Tutorial: Solar Power Selection Guide for Maker Projects

Selecting a Solar Power system for your project is a confusing thing the first time.   This tutorial is an introduction to sizing solar power systems to fit the computer projects being powered.

We will be looking at 5 different options for powering solar projects at SwitchDoc Labs.   They are:

• SolarMAX2 – coming in September on Kickstarter – better, faster and cheaper!
• SolarMAX Lead Acid (Retired)
• SolarMAX LiPo (Retired)
• SkyWeather Solar Extender Kit
• OurWeather Solar Extender Kit

SwitchDoc Labs also sells three stand-alone solar power controllers, SunAirPlus2 and SunAirPlus3 and SunControl.

 

Full Solar Selection Chart

 

 

What the Columns Mean:

• Solar Panel Nominal Voltage – What voltage of solar panels supported
• Maximum Solar Power Charging Power/Current – The Maximum amount of power the charging system can use from the solar panels (Note: Bigger panels are still useful – see below)
• Battery Type – LiPo or Lead Acid batteries
• Solar Charging Environment – What is the potential solar power available for your particular placement and environment (see below)
• Panels Included – Any solar panels included in Kit
• Data Collection – All of these systems collect data on solar power voltage/current, battery voltage/current and load voltage/current
• Computers Powered – What kinds of small computers that can be powered by these systems

What is my Solar Environment?

Your particular solar environment is dependent on your climate, how you place the panels and your latitude.   High means a very good solar environment is needed for good results (although you can still run your computers – they just turn off more), Medium is a more typical solar environment and Low means it will work just about anywhere as long as you can position your panel well.

To get a handle on what your climate for solar panels here are some resources:

• Cities Ranked by Sun in the United States
• NREL PVWatts Calculator  This tool allows you to calculate the number of kWh / Year you can expect at your location.   You can scale it down for your system (if you are looking at a 100 W panels, we calculated a total of 125 kWh here at SwitchDoc Labs.).  Doing the same calculation for Curacao (the home of Project Curacao3) yields 171 KWh.

What do Kilowatt Hours Mean to My System?

A SkyWeather Pi 3B+ system will consume about 2.5W of power.  A SkyWeather Pi Zero W system will consume about 1W of power.  Let’s use these for our examples below.

Let’s make an assumption that your system uses 2.5 Watts of power and you are running it 24 hours per day.  Calculate your kWh per year by multiplying the kWh/year = (SystemWatts /1000)*24Hours/Day*365Days/Year.

For our 2.5W system (about what a full SkyWeather system uses) this gives your kWh/Year Required = (2.5/1000)*24*365 = 21.9 kWh/Year.

Looks like we can build a system even at SwitchDoc Labs to run this system 24/7 using SolarMAX LiPo or SolarMAX Lead Acid.

To calculate if your system can perform at this level, you use the wattage values of the solar panels you are using and perform the same calculation.     Note that just because you have a 100W solar panels, the maximum you can generate from your system is the Maximum Solar Power Charging Power column (72W for SolarMAX Lead Acid for example).   Using SolarMAX Lead Acid at SwitchDoc Labs as an example (72W Max), using the NREL PVWatts tool, we get 90 kWh/Year generated.   123 kWh generated > 21.9 kWh needed so we should be able to build this system and run it 24/7.

Using the SkyWeather Solar Extender Kit with it’s four 2W Panels (Total 8W) we can generate (from the NREL PVWatts tool):

10 kWh/Year for the 8W of panels with the SkyWeather Solar Extender Kit.   This means that you can expect our example system to run roughly 1/2 of the time on Solar Power at SwitchDoc Labs using these smaller panels.   We can improve this by setting the array tilt to 45 degrees but only to 10.4 kWh.    Note for usage of the NREL PVWatts tool:   It won’t allow you to enter 0.008 kW directly into their calculator.   Enter 0.08kW (80 watts) and divide the resulting production by 10.   

Note:   The SkyWeather Solar Extender Kit is designed for a SkyWeather powered by a Raspberry Pi Zero W which will reduce the power required by the system down to about 1W.    This drops our power required by the computer system to 8.76 kWh/year required which means our SkyWeather Pi Zero W based system would work!

SkyWeather2 really requires a Raspberry Pi 3B+ or a Raspberry Pi 4B.

Using these tools will only give you an approximate estimate because there are additional losses in this tools  as it assumes you are using an AC based system and we are using a DC based system, but it is still about correct for these ball park estimates.

 

Note:  The difference between Power and Current

If you are going to compare currents with different voltages, then you need to compare power.   Power is measured in Watts.  Power is the product of current and voltage (Power = Voltage X Current).  P = VI.   Of course, if all your currents have the same voltage, then you can just compare currents.   Looking at the Raspberry Pi chart below, and using the Watch 1080p Video (this column by the way is pretty close to what it takes to run a SkyWeather weather station on a Pi), then running a Pi3B+ consumes 2500mW (2.5W) – Power = 510mA * 5V or 2500mW.   1000mW = 1W, so we get 2.5W.

 

How Much Power do you Need?

One of the first things that comes up in a solar powered design is how to size our system.  The three main questions to be asked and answered are:

1. How much power will you need to run your project?
2. How long do you want to run your project without Sun?
3. What is your local sun environment?

The first thing you need to do when designing a solar powered system is to determine the power requirements for your solar powered design.

 

How much power will you need to run your project?

Since your project power consumption is going to be different than other projects, it is always best to measure your own power on your own setup.  Here is an article that shows you how to do that.

 

A typical SkyWeather project will consume about 200mA (1W) with a Raspberry Pi Zero W and about 520mA (2.6W) with a Raspberry Pi 3B+ (as in Project Curacao3).   Why not always use a Raspberry PI Zero W?  Sometimes you want to do other things with the processor like a local web server (Graphana) ,  machine learning or AI work on the incoming pictures.   We are doing AI with Project Curacao3.

For Raspberry Pi Systems, here is a good start with  a chart from RasPi.tv showing Raspberry Pi current when doing various tasks:

 

By the way, it’s crucial that your solar power system can handle the startup currents for the Raspberry Pi family.  They are substantial!  Look at this article.

Power Consumption of Arduino, ESP8266 and ESP32 Systems

These numbers will vary quite a bit from design to design because of the huge number of boards out in the market place.   Here are some estimates for Arduino based units.

And for the ESP8266 (1 CPU Core) / ESP32 (2 CPU Cores) based systems (very dependent on WiFi usage and use of low power mode):

• ESP8266 – 0.3W to 0.85W (60mA – 170mA)
• ESP32 –  0.35W to 1.35W (70mA – 270mA)

You can really, really reduce the power required on the ESP32 device by using power management modes in many applications.  You can do the following things to reduce the consumption on the ESP32:

• Keep the use of the CPU Cores down
• Try deep sleep, but there are number of reliability problems with deep and light sleep on the ESP32.
• Manage using only one Core
• Maintain Radio Silence (Keep the Radio off!)
• Stay away from BSD-Style Sockets
• Put the right matched antenna on the ESP32 (50 Ohms)
• Switch Off peripherals on the chip
• Use the RAM Cache – Flash memory is power hungry
• Turn off Logging to your UART
• Turn off external peripherals
• Use the ULP (Ultra Low Power co-processor) – It is really cool, but really complicated to use