Tech Talk: Solar Panel Wattage Selection for your Emergency or Off-Grid Needs

It is good to be prepared for Electrical Grid Interruptions, or to have already taken the steps to be Off-Grid. Selecting the correct Solar Panel Wattage for the battery bank or other emergency or off-grid needs will help avoid trouble while using the Solar Power setup.

Solar Panel Types

The type of solar panel will also be important, there are two types most popular for off-grid and residential. Mono-crystalline solar panels are more efficient with full sun coverage. Poly-crystalline solar panels are more efficient when there is partial sun coverage of the solar panel - for example, if the solar panel is partially covered with snow or shade from a tree or other object.

Ideal sun coverage time is when the sun is hitting the solar panels unobstructed and at a perpendicular angle. At least two hours a day would be ideal for most setups. More solar panels would help with places with lower light levels, but that would come down to the cost and return factor.

Solar Panel Charge Controllers

There are two major types of solar panel charge controllers. PWM or Pulse Width Modulation charge controllers perform decent with a smaller battery setup and limited low light conditions, see picture above. You can see the large heat sink fins of the MPPT Charge Controller below. The easiest way for me to think of how it works is to consider it a capacitor that gathers the lower voltage until it's larger, then pushes it into the battery bank all at once. Lower voltages from low sun coverage would have trouble putting any charge into a large battery bank with a PWM charge controller.

Unsurprisingly MPPT chargers are more expensive - MPPT stands for Maximum Power Point Tracking. Both types of charge controllers convert the voltage down to 14 volts, the ideal charging voltage for a 12 volt battery setup.

A quick way to estimate total wattage for the solar panels goimg by the size of battery bank would be to have close to the same solar panel wattage as your battery bank amp hour total. So with a 800 ahm hour battery bank I would want at least 800 watts of solar panel power. Your local weather patterns and distance from equator may vary, I live in Southern Ontario Canada.

Case Study 1: The Cabin

• 960 watts of 12 volt solar panels
• 800 amp hours 12 volt battery bank
• 60 amp MPPT 12 volt charge controller

The cabin battery bank charges up from low in about 2 hours. After the battery bank is full the excess power is free to use to charge USB battery banks, devices, fridges, and other under 800 watts or so appliances comfortably with the correct size inverters or voltage converters - drawing a minimal amount from the battery bank. Higher power tools and appliances would be able to run for short periods of time on this setup. For longer of course with more solar panels.

Case Study 2: Comparable Performance from a 70 amp Battery Charger

Charging the 800 amp hour Cabin Battery Bank. The battery charger that is ran off the gas generator is designed not to over charge or charge the batteries too quickly. The temporary backup gasoline generator costs on average $1 per hour to run.

The battery charger connected to the gas generator's household voltage usually starts out at around 45 amps when the batteries are low. Low is 11.8 volts with nothing running off the batteries, 11.5 volts is empty. As the batteries become closer to full, the amperage trims down to approaching zero until it shuts off automatically with a fully charged battery or bank - 12.8 volts for a battery setup with no inputs or outputs is Full.

It takes almost 2 hours to fully charge with the gas generator powered battery charger if the 800 amp hour Cabin Battery Bank is low to start with.

Case Study 3: The Camper

• 200 watts of 12 volt solar panels
• 140 amp hours 12 volt battery bank
• 30 amp PWM 12 volt charge controller

The camper setup with no fridge would be great for laptop charging, 20 volt tool battery charging, USB batteries, AM/FM radios, 2-way radios, phones, tablets, and devices of all sizes.

Below is a picture of the camper battery and PWM solar panel charge controller.

The Camper batteries charge to Full from low in 2 hours - with full sun.

Performance Recollection:
This one time a guest brought a blow up queen size air mattress and electric air pump to the camper. The pump sounded like 1500 watts of power usage, like a hair dryer. At first I had it plugged into the 300 watt 12 volt to 120 volt inverter, but I had never seen the voltage roll down from 13.4 volts to 0 volts so quickly before. We used a 3000 watt inverter and it got the job done. I'm sure the batteries were drained of substantial power, the voltage stayed dropped to 12.1 volts afterwards - until the solar panels filled the batteries.

One of the most curious occurences we noticed with the MPPT solar panel charge controllers was that we saw charge pulses to the batteries at night - when the Full Moon was positioned just right.

You may find these previous posts useful:

-https://steemit.com/homesteadersonline/@jackdub/off-grid-how-to-calculate-the-battery-bank-size-for-your-off-grid-system-or-emergency-power-needs
-https://steemit.com/homesteading/@jackdub/my-solar-powered-cabin-setup
-https://steemit.com/homesteadersonline/@jackdub/off-grid-the-winter-months-our-first-8-months-of-off-grid-living-part-2

Have a great day!