That's non-metric thinking. In the metric system, you could simply use the cubic meter.
A hectare is 10000 square meters. A centimeter is 0.01 meters. Thus, if 1 centimeter of rain falls on a 1 hectare field, then that's simply 100 cubic meters of water.
The nice thing about this is that you can easily compare quantities from different contexts. A cubic meter is a cubic meter, whether the water came from a pipe or from the sky.
But what does it mean to have an acre-foot of piped water? Nobody specifies pipe cross-sections in acres. Or a barrel of rain falling on a 1-acre field. How many inches is that equivalent to?
It's not just water. Look at HVAC. In the US, natural gas is billed in therms, and electricity is billed in kilowatt-hours. In metric countries, both electricity and natural gas are specified in kilowatt-hours.
And why not? Energy is energy. If you have a heat pump with gas backup, then you could use gas and electricity interchangeably to heat your house. If you've got a residential cogen system, like they have in Japan, then you could buy either gas or electricity to run your television.
> It's not just water. Look at HVAC. In the US, natural gas is billed in therms, and electricity is billed in kilowatt-hours. In metric countries, both electricity and natural gas are specified in kilowatt-hours.
It's funny you use that example, because kWh are silly, unnecessarily convoluted units themselves :-) the "proper" unit for this dimension is the joule, and 1 kWh = 3.6 MJ (1000 J/s ∙ 1 hr ∙ 3600 s / hr ∙ 1 MJ / 1e6 J).
There actually is, however, some value in having different units for methane vs electricity, since you have to run methane through a suboptimal furnace to harness its energy. Granted, modern gas-powered furnaces are usually more than 90% efficient, but there's still some loss. And since each customer has a different furnace with a different eta, it's usually not useful to directly compare natural gas tariffs vs electricity tariffs even if you do have a cogen unit.
> There actually is, however, some value in having different units for methane vs electricity, since you have to run methane through a suboptimal furnace to harness its energy. Granted, modern gas-powered furnaces are usually more than 90% efficient, but there's still some loss.
Well, heat pumps transfer more than 1 kWh of heat transfer for every kWh of electricity. You have to account for the performance factor anyway, whether you're using natural gas or electricity.
The point is that, once you've divided by COP or AFUE, you get price per effective kWh, which you can then compare directly. As opposed to getting a price per therm, which you must apply a conversion factor to.
With cogen, you get both electricity and heating out of the unit. If it's all in kilowatt-hours, then you simply add them up and compare to the kWh of the incoming natural gas to get efficiency. But how does a therm of electricity translate to volts and amps?
As for the kilowatt-hour, I think the reason we don't use megajoules is that metric time never took hold. If we had metric hours, minutes, and seconds, then the kilowatt-hour would be as awkward as the hectare-centimeter.
A hectare is 10000 square meters. A centimeter is 0.01 meters. Thus, if 1 centimeter of rain falls on a 1 hectare field, then that's simply 100 cubic meters of water.
The nice thing about this is that you can easily compare quantities from different contexts. A cubic meter is a cubic meter, whether the water came from a pipe or from the sky.
But what does it mean to have an acre-foot of piped water? Nobody specifies pipe cross-sections in acres. Or a barrel of rain falling on a 1-acre field. How many inches is that equivalent to?
It's not just water. Look at HVAC. In the US, natural gas is billed in therms, and electricity is billed in kilowatt-hours. In metric countries, both electricity and natural gas are specified in kilowatt-hours.
And why not? Energy is energy. If you have a heat pump with gas backup, then you could use gas and electricity interchangeably to heat your house. If you've got a residential cogen system, like they have in Japan, then you could buy either gas or electricity to run your television.