a question about "that"

[JuniorAA799]

Here is an example:

people tend to overestimate the amount of energy used by equipment and underestimate that used by unobtrusive equipment.

What does the "that" refer to?I think it refers to "the amount (of energy)",but the phrase following the "that" (used by unobtrusive equipment) is the modifier of "energy" rather than "the amount of energy".Can the modifier following the "that" modify something inside the "that"?How can it be possible?

[RonPurewal]

Here is an example:

people tend to overestimate the amount of energy used by equipment and underestimate that used by unobtrusive equipment.

What does the "that" refer to?I think it refers to "the amount (of energy)",but the phrase following the "that" (used by unobtrusive equipment) is the modifier of "energy" rather than "the amount of energy".Can the modifier following the "that" modify something inside the "that"?How can it be possible?

"Used by unobtrusive equipment" can refer to "the amount of energy".
Corporate offices generally use a large amount of energy during the daytime.. --> An amount of energy can be "used".

[JuniorAA799]

It seems that the word "number" can also be used in such cases.I have found many such sentences in official materials.Here is a correct sentence in prep software:

In 1914 a total of 469,000 cars and trucks were produced in the United States, but in 1929 almost twice that number of trucks alone came off the assembly lines.
(Though I still think "twice that number came off the assembly lines" is a little strange,now I just try to accept it.)

But I haven't find any sentence using word other than "amount" or "number".

Can I just consider "amount" and "number" special nouns?Is there any other word that can be used in such cases?For example, "increase"?

[RonPurewal]

This might sound like a stupid answer, but it's the only answer I can really conjure up: Just think about what the sentence is saying.

If you're talking about a number, quantity, or amount, then you can use words like "number", "quantity", and "amount".
If you're talking about an increase, then you can use words ilke "increase".

If you are asking something more specific, please clarify. Thanks.