# Formula’s (Energy Harvesting)

For harvesting you need a bit of calculating, otherwise you fall into the trap of confusing big and small energy.

For instance: heating water is big energy, so is moving mass of kilograms. Doing a measurement with a sensor, sending bits of information, calculating is much less energy (most of the time).

Law of Ohm:
V = I * R,
This looks simple, but becomes somewhat more complex with resistors parallel or in series, with capacitors or in a complex circuit with voltage dividers or other fancy electronic components.

Energy formula:
P = V * I (in Joule*second = Watt)
The energy can also be given in current-hour: a lipo battery of 1500mAh. This means that you can get 1500 mA in 1 hour at a voltage between 4.1 and 3.6.
Or 750mA for 2 hours.
So for one LED at 50mA, “slightly” above the 3V for the LED you can run it 1500/50 = 30 hours.
An attiny85 at 5microA (between active 300microA and sleep 0.1microAmp) can run … 300000 hours on this lipo…34 years? (Let’s check! See you after 34 years Energy of a capacitor:
E=1/2*C*V*V (in Joule)
example: super capacitor of 2.5V, 10F fully charged: 31 J
Compare:
LED always on + Attiny85 runs at 50 mA, 2.5V on this supercap.
P = 0.125 W (J*s), which means my circuit with 1 LED can run for 248 seconds with LED always on (248 seconds = 6 minutes) on this super cap from above (well, the voltage of the cap will drop decharging but this drop can be taken care of by … another LTC3105 Accordingly if you blink (say 1000 milliseconds) once a minute: 1/60 of the time: 0.0021 W, will run for around 4-6 hours on 31J.