John L Errington MSc

John Errington's Experiments with an Arduino

Light power measurement and heater control

As I have a 3.6kW solar panel installation it makes sense to take advantage of the free electricity whenever possible; so I try to time my use of high power devices (shower, washing machine, etc) to match the highest power output from the panels. The aim of this project is to detect the times when power is being generated, and turn on a small freestanding heater, as in the experimental circuit shown here.

The solid state relay includes an opto-isolator, to protect the arduino (and the user) from mains voltages.

When in use, solid state relays produce some heat.
The voltage drop across the device in the "on" state is about 1.5V so each amp produces 1.5 watts. This heater is rated at 2300W 2300/230 = 10A so we anticipatea maximum 15 watts of heat from the SSR.

The system will be connected in the same way as a mains adaptor, using a line mounted mains socket to feed the heater.

As the heater wont be needed in the high summer its planned use will be during periods when the solar panels will be producing less than 2kW so the heater will be used on its setting of 600W or 1500W so here we expect only 5 or 10 watts.

The easy way to deal with this is to build the unit into a metal box, taking care to ensure the mains connections are fully insulated and the box is earthed. The MEGA is used for experimentation, a cheaper and smaller "nano" will be used for the final build.

Is it worth doing?

The diagram below shows the number of times a power of more than 600, 1200, or 1500 watts was available from the solar array.

free power during cold months

These data show that over this period a 800W heater could be operated from FREE solar electricity for a total of 372 hours, giving 0.8 * 372 = 298 kWh of free heat. A 1.6kWh heater would only be able to be operated for around 166h giving
1.6*166 = 266kWh;, while a 2.0W heater would be on for only 90h yielding 2.0*90 = 180kWh.

With electricity at £0.16 per kWh 298kWh = £48

Cost of unit:

Arduino Nano (compatible)   £6
Solid State relay   £2.50
Aluminium case   £6
Solar panel - from broken calculator, garden light etc.   £0
5V power supply (USB charger type)   1.40
Assorted bits - wire, grommets, fuse, plug etc allow   £2.50
Total (not including labour)   £18.40

arduino nano compatibleThis is the Nano compatible that will be used. Like the Mega and Uno it has Analog inputs that will be used to monitor light power, and digital / PWM outputs. Power is provided by a mains - 5V USB 'brick' & USB cable.

SSRssr circuit


This is the Solid State Relay that will be used to switch the heater. Its rated 25A at 24 - 380V AC. The switch is turned ON by a voltage of >2.4V driving a current of around 8mA through the LED's. The zero crossing circuit ensures the switch only turns on when the mains voltage is near zero, which prevents interference arising from spikes on the line.