battery packs in any given appliance, that’s what you’ll get (assuming you don’t start a fire). but suppose there are two competing battery-powered lawn mowers. One uses a 20-volt battery pack rated for 10 amp-hours, the other uses a 40-volt battery pack rated for 5 amp-hours. In other words, same total energy content in each battery. Both mowers provide the same mechanical power output to the blade. I would expect one of two things:

  • The 40-volt mower will have a longer run time because the current flow is lower, reducing the I2*R losses in the wiring, making that energy available to power the blade.


  • The 40-volt mower will have the same run time as the 20-volt mower, but it will be lighter and cheaper because the manufacturer was able to use thinner, lighter wiring without increasing the I2*R losses above what is seen in the 20-volt mower.


  • The 40-volt mower is a piece of garbage that they’re trying to hype up by attaching bigger numbers to it.

Always a possibility.

All things being equal, a 40V battery will have less capacity than a 20V one, due to the necessity for more cells. Probably a small effect, but it’s there.

How does that affect a small, say 3500 watt generator? I mean, the steady-state current draw might only be 300 watts or so for a refrigerator while the compressor is running, but the inrush current may be quite a bit larger. Does the generator have to be able to supply the full inrush current, or is it sort of like the circuit breakers and can handle that sort of thing?

The generator will briefly slow down, converting its rotational kinetic energy to electrical energy (just as, of course, the motor is converting it back to rotational kinetic energy).

Presumably, the generator is larger than the motor, so it won’t have to slow down much.

It should also be kept in mind that charging a high number of cells in series can be problematic. Unless the charger is monitoring the voltage across each cell, it’s impossible to ensure the cells are balanced. This can cause all kinds of problems. We’ve run into this issue for military systems at work.

The generators themselves will have a peak (often called “starting”) watts figure as well as a continuous one. As you might predict, the peak watts figure is typically the one you see printed in large letters on the box.

As Chronos mentions, the rotational inertia of the generator also helps you out a bit. The generator peak watts are often only ~25% higher than the normal watts, but most likely they can handle more than that in the very short term (fractions of a second). Also, with high load you’ll get voltage and frequency droop, which contribute to the load itself pulling less (possibly to the point of making it unable to actually start).

If the device doesn’t trip the breakers, you’re probably safe. The breaker is designed to give some allowance for overcurrent, within the limits of the device it’s protecting. Not that I’d necessarily use that as the primary way of determining the limits, especially on some cheap-ass Harbor Freight special. But if the generator has a few times the capacity of the fridge and it works, you’re probably okay.

Likely it doesn’t have more cells - just the same number of cells, but wired to double the series arrangement (so as to double the whole-battery voltage). Cells on Li-ion power batteries are limited in size by cooling requirements; you have to get air in there to take away waste heat when they’re charging or discharging. A battery-electric car’s propulsion battery is actually filled with thousands of individual cells, each about the size of an adult man’s finger.

In the context discussed - hand-held power tools, it is a matter of twice as many half sized cells. But in general, yes, there are all manner of ways of connecting cells, and designs will settle on the most cost effective cell as the building unit. Hand held tools are volume and weight limited to the extent that that luxury doesn’t currently exist.