Whether powering an outdoor asset tracker or an indoor BLE beacon, understanding the balance between power consumption and power generation is critical to an energy harvesting solution.
A properly sized PV module not only depends on device consumption but also power margin. Margin, in this case, is the headroom that ensures a device still operates during a worst-case scenario, unexpected events, or real-world uncertainty.
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Where to Start?
There are a lot of inter-dependent variables that significantly affect generation and consumption. These variables offer levers to pull when putting together a viable use case but can be complex and overwhelming.
Additionally, we must also define the amount of light the PV module will see each day.
The tables above provide a good starting point for common lighting scenarios in outdoor and indoor environments.
A significant portion of power margin is indirectly built into this metric. As you can see, light level and exposure profiles change radically depending on a device’s location, objects in the environment, and the device’s use.
With daily power consumption and hours of sun/light, we are ready to apply our first sizing strategy.
The formula above is a straightforward and common way to size your PV, including an appropriate factor for power margin.
Dividing daily consumption by hours of light results in the minimum required PV power to break even. In this setup, the margin is simply a multiplier of the minimum break-even point.
This multiplier is typically between 1.2 (if you live life on the edge) and 10 (if you always quadruple tie your shoes). If you are unsure what is right for you, start with 2x and adjust after testing.
Consumption Strategy Example – ESL
For this example, we will examine an electronic price tag (ESL) used in a grocery store environment.
ESL power consumption is primarily dependent on how often the screen is updated. In this case, we assume two updates per day and daily consumption of 250uWh. We can also consider consistent lighting for at least 16 hours daily. With a 2x power margin factor, the PV size is calculated below.
PV Size [uW] = 2 x 250uWh / 16h = 31uW.
Retail environments are relatively bright, averaging 500–1000 lux, where a small PV module can reach 31uW easily.
PV Size [W] = Margin x (Battery Size [Wh]) / (Desired Charge Time [h])
Battery capacity can also be used to drive the size of the PV.
This method ensures the battery is fully charged after a given time, whether one hour or one day.
This strategy is ideal for devices with highly irregular lighting patterns that may need to operate through extended stretches of darkness or shade and recover from these events quickly.
Because this strategy often results in much more power generation than device consumption, the margin can tend to be lower. Starting with a 1.2x multiplier is adequate to cover typical conversion inefficiencies and variations.
Battery Strategy Example – Asset Tracker
For this example, we will be looking at an industrial asset tracker where day-to-day illumination can be highly variable, and the device could be stored in darkness for days or weeks at a time. For this use case, it is essential to fully charge the battery in one day after extended darkness.
The device uses low-power LTE-M cellular connectivity at a modest latency to achieve ~15mWh consumption per day. A 450mWh (120mAh) battery ensures the device could last up to 1 month without solar input.
Used year-round in the continental US, one sun hour per day was chosen as a worst-case scenario, and a 1.2x margin was applied.
PV Size [mW] = 1.2 x 450mWh / 1h = 540mW
PowerFilm’s highly durable thin-film PV was integrated into the device enclosure and was well suited to stand up to the harsh conditions expected in this application environment.
A properly sized PV module depends on consumption, environment, and what margin you are comfortable with for your application.
The strategies discussed in this post are a great starting point giving you a leg up on defining a viable solar energy harvesting use case.
Please contact us if you have additional questions about putting together a power budget or properly sizing a PV module for your application.