Sizing Solar Power for Off-grid Field Studies – Part I

Part one of a three part series.

This article takes you through the necessary considerations for a solar-powered field station.


Selecting a solar panel, charge controller, and battery suited to supply power to a remote data collection site can be a challenging task, even for an experienced user. One of the problems is that most of the guides available for selecting photovoltaic (PV) solar power options assume that it is a permanent installation for a home or business rather than a temporary, unmanned installation. With this article, we hope to present some of the challenges and solutions to providing reliable power for autonomous field stations.

Figure 1: Typical Solar-Powered Remote Installation

Figure 1: Typical Solar-Powered Remote Installation

IMPORTANT DISCLAIMER

Even though we are exclusively describing independent systems not connected to the electrical grid, you should still ensure that your installation is in compliance with whatever electrical, safety and building regulations apply in the jurisdiction in which you deploy it. The following guide does NOT constitute professional consultation. Solar panels and deep cycle batteries can produce potentially dangerous electrical currents capable of starting fires or causing injury and death. Always exercise caution and consult an expert if you are unsure about anything.

Determining Power Requirements

The first step is to calculate the power needed for your station (electricians call this the load). Each device to be powered should list in its specifications either its average power (in watts) or average current (in amps or milliamps). If you only have the average current, then multiply by the system voltage (typically 12 V) to get an average power, then add up all devices to get the total average power load in watts. Multiplying by 24 gives the average daily power consumption in watt hours (for a small installation, this is a more appropriate unit of measure than kilowatt hours).

Sample Load Calc

Determining Battery Requirements

Now that the daily energy consumption is known, we can determine the battery required. A deep-cycle, sealed lead-acid battery is typical for this type of application since it operates over a wide temperature range, is usable in any orientation and, unlike a flooded lead-acid battery, it requires no maintenance. In the future, lithium-ion battery packs (typically found in laptops and cellphones) may be a good choice due to their lower weight per capacity, but currently these are expensive, require specialized charging systems, and are difficult to ship due to safety regulations. Sealed lead-acid batteries are available in two types: absorbed glass mat (AGM) and gel cells. Both are appropriate for unattended operation, with AGM batteries being slightly less expensive and gel cells allowing a slightly higher ambient operating temperature. Whichever you choose, be sure that your charge controller supports that type of battery (more on charge controllers later).

Sample Battery Calculation

Calculating Battery Capacity

Battery capacity is typically given in units of ampere hours (Ah). To calculate the ampere hours needed, take the average daily energy consumption calculated earlier and multiply by the number of days of autonomy you would like the system to have (i.e. the number of days in a row that the location might have complete cloud cover). To maximize battery life, the battery should not be discharged more than 50%, so multiply the resulting number by 2. Then derate 10% for inefficiencies like self-discharge. If the ambient temperature during your deployment will be less than 25 °C, you should also derate the battery capacity for temperature effects. Each battery manufacturer will have its own specifications, but the one shown in the example is typical for AGM batteries. Finally, divide the result by the system voltage (12 V) to find the desired capacity in Ah and chose the battery with that rating or higher.

Battery Temp Derating Table

In this example, 45.4 Ah are needed, so a battery with a 50 Ah capacity would be adequate.

Part 2: determining the solar energy available at the deployment site…

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