No Stupid Questions

Where I live most people actually do think of household electricity in terms of instantaneous current draw. The power grid is insufficient, so you get rationed grid power throughout the day depending on what area you live in, with the rest filled in by mob-run local power generators.

You pay a subscription based on the maximum amperage and you have to manage your power use accordingly.

🤓☝️ Um that’s functionally the same as a power limit

Yes, but it’s ampere based because it’s managed by a breaker out on the street, and the ampere limit is colloquially understood. “I’m paying for 5A, the bastard won’t give me more” is a perfectly understandable statement. When I was 7 years old I already understood that “we only have 5 amperes during most of the day” and that it meant the microwave wasn’t available during that time. And since the breaker is out on the street, you learn your limit very quickly, since you have to get dressed and go down a few flights of stairs in the freezing cold to turn it back on. If you have an elevator in your building, it sure as shit isn’t running when the generator power is active.

Annoying when you’re using solar to escape this hell electrical system and everyone has to re-learn to think in terms of Watts/VA. I have a table printed out stuck to the wall to “convert” between amps and Watts at 230V. Do you want to explain to grandma which devices are intuitively at 1kW≈1kVA and which are not? No? Then let her keep using amperes, it’s fine.

Yes, the power generators run off diesel, yes, the diesel fumes and generator noise is a problem, yes, we get price gouged by both the generator mobs and the government grid, yes, I hate dieselpunk and think diesel is the most disgusting fuel. The generators give you a much closer wave to 230@50Hz though, so it has that over the grid. Was solar the most expensive thing we’ve ever paid for? Yes. Does it make me feel like a king with a 24 hour battery-backed microwave? Also yes

It depends on what's useful to know.

A microwave is a heating device. It's not useful to know you're using a 7a microwave on its own. Is it 120v or 220v? What's important is the wattage, as an indicator of how much heat it can put into food in a given time. A 700w microwave is going to take longer than the instructions say but could be a 3.5a euro oven or a 7a north american oven.

With lights, wattage ignores the change in voltages as well. But it relied upon the tungsten incandescent being ubiquitous. The socket type defines the voltage, so you just want to know if it's a soft 25w reading light or a 100w for a garage bay. But now, with the prevalence of fluorescent and then LED lights, wattage has become almost irrelevant. They usually list actual wattage in pale text and "incandescent wattage equivalent" in bold. I'm happy to say I'm finally seeing bulbs state actual lumens now, which is what really matters to the end user. LED lighting is now the least of your electric bill worries.

With a car battery, you're seeing the options in a later stage of market uniformity. Cars used to very commonly have 6v systems, so the 12v system was distinct. Large trucks use 24v (though I think with dual 12v batteries). But for you buying a car battery, just about all passenger cars are 12v. It's a specific size like "group 65", so it's a 12v of certain size and terminal placement. You do have some options for amperage, listed as CCA. You can't give more amps to the starter, but rather the battery lasts longer per charge and drops voltage less when under load.

But, you will actually see amps listed on power tools. I guess they traditionally had very little export, so the voltage is constant for the market. You can compare amps and take a good guess how two saws will compare. Even still, many tools now list actual specs like rpm and torque

devices draw the amperage they want so there is not much risk of over amperage but plug something that wants less volts into a more volts source and. erm. not good. wats are bigger because its more a measurement of total power although I see people arguing we should use joules or such.

Many batteries these days are rated in Ah, so that's something.

One reason is because most power supplies are voltage supplies and not current supplies. Though they do report the max current they can supply. Also voltage is in those cases (arguably) more important. The wrong voltage and it probably won't work at all or even breaks. Whereas to little current, it probably still does something, though at reduced power or it cuts off at some point.

Because current (amps) has nothing to do with energy. Formally, an ampere is the current of 10^19^ electrons moving through a given point in 1.6 seconds; in more reasonable terms, it's 1 coulomb per second. The amount of energy in those electrons doesn't matter to the amount of current, but energy is very relevant to making machines do things.

Potential (volts) does include energy; specifically, 1 volt is 1 joule per coulomb. Add more energy and you get more volts, but the current remains the same. So volts are more relevant to how much use you can get out of your electrons.

Power (watts), meanwhile, tells you how effective your machine is at extracting that energy. 1 watt is 1 joule per second. Suppose you are running a 6W heater. Every second, that heater converts 6J of electrical energy into heat energy, while the current remains the same.

Thus, knowing current is important for electrical engineering, but potential and power matter more for operation.

We used to see it on phone chargers. 5V with different amperages. Nowadays it's mostly given i watts, though.

While constant current sources exist, they're very uncommon. A battery's voltage is constant (or at least we consider it so), but the current it needs to supply is dependent on the load. In general, we consider current to be a result of voltage, to the extent that some teachers prefer that Ohm's Law be written as I = V/R (rather than V=IR) to show this relationship more clearly (where I is current, V is voltage and R is resistance).

Popularity, tradition, laziness :)

The danger of electricity is the first thing about it that you learn as a little child. And later you learn: the more voltage, the more danger. Even such people who never learn anything else about electricity know that all their life. Now the voltage has an unbeatable popularity.

Power sources with a fixed voltage were invented before such ones with a fixed amperage. Therefore the voltage was used as "the" number to tell the size of it. People like to be lazy, they like to have one single number to tell the size of things. Accordingly, for the devices that use electricity the traditon was established to say only the voltage (often it must fit to the fixed voltage of the source) and then be satisfied.

Amps are the variable part of the equation...

There are other parts of the equation, every one being constant would make every electrical component binary. Either full power or no power.

That's why we really only see variable amperage on battery charges to force a slower charge rate for the health of the batter. On something like a radio, you could think of the volume knob as amperage control. The more power, the louder the sound comes out of the speaker.

A steady amperage current would "lock" the volume at one setting forever.

Interestingly, in your example you put car battery. However CCA, or cold cranking amps is like the only thing interesting about the battery, other than physical size.

I don't have an answer but this video explains a lot of this stuff in a really good way:

https://youtu.be/X_crwFuPht4

Also:

220v: 4.5A

110v: 9A

Everyone is aware of amps when their breakers trip.

If you know the voltage of an outlet you know the amps of a 1000w microwave.

Because in most relevant scenarios amperage is essentially just a function of voltage and resistance/impedance

Amps are used when deciding the wiring. So that afterwords you know if your plug matches, it works in that outlet.

In a nutshell, voltage incompatibility is generally more damaging than current mismatch, typically in a frightening or energetic manner. Many Americans tourists find this out when they bring their 120v AC hairdryers to an overseas hotel with 230v AC power. If there is only room for one number to be emblazoned on an outlet or plug, it should be the rated voltage, first and foremost.

For current protection, we've had thermal fuses since the 1890s, and thermo-magnetic circuit breakers since the 1940s. There are even more fancy transistor-based current protections available for industrial equipment that can shut off extremely fast. In a sense, protection against over-current has basically been solved, in the scenarios where there's enough of a risk of humans or property.

Whereas voltage mix-ups still happen, although consumer electronics are now moving to automatic voltage detection (eg an 18v electric drill battery charger refuses to charge a 12v battery) and through actively negotiated power parameters (eg USB PD). And even without human error, under- and over voltage transients still happen in residential and commercial environments, leading to either instant damage or long-term product degradation (eg domestic refrigerator motor drive circuits).

It should be noted that a current starvation scenario, such as when an ebike is current-limited per regulations, does not generally cause a spike in voltage. Whereas in a voltage starvation situation, resistive loads will indeed try to draw more current in order to compensate. Hence why current protection is almost always built-in and not left to chance.

Amperage is only relevant for a short moment, just before you go through the pearly gates.