As appealing as that narrative is (just utilize waste electricity, its cheap/free!) a plant (making anything, not just this chemistry) that only runs 50% of the time for e.g. nighttime electricity costs twice as much[0] as one that runs all the time.
To the parent re: efficiency and economics, the supplemental info: https://pubs.acs.org/doi/10.1021/jacs.9b07310 (as a rule, free/open access w/ 90% of the utility of a given paper) has the half cell potential at about 0.6V[1] vs RHE, which puts a whole cell potential at about 2V. So count the electrons to convert CO2 to propanol, take Coulomb's constant to get amps, convert to watts, and do the stoichiometry to get kwh/tonne propanol.
This is all ignoring throughput and CAPEX utilization, which is almost certainly poor (but not unexpected at this stage of research).
It is also worth noting that they are using a 3 dimensional, convoluted electrode (carbon cloth) to support their catalyst which may be responsible for the claimed 'trapping of CO'-effect, rather than their specific catalyst. Notably C3's have been observed before in CO2 reduction from less involved catalysts.
tl;dr Probably not very efficient at all, I'm too lazy to do the math. If it was efficient, it would probably still be wildly uneconomic to make propanol via this route.
[0]per unit of production, revenue
[1]Don't forget to account for side product production, per the supplemental information they are making about 600 moles of H2 per mole of propanol (hard to read the chart).