How Much Battery Is Enough?
So, you've decided to go off the grid and utilize batteries. Maybe even add some solar? Good for you! But now you're faced with the daunting task of sizing the batteries for your off-grid system. Don't worry, it's not actually as complicated as it sounds. Let's walk through it.
Before we get going though, I want to really impress on you that you want to size your batteries BEFORE you start buying solar panels! You need to know how much battery to charge before you buy expensive controllers and panels.
OK So how do I choose the batteries for my system?
If you're simply upgrading from an existing lead setup, to Lithium/LiFePO4, you could check your existing battery's Amp hour rating and decide if that's good enough. If you were happy with the performance before, you can simply buy the closest size (but go up, not down) in a lithium and call it a day (but do check that your chargers will work ok).
But what if you want more? Or you don't even know what you need to run....
Well, you could hop on to our website and just buy some big ones right? I mean, you could... but you won't really know what you're getting. You probably could get away with this approach, but wouldn't it be better to know how your system will perform before you spend your money?
Before you actually start choosing batteries, it is best practice to determine your daily energy consumption first. This includes all the appliances and devices you plan to power with your battery. Are you a minimalist who only needs to charge a phone and a laptop, or are you running a full-blown home theater system with a fridge and a microwave?
Be absolutely honest with yourself here. We won't judge, but your battery will!
You need to figure out how much energy all the 12V devices, and all the 120V devices will consume every day. If you already understand Watt hours (or Amp hours) and know your devices, you can just add it all up for the day.
Once you have a rough estimate of your daily energy consumption in watt-hours (Wh), you'll need to factor in the number of days you want your system to run without sunlight (or just being away from shore power). This is where things can get a bit tricky. You'll need to do some math (or use a fancy online calculators) to figure out the total capacity of the batteries you'll need.
A great option, for your inverter powered 120V stuff, is to use an energy meter (like a "Kill-A-Watt") to actually measure the consumption of your devices over a 24, or 72 hour period. Accuracy in your estimates will pay dividends when it comes to your overall satisfaction.
How to use a Kill-A-Watt device to measure consumption?
Ok. Make a list of the devices you plan to power. Include everything like toasters, kettles, phone chargers, lights, etc. Make a table on a sheet of paper, or a spreadsheet in your computer to keep your notes.
I strongly suggest ordering a Kill-A-Watt type energy monitoring device to use for this stage of planning. It's like a Fitbit for your appliances, giving you real-time data on how much energy they're sucking up.
Simply take turns with all your devices, plugging them into the energy monitor for a day (or two, three, or even a week), and voila! You'll see how many watts each one is using over a given time period (measured in Watt hours). Use each device like you normally would. Record the 24h consumption in your spreadsheet at the end of each test. (Don't forget to clear the memory before each new device is connected)
With the Kill-A-Watt, you can identify energy hogs that may not work well on a battery powered system, and this can help you make informed decisions about which appliances to use, or ditch. (Spoiler alert: Anything with heaters in it, can be a huge power hog)
For example, I was surprised to learn that my coffee maker was using about 400Wh of energy per day (This equates to about 30 amp hours of 12V battery). That's to make one pot, and keep it warm for an hour or two. When it broke, I replaced it with a thermal carafe style (no heating pad for the pot), and now it only actually uses about 170Wh per day (approx 14 amp hours). That was a pretty substantial reduction, especially when factored over multiple days.
Ok so now can I choose batteries? How do I choose them?
Yes. Now you can choose your batteries.
Take all the watt hour power numbers you have recorded for your devices. Calculate the average for a 24h period for each, and add all the devices up. That gives you a daily consumption number. (Watt hours per day or Wh/day)
Now, decide how many days you want to be able to run off the batteries, with no external power input. For a solar setup, the usual number to use here is 3 days. But you can adjust that longer if you want more reserves. For example, if you're in a camper, and want 7 days, then use 7 days.
Multiply your 24h average consumption by the number of days you want to run. This is your target number for a battery bank.
Assuming that you're planning to use LiFePO4 style batteries, you can skip to the next step. If you're planning to use lead (FLA, AGM, etc.) then you need to multiply your number by 2. This is because you don't want to regularly discharge your lead batteries past 50%, or you risk damaging them (which drastically shortens their lifespan).
Still with me? We are almost there! I promise!
Since we have to use an inverter for our 120V electrical power, and nothing is perfectly efficient, I recommend adding 20% to your calculated numbers. Depending on the inverter you choose, this may be overkill, but given a choice it is almost always preferable to have extra storage instead of coming up short. Have you ever heard anyone complain that their devices are still on and working? Yeah, neither have I.
Alright, final step. Take that number you've calculated, and convert it to amp hours.
Wh ÷ Battery Voltage = Amp Hours (Use 12.8V for LiFePO4, 12V for Lead)
Example: 2300Wh ÷ 12.8V = 179.7Ah minimum LiFePO4 battery size (~192Ah for Lead)
Round up to the next size of battery and you're done! In this case it's probably a 200-230Ah model.
So there you have it, you have sized your batteries for your off-grid system and used a Kill-A-Watt device to measure consumption. With a bit of planning and the right tools, you'll be well on your way to living that sustainable, off-grid dream.