The essential answer
Energy per liter divides energy consumed over a stated interval by water recovered over the same interval. The result depends on the system boundary: a fan, electrical field, pumps, controls, heating, and idle operation can all contribute. Gross energy per liter and energy per additional liter answer different questions. Neither metric alone establishes economic or environmental superiority. Report the water denominator, time window, and included loads so another reader can reconstruct the calculation.
How to reason about it
Power is a rate, while energy accumulates over time. A constant 10-watt load operating for one hour uses 10 watt-hours. For variable loads, integrate measured power over the event. Water retained on the collector should not silently enter a bottle-yield denominator unless the metric explicitly includes storage. A passive baseline is useful when asking whether powered assistance earns its extra energy through additional water. The cited fog-harvesting experiment evaluates energy associated with electric assistance; this page's arithmetic is independent of its reported performance. Compare like boundaries, especially when one system harvests pre-existing liquid fog and another condenses vapor. Those mechanisms face different thermodynamic and environmental conditions. Also report low-yield intervals: energy intensity can rise sharply when fog weakens but auxiliary loads continue operating.
ILLUSTRATIVE · NOT A REPORTED RESULT
An example to work through
Assume a device draws 20 watts for 3 hours and delivers 2 liters. Energy is 60 watt-hours, so gross intensity is 30 watt-hours per liter. If a matched passive baseline delivers 1.5 liters, the additional yield is 0.5 liter. Assuming the entire 60 watt-hours is additional energy, incremental intensity is 120 watt-hours per additional liter. Both calculations are correct but describe different questions. They exclude embodied energy, maintenance, and water treatment, which would need separate accounting for a broader comparison.
Where interpretation fails
Quoting only the emitter's power while a fan supplies most of the airflow understates the system input. Another error divides a short peak production rate into a full-day power claim. Energy and water must cover the same interval and the same operational boundary.
The next decision
Publish a load inventory, integrated energy, recovered volume, and baseline definition. Calculate both gross and incremental intensity when powered assistance is the design change. If the water increment is near measurement uncertainty, report that instability instead of presenting a precise efficiency advantage.
Sources & context
These sources support the concepts noted below. Our synthetic examples and research framing are educational interpretation; they are not findings from these organizations or validation of our ventures.
- Fog harvesting: combination and comparison of different methods ↗
Experimental energy accounting for electrically assisted fog harvesting.
Source check: Search-result abstract or official excerpt reviewed; full paper methods not audited
Educational research context, not a validated device specification or clinical protocol. Read our editorial and correction policy.