Question from Kelly: After reading your articles about taking part of a house off-grid and going solar, I took the plunge and converted the game room in my basement to solar.
I installed several new lights and outlets and connected them to my inverter and everything works great, but the 12V 100Ah LifePo4 battery runs out of juice before morning.
The only things I have running off the battery at night are a refrigerator and a dehumidifier, both of which keep shutting off at some point during the night.
What can I do to make the battery charge last until the sun comes up the next morning and starts charging the battery again?
Rick’s answer: Kelly, there are several variables that directly affect how long the battery bank in an off-grid solar setup will retain enough charge to keep the appliances and gadgets that are connected to the inverter up and running:
1 – The total wattage that’s consumed by every device that’s drawing power from the system.
2 – How much total battery capacity your system has available (in watt hours).
3 – The total wattage supplied by your bank(s) of solar panels.
4 – The amount of sunlight striking your solar panels during the daylight hours.
5 – Obstructions (if any) that block the sunlight from striking your panels.
Determining the total wattage consumed by each connected device
Find out how much wattage is consumed by each connected device and multiply that amount by the number of hours that device is used in a day’s time. That will give you the number of watt hours required to power that device for one day.
Note: This inexpensive meter (affiliate link) will give you the exact wattage consumed by each device.
Now, add all of the watt hours on your list together to get the total amount of watt hours your system will need to provide on a perfect “solar day” (more on solar days below).
Determining the total battery capacity your system has available (in watt hours)
This one is easy. Since your system has one 12V 100Ah battery that means your system’s total battery capacity is 1,280 watt hours (12.8V x 100Ah).
If you had two 12V 100Ah batteries the total capacity would be 2,560Wh (1,280 x 2).
Three batteries would give you 3,840… you get the picture.
Determining the total wattage supplied by your bank(s) of solar panels
Note: The following assumes that all the panels are the same in terms of wattage, open circuit voltage and amperage.
The wattage provided by each panel should be listed on a label that’s affixed to the back of the panel. Add the wattage provided by each panel in your system together to get the total amount of watts that will potentially be available to your system under ideal charging conditions (see the info on solar days below).
Determining how much sunlight will strike your solar panels during the daylight hours
If you live in the northern hemisphere, the farther north you live the fewer daylight hours your panels will have for producing electricity.
Conversely, if you live in the southern hemisphere, the farther south you live the fewer daylight hours your panels will have for producing electricity.
Directionally, you want to have your panels facing due south (or as close to due south as possible).
The “tilt” angle of the solar panels also affects how much usable sunlight will strike them in a day’s time. Ideally, you want your panels positioned at the tilt angle that allows the sunlight to strike them at as close to a 90 degree angle as possible.
You can determine the optimum tilt angle for your location (and for each season) by using this handy online calculator.
Checking for obstructions that block sunlight from striking your panels
There are several potential obstructions that could prevent the sunlight from striking your panels at various times of the day.
For example, is there a tree or a building that will block the sun from striking your panels at any time during the day? If so, that could severely affect the total amount of sunlight your panels can use to produce electricity for charging your battery bank.
If you discover that your solar panels are being shaded by a tree (even for just a part of the day), you can either have the tree removed or move the panels to a location that will receive full sunshine for the entire day.
If your panels are shaded by a building your only realistic option will probably be to relocate the panels to a better location.
Understanding solar days
When it comes to solar power generation, not all days are the same.
If it’s cloudy that will filter out a portion of the sunlight and prevent it from reaching your panels. And if it’s raining the sunlight will be filtered even more.
This filtering of sunlight can greatly reduce the amount of watts your solar panels can produce at a given time.
For example, a 600 watt string of panels might only produce a total of 400 watts, 300 watts or even less on days that are cloudy. And if it’s raining they might well produce as little as zero watts.
Fewer watts coming in from your solar panels means less wattage available for powering your devices and charging your battery bank.
A day with no clouds and full sunshine is a perfect solar day, meaning your solar panels will produce as much power as they can possibly produce over the course of the day.
A day with intermittent clouds or scattered periods of rain is a so-so solar day, meaning at least some power will be generated.
And a day that’s heavily clouded or rainy for most or all of the day is a poor solar day, meaning little to no power will be generated on that day.
If your location is frequently affected by so-so, or even poor solar days you’ll need to adjust your system accordingly to ensure that your battery bank has enough stored energy to power your devices for at least a couple or three days until the solar conditions improve.
How to ensure that you’ll always have enough battery charge to keep your gear up and running
First of all, you need to make sure you have enough battery capacity in your system to supply the amount of watt hours needed to power your devices for at least one night, and preferably for two to three days.
As I mentioned above, you’ll probably want enough battery capacity to power your devices for at least two to three days in case you have several days during which there is a reduced amount of sunlight due to clouds or rain.
How do you increase battery capacity? By either adding additional batteries or replacing your current battery with one or more batteries with a higher capacity rating (200Ahs instead of 100Ah, for example).
Next, you need to make sure you have enough wattage coming in from your solar panels to fully charge your battery bank, even on days with reduced sunlight.
This is done by “over-paneling”, i.e. adding an extra solar panel or two to your system so your batteries will receive the same amount of total wattage from the panels on so-so solar days that they would otherwise receive on a perfect solar day.
When determining how many solar panels you’ll need, be sure to make that calculation based on the number of hours of daylight your panels will receive on the shortest day of the year.
Many folks with off-grid solar installations have enough solar panels installed to easily get them through the night during the summer but not on the shorter winter days. You’ll need to plan your solar panel array accordingly.
I hope this helps, Kelly. What I recommend is evaluating your current system with the above info in mind and making adjustments as required. With any luck the adjustments might be relatively minor.
And now, one final thought…
Off-grid solar power generation is actually pretty simple in general, but there are many factors at play that are beyond the scope of this post.
If you’d like to learn more about building small-scale off-grid solar generation systems, I strongly recommend that you check out the videos from Bob at Projects in Paradise 808, a fantastic YouTube channel specializing in building multiple small solar power systems to power separate areas of your home via their own dedicated solar system.
As you will see, Bob is very nice and down-to-earth. I have learned a lot from him and I believe you will too. Check out his videos!