A low-power application is a schedule of demands. Start with a load table: how long does the device sleep, measure, calculate and transmit, and how much power does each state require? The daily energy is the sum of power multiplied by time in each state.
An illustrative budget
Imagine a device sleeping at 2 microwatts for 23.9 hours and operating at 1 milliwatt for 0.1 hours. Its daily requirement is 0.1478 milliwatt-hours before conversion losses. Average demand is about 6.16 microwatts. These are invented design inputs for arithmetic, not measured sensor specifications.
The source must also handle the instantaneous demand. A source that meets the average may still brown out during a burst. Storage, voltage conversion and startup behaviour must therefore be tested as part of the same system. Their losses reduce the energy reaching the load.
Area is only a first constraint
For a hypothetical sustained density of 1 microwatt per square centimetre, an ideal 1-watt output would require 100 square metres. Real design would additionally need spacing, losses and environmental variation. This arithmetic is not a forecast for any published cell.
Evidence boundary: all numbers in this article are transparent hypothetical inputs. The practical precedent is the published microprocessor experiment, whose exact boundaries are discussed separately. Neither the example nor the area calculation establishes household electricity, continuous outdoor operation, or manufacturing feasibility.