July 5, 2017 - So you’re looking to go off the grid, or maybe just build a backup power system to ensure you can survive a long-term grid down situation? If you’re like most people who email us, you probably have a number of questions on how to do it, how much it costs, and what you will need to be successful.
- How many solar panels will you need to power your home?
- How many batteries will it take to keep things running
- What can you realistically power with an off-grid solar setup?
I’ve read through thousands of articles and forums, and talked to hundreds of experts; one thing that never fails is the topic of solar can be downright confusing.
I’m not going to over complicate this article, or bog you down with theoretical examples and difficult mathematical equations. We are going to look at baseline figures on how much power you consume, how many batteries you need to power your home, and how many solar panels you need to maintain those batteries and guarantee you have the power you need when you need it.
The first step in determining how many solar panels and batteries you need to power your home is finding out how much power your home is actually using. There is a mathematical way to determine this, which will get you close; or, there is a simple way to determine your needs that will only cost you about $20.
Estimating your power consumption:
Method 1. The first way looks at the manufacturer’s guidelines to determine power draw. The wattage of an appliance can usually be found on a label attached to the power cord. It’s listed in either amps or watts; you want to know the watts — don’t worry if it doesn’t tell you there is a simple formula for figuring it out.
Volts x Amps = Watts.
For simplicity sake, the average household wiring is 120 volts; chances are, your appliances and anything you plug into the wall runs at 120 volts. Trust me on this one.
So let’s say the tag on your appliance says it draws three amps; you simply multiply 120 x 3 giving you 360 Watts! Now you know how many watts that appliance uses per hour. (Side note, some appliances have a startup draw which can be higher, but for now, we are keeping things simple. We will worry about that later.)
Now we need to figure out our average daily use:
So we take the 360 watts and multiply that by the hours a day we run that specific device and come up with a daily wattage figure. So if we run that device 3 hours a day, we will need 1080 watts a day.
In the case of something like a refrigerator that may draw 400 watts an hour, we need to multiply the number by 24 hours coming up with 9,600 watts a day.
Method 2: This is the method I suggest, because it’s going to give you real-world data, account for your startup draws, and give you numbers based on your unique environment and how you live.
Buy a Kill A Watt Electricity Usage Monitor. For $20 you can buy a device that will measure exactly how much power your appliance uses throughout the day. Run this on each appliance for a week and you have real world data that will guarantee your math is not only right, but it factors in how your family uses these things in the real-world. Screw estimates - let’s get this right from the beginning.
Now that we have our numbers, we can start figuring out what we realistically need and how much our off-grid system is going to cost us. Ok, so I hope I don’t lose you here; but when we are looking at batteries, we need to calculate what we need in AMP hours (AH) – this is how batteries are rated.
Amp Hour (AH) gives you a measurement of battery capacity. In other words, it tells you how much energy can be stored by the battery.
So to come up with the number, we are going to divide the total battery capacity required by the voltage of the battery. So in the case of a 12-volt deep cycle battery, we are going to divide our 9,600 number (watts a day for the refrigerator) by 12 (volts), which tells us, we need 800 amp hours.
So assuming we found some deep-cycle batteries rated at 200 amp hours, we would need four batteries for our system.
Ok so here is where things get a bit complicated:
I wanted to write this article in the simplest terms I could, that way everyone can understand the basic concept, but I know if I leave this out some smart guy is going to come here and start complaining in the comments section, so here it goes.
These are all rough estimates. In the example above I’m going to want to add at least another battery to account for things like discharge, days where the sun might not be shining, etc. I also know that these AMP hour specifications are only a gauge of the relative capacity of a battery.
Peukert’s Law: Here’s where things get a little screwy. A common misconception is that a 100 AH battery will give you 100 amps for 1 Hour. WRONG!
There is a phenomenon referred to as Peukert’s Law. To get the real storage capacity of the battery, you need to understand that the amount of energy stored is dependent on the battery’s discharge rate. The greater the rate of discharge, the lower the delivered capacity will be.
Unfortunately, most manufacturers know that at this point in the conversation most people’s eyes glaze over, so they don’t advertise a Peukert’s exponent. Now consider Peukert’s Law can be affected by things like temperature and battery age, and you can throw your estimates right out the window. See why I tried to keep the article simple?
So for people that don’t want to get to bogged down in the equations, I always suggest overestimating your needs a bit.
Ok, I hope I didn’t lose you there; but don’t worry, we are almost home. Now we can estimate how many solar panels we need to maintain our system.
Let’s say you have a 100-watt panel. You want to take that number and multiply it by the number of direct sunlight hours you have in a day. So let’s say you average about 6 hours of direct sun a day, that one panel can produce 600 watts a day.
Panel watts x hours of sun = Daily Output
Next, take your daily wattage needs, in the case above 9600, and divide it by the daily panel output.
Daily Need divided by Daily Output = Number of Panels
So in the case above, to achieve my 9600 watts a day, I would need sixteen 100 watt panels. Now, of course, this will depend on the wattage of your panels, but you should be getting a better idea of what you need to power your setup.
For the 9600 watt system, I need sixteen 100-watt panels and four 200-AH batteries.
Remember these are baseline figures to help with your estimation; in the real-world, these are only numbers and just like all areas of preparedness you need to compensate for failure points. It’s better to go a little bigger and overcompensate, because when it comes to off-grid power there are hundreds of factors that we can’t account for that are going to popup at some point down the road. If a ten-day storm comes through and you lose the sun, you need to have a backup plan!