eli5 the difference between watt-hours and Amp-hours?


I’ve seen electricity being measured for like houses and stuff with kWh, but i always see on batteries it’s measured with mAh. what’s the difference? also, is there such thing as Volt-hours?

In: 6

No, there’s no such thing as volt-hours. Your home should always be at 120 volts (north america), even if nothing is plugged in at all. Voltage is always present on the wires assuming nothing’s wrong (power outage, etc).

Logically, as long as the voltage doesn’t change, volt-hours and amp-hours are functionally identical up to that voltage multiplier. In practice it’s a matter of how you choose to measure the power. A battery would normally measure its output in amps because a sensor monitoring the battery would measure it that way. In your home we use watts because appliances are usually rated in watts, allowing for fairly easy math.

Does it matter? Not really, if you know the formula.

Think of it like a backpack – watt hours measure how much stuff you can fit in the backpack and how heavy it is, while amp hours just measure how much stuff you can fit in the backpack. So watt hours tell you how much energy the battery can hold and how long it will last, while amp hours just tell you how much charge the battery can hold.

It’s easy to get confused. Amps are important because they represent the flow of electricity (and as a direct result, the size of wire needed to safely conduct that flow), regardless of the voltage. Your house likely has electricity available in both 120 volts and 240 volts. Watts are the result of volts, multiplied by amps. So, you might have an appliance that consumes 1200 watts but that doesn’t tell you the whole story. If it’s 120 volts, it draws 10 amps but if it’s 240 volts, it draws 5 amps.
The examples given are for one moment in time. A fridge might draw maybe 500 watts but only when it’s compressor is running, which isn’t all the time. This is where the element of accumulating time plays a roll, and where you’ll see the ‘h’ added to the end of the kW or mA. One kWh is equivalent to 1000 watts, running for one hour. The fridge I mentioned earlier, if it’s compressor was running 50% of the time, would take 2 hours to consume 1000 watt/hours (1 kWh).
Batteries is a bit more confusing, mostly because the voltages are all over the place. Cell phones are usually 5 volts. AA batteries are 1.5 volts. Car batteries are 12 volts. I’m not sure why they can’t just list all batteries as watt/hours instead of making us look up the voltage and do the math. I think they like to intentionally muddy the waters for the average person. I’ve never seen the term volt/hours used.

Wh = V * Ah.

I work for an ebike retailer. Our standard batteries are 36V 10Ah = 360Wh. A 48V 7.5Ah battery would also be 360Wh.

My understanding is that batteries are usually described in Ah because you’ll typically comparing them to other batteries with the same voltages; you can’t use a 48V battery with a 36V system, so if your 360Wh battery turns out to be 48V 7.5Ah instead of 36V 10Ah you’re SOL. But Wh is a more complete description of the amount of electrical energy your battery can hold, or your household’s annual power consumption or whatever.

EDIT: The water analogy is a useful way to understand electrical stuff. Voltage is like water pressure, current (amperes) is like flow rate, charge (ampere-hours or coulombs) is like water volume. Watts are like the amount of power the flowing water supplies (imagine it’s turning a water wheel or something) – pressurised water can supply more power. X Watt-hours are just ‘supply X amount of watts for 1 hour, or 1 watt for X hours’.