Hoy tengo más energía de la que pensaba.

Because Spanish often leaves out subject pronouns when the verb already makes the subject clear.

• tengo already means I have
• so yo is not necessary

The sentence could be Yo hoy tengo más energía de la que pensaba, but that would sound more emphatic, as if you were stressing I in particular.

In Spanish, tener is the normal verb for many physical or personal states that English often expresses with to be or to have.

So you say:

• tener hambre = to be hungry
• tener sed = to be thirsty
• tener sueño = to be sleepy
• tener energía = to have energy

Using estar here would not sound natural.

Here energía is being used as an uncountable noun, like energy in English.

So Spanish normally says:

• Tengo mucha energía
• Tengo poca energía
• Tengo más energía

The plural energías exists, but usually in other meanings, such as types of energy or set expressions like energías renovables.

Because the sentence is talking about energy in a general, non-specific sense.

Spanish often omits the article with uncountable nouns when speaking generally:

• Tengo paciencia
• Tiene hambre
• Tenemos tiempo
• Tengo energía

If you used an article, it would usually refer to a specific energy already identified in context, which is not the case here.

Más has a written accent because it means more.

Without the accent, mas is a different word meaning but, and it sounds formal or literary in modern Spanish.

So:

• más = more
• mas = but

In this sentence, it must be más energía.

Because in Spanish, when a comparison like more than I thought is followed by a whole clause, the structure is often:

• más ... de lo que ...
• or, when a noun is involved, más ... de la que / del que / de los que / de las que ...

So:

• más energía de la que pensaba

This is different from simpler comparisons like:

• más energía que ayer
• más energía que tú

There, que is followed by a noun, pronoun, or short comparison, not a full clause.

Because la matches energía, which is feminine singular.

Spanish often uses an agreeing article in this pattern:

• más energía de la que pensaba
• más tiempo del que pensaba
• más razones de las que esperaba
• más libros de los que creía

So la is there because the comparison refers back to energía.

Pensaba is the imperfect of pensar, and it is very natural here because it refers to your previous idea or expectation, not to one single completed thought.

So the sense is something like:

• than I had thought
• than I was thinking
• than I expected

If you said pensé, it would sound more like a specific moment of thinking:

• más energía de la que pensé

That is not impossible, but pensaba is more natural in this kind of statement about a prior impression or expectation.

Yes. Spanish often leaves repeated information unstated when it is obvious.

The full idea could be something like:

• Hoy tengo más energía de la que pensaba tener
• Hoy tengo más energía de la que pensaba que iba a tener

But native speakers usually omit that extra part because the meaning is already clear from context.

Yes. That version is also very common and natural.

There is a small difference in feel:

• de la que pensaba ties the comparison more directly to the noun energía
• de lo que pensaba sounds a bit more general, as if referring to the amount or overall idea

In everyday speech, both are widely understood and used.

Yes. Spanish word order is flexible.

These are all possible:

• Hoy tengo más energía de la que pensaba.
• Tengo más energía hoy de la que pensaba.
• Tengo hoy más energía de la que pensaba.

The first one is the most neutral and natural. Putting hoy later can slightly change the emphasis.

A rough guide would be:

oy TEN-go mas e-ner-HÍ-a de la ke pen-SA-ba

A few key points:

• hoy: the h is silent
• energía: the stress falls on gí
• the g before i in energía sounds like the Spanish j, a throaty sound in Spain
• que sounds like ke

So energía is roughly e-ner-HÍ-a, not en-ER-gia like in English.