We tend to assume the answer is obvious: swap diesel for batteries, emissions fall. But shore-charged electricity isn’t clean by default. It’s as clean as the grid behind the socket. On a coal-heavy grid, replacing diesel combustion with grid electrons can move the emissions rather than remove them — and in some cases increase them.
I wanted to know exactly where that line sits. So I worked it out from first principles and reduced it to a single closed-form expression:
R = 1 − 1.308 · g · (1 − s)
where R is the CO₂ reduction from going electric, g is the grid’s carbon factor (kg CO₂/kWh), and s is the share of energy from onboard solar. The result is independent of vessel size, route, or operating profile — it depends only on those two numbers.
That gives clean break-even thresholds. Propulsion turns favourable below g(1−s) = 0.76. Auxiliary loads — met on a diesel vessel through a generator set — tolerate a dirtier grid still, with break-even near 0.94. Map this against national grids and three regimes fall out: clean grids where electrification always helps; medium grids (India sits here) where it’s marginal without solar but clearly positive with it; and dirty grids where you need a meaningful solar share just to break even.
The whole thing is on a symmetric well-to-wake basis — upstream production plus combustion for diesel, upstream generation for grid electricity — so it’s a fair comparison on both sides.
Now I want it stress-tested before I submit it.
This is a short, deliberately simple model. Simple models are useful precisely because they’re easy to argue with — so I’d genuinely value the argument. A few things I’d especially like pushback on:
- Are my efficiency assumptions (drivetrain, alternator, battery) realistic for the ferries you operate or design?
- Is the ~19% well-to-tank uplift on diesel right for your region?
- Where does treating the grid factor as a constant break down for you? (Time-of-day charging is the obvious gap.)
- What real-world factor have I left out that would change an operator’s decision?
If you operate ferries, design them, work on class or regulation, or model grid emissions — I want your honest critique. What’s wrong, what’s missing, what’s oversimplified?
The paper and an interactive calculator are linked. Comments open. Be blunt.
Sandith Thandasherry 