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She's much more energetic today than she was yesterday.
Literal
She [topic-は] yesterday [than-より] today [genitive-の] side [subject-が] much energetic [is-です].
The classic AよりBの方が pattern packs comparison into a fixed slot: より marks A as the standard ('compared to A'), の方が ('the side of B') promotes B as the more so. 方 ('side') turns the comparison into a choice between alternatives — 'on the side of today, things are more lively.' ずっと ('much, by far') intensifies the contrast, and 元気 (健気) covers a wider semantic range than English 'energetic,' shading into 'cheerful, in good spirits, healthy.' The polite copula です anchors the register.