Every printed book carries a roughly fixed production energy cost Eprint (papermaking, printing, binding, distribution) paid again for each copy. An e-reader instead pays one large one-time manufacturing cost Edevice, then a small marginal cost Edigital per book read (battery charging + data transfer, amortized over the device's life).
Cumulative print energy(n) = n · E_print
Cumulative digital energy(n) = E_device + n · E_digital
Crossover (break-even) book count:
n* = E_device / (E_print − E_digital)
Below n* books, the printed-paper path has used less total energy than the still-unamortized device. Past n*, the printed path keeps accumulating linearly while the digital path barely rises — the reader has "paid off" the device's embodied energy and every further e-book saves roughly (E_print − E_digital) of energy versus buying it printed.
- E-reader embodied energy — total manufacturing + supply-chain energy locked into building one device.
- Printed-book energy — energy per physical copy: pulp/paper, printing press run, binding, shipping.
- Digital reading energy — battery charge + wireless download energy attributable to one book on the device.
- Reading pace — books per year, used only to convert the crossover book count into an elapsed-time estimate.
- Play — advances the simulation one book at a time; the bar-chart panel and the cumulative-curve panel update live, and a green crossover marker lights up once the crossover book is passed.
- Drag on the chart to pan left/right through all 120 books; Fit view snaps back to show the whole range.
This is the same "energy payback" logic used to compare embodied vs. operational energy for buildings, EVs vs. combustion cars, or reusable vs. disposable packaging — a one-time manufacturing cost traded against a small recurring cost.