Whether powering an outdoor asset tracker or an indoor BLE beacon, understanding power consumption and generation is critical to an energy harvesting solution.
Making a power budget will help you balance your device's power use over time, just like you would balance your spending, saving, and income each month.
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Power Budget
A power budget includes three main components; power generation, power consumption, and energy storage. These are analogous to income, spending, and saving in a financial budget.
Over a given time, typically days or weeks, the budget has to be balanced, and average power generation must be greater than or equal to the power consumption. If this is not true, your device will run out of power and stop working.
Depending on your application, you may also want to store additional energy so your device can continue to operate when there is no light available or during adverse light conditions or events.
Whether you organize all this information in a neat excel sheet or scribble it down on a napkin, putting together a budget is straightforward once you understand each component.
Power Generation
Power generation depends on light conditions, light duration, and panel size. The tables below show a variety of common lighting conditions and a corresponding daily power budget per unit area of PV [uW per cm^2].
To calculate your power budget, find the scenario that matches your application closest and multiply the power budget per unit area by the surface area available for PV in cm^2. This value represents the daily generation side of the power budget, our ‘income.’
Alternatively, you can create your own scenario to match your application. Power density (uW/cm^2) will scale almost in proportion to the light level. Multiply the light duration, in hours, by the power density to get the power budget per unit area. Then multiply by available surface area to get a more accurate daily power generation budget.
Power Consumption
Power consumption can be divided into two categories, sleep and active. Some devices may have multiple sleep modes or active modes to account for, so you can dig as deep as needed or stay high level for simplicity.
Sleep current is the current draw of all circuit components when the device is not active. This value can often be found on a datasheet listed as “quiescent” current for individual components.
Active current is the power drawn from the device while it's functioning. That could be taking a measurement, transmitting data, or waiting to receive a signal.
Datasheets provide good insight for estimating these values, but ultimately they are driven by programmed functions and how the device operates. Measuring sleep and active power levels directly will provide the most helpful information for a power budget.
The plot below of a BLE transmission event was measured using an oscilloscope and a precision inline current amplifier. As you can see, the device is sleeping, wakes up, transmits data, then goes back to sleep.
Figure 2, power profile of a BLE data transmission event showing how devices wake up momentarily to send or collect data before returning to an ultra-low power sleep state.
To calculate the consumption side of the power budget, multiply the sleep and active current values by the system or battery voltage to convert to power. Then multiply by the time spent in those modes each day and add them together. This is shown in the equation below.
Sleep Power × Sleep Time + Active Power × Active Time = Total Power Consumption
This value represents the amount of energy your device consumes in one day, which we can compare with power generation when we start to balance the budget.
Low power BLE and cellular devices/sensors will typically have daily consumption levels in the 100-1000uWh, which matches well with many typical indoor environments. Higher power gateways and more active sensors can quickly consume 1mWh - 10Wh+ daily and are better suited for outdoor harvesting environments.
Balancing the Budget
To make the power budget work, it has to be balanced. Now that we understand the individual components, we can see the different levers available to adjust generation and consumption.
Margin is also vital to consider in this step. Since the lighting conditions of IoT devices are often highly irregular, we usually recommend starting with a 2x margin or generating twice as much power per day as your device consumes. Choose a margin that you feel comfortable and confident with for your specific application.
If you find that you are generating much more power than your device requires, you could increase the functionality of your device or reduce the size of the solar panel.
If your device's power consumption is higher than power generation, find ways to mitigate active or sleep current consumption. Increasing the size of the PV module or increasing the worst-case lighting conditions can also improve power generation.
Energy Storage
Energy storage can be like a checking account or an emergency savings fund. Excess energy stored during the day can be used to power the device while the lights are off or the sun is not out. There are many different types, sizes, and storage elements, including Li-ion, LiFePO4, supercapacitors, solid state batteries, NiMH, etc. Each has its own set of benefits and trade-offs.
Smaller batteries can be helpful as a shorter-term buffer. Supercapacitors and solid state batteries can be used to extend the operation of a device overnight or for several days without the need for large batteries.
Larger batteries come in handy for an extended lifetime under adverse lighting conditions. Some devices may go into storage for weeks or months, and without proper storage capacity, they will stop operating. When using increased storage, allocate a certain amount of energy per day for excess charging in the power budget.
Summary
A power budget is a great tool when implementing an energy harvesting solution. It ensures that your device will stay on when needed and will not run out of energy. It can also help plan for unexpected or adverse lighting conditions, selecting a storage element that can keep your device powered up.
If you have any questions about creating a power budget for your IoT device please contact us. We look forward to innovating together.
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