Battery Basics – RV Battery Sizing (Capacity)
We’re going to give you 3 ways you can size your RV battery capacity. Getting your RV battery size right is going to give you the freedom to explore and save you money down the road. Let’s go over how to size your RV battery and why it’s important.
Battery Basics – A Series to Learn About Batteries
Battery Basics – Types of RV Batteries
Battery Basics – How to Care for RV Batteries
Battery Basics – True Deep Cycle vs Dual Purpose
Battery Basics – How to Wire RV Batteries
Battery Basics – RV Battery Sizing
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Terminology We’ll Use
- 12-Volt & 6-volt – RV batteries come in 2 main voltages, 12-volt and 6-volt. RV electrical systems are generally 12-volt in the USA. A single 12-volt, or 2, 6-volt batteries are needed for an RV.
- Battery Bank – This just means more than one battery. It could be 2, 3, 4, 12, etc. It can also reference 12-volt or 6-volt batteries.
- Amp – Amp is short for Ampere. Ampere is a measure of electrical current that is standardized. It has a relation to volts and watts. It’s a standardized measurement found on many devices and batteries.
- Amp-hours – This is a common rating system for quantifying the number of amps over a period of time. It’s also how many deep cycle batteries are rated (80 Amp-hour, 100 Amp-hour).
- Volts – A volt is another measurement for electricity. It also has a relationship with amps and watts. Volts can be specific for a direct current (DC), or Alternating Current (AC) system. Batteries use DC.
- VDC (Volts Direct Current) – Direct current is a term to differentiate between types of voltages. Batteries are a direct current while powerlines and household power are Alternating Current or AC.
- Watt – A watt is a measure of power. It relates to amps and volts. A watt is a good measurement of electrical work across DC and AC devices.
- Watt-Hours – The total amount of watts used in an hour. Some devices are rated in watt-hours and it’s useful for calculating power consumption.
- Draw or Load – This refers to the electrical consumption of a device. Light load, small device, heavy load, bigger device.
Why is RV Battery Capacity Important?
Your RV batteries are like a gas take for your car, but they power the electrical devices in your RV. If your battery is too small, your electronics won’t last long. If your batteries are too large, it could be a waste of money, space, and added weight you don’t need.
If you do any amount of boondocking, dry-camping, or camping off-grid you’re going to want to make sure you have the proper size battery bank. Your RV battery powers your lights, fans, water pump, TV, and recharges your phone, computer or tablet if you have them. Combine your battery with a power inverter and you’ll have the capabilities of running household devices like a coffee maker, microwave, toaster, or electric blanket.
What do we Mean by Battery Size?
When we say battery size we mean battery capacity. Battery size is a common term people use when they aren’t sure what they need for an RV battery. The physical size of the battery and capacity aren’t consistent. Not all batteries that have the same size can be used for the same application. For instance, car batteries won’t last very long in an RV application even though the physical size can be the same.
See our page on Types of RV Batteries for a deeper explanation.
How is Battery Capacity Rated? (Deep Cycle Battery)
Deep cycle lead-acid batteries are recommended for RV use. They are built differently than lead-acid batteries for your car and have a different way to identify performance. Car batteries are commonly rated in CCA or Cold Cranking Amps, while deep cycle batteries are rated in Amp-Hours.
The amp-hour capacity rating means 1 amp for 1 hour or basically, amps times hours. It’s a standardized way to understand how that battery should perform. You’ll commonly see the amp-hour (Ah) rate printed on the battery such as 100AH or 150AH.
Standardized Time Rating (@20, @100 rates)
It’s worth noting that you might also see some info regarding Amp-hour capacity with an @ symbol and either 5, 20, or 100 – sometimes 24. This is a way to rate the capacity of the battery based on when it dies. It’s also referred to as “20-hour rate”, then a number of amps.
This @20 rate is how many amps a battery like this supplied for 20-hours until the voltage of the battery was below 10 VDC. So an 80 amp-hour battery supplied 4 amp per hour for 20 hours before being depleted.
Careful with the @100 hour rate. Some manufacturers use this rate to make the battery look better than it is. Due to how batteries are built and the resistance characteristics, a lower power draw on a battery will allow it to last longer than if you were to pull a heavier load and make it seem like you have more capacity.
A battery that has a 1 amp draw will see less stress and last longer if you compared a battery with a 25 amp draw. The relationship is not constant. Kind of like walking and running. You might be able to walk for miles and miles, but you can full out run for that long – you’re tank goes empty faster with heavier use.
If a manufacturer uses a @100 hour rate but not the @20 hour rate they are trying to make their battery look better than the competition.
Lithium Battery Capacity Ratings (Watt-hours)
Lithium batteries are become very popular and are a go too for many RVs. Some manufacturers of these batteries are throwing in another rating along with an Amp-hour rating or sometimes just by itself, and I think it’s useful.
I’ve been seeing lithium manufacturers add a Watt-hour rating to their batteries. Many lithium-type RV batteries are built at 100 amp-hours but also include a rating of 1,200 Watt-hours. That is 100 amps x 12-volts = 1,200 Watt.
When is RV battery Size not important?
There are times when the size of your RV battery doesn’t really matter. If your RV stays plugged in all the time there isn’t much need to worry about the battery capacity.
When the RV is plugged in, also called shore power, the battery is constantly fed power from what’s called the RV converter.
Your RV has a 12-volt DC system and a 120-volt AC system. The 12-volt side of the system is connected to the battery. When the RV is plugged into shore power the 120-volt AC power is converted to 12-volt DC power and everything is powered from shore power. If you ever planned on operating just off the battery sizing your batter properly would be wise.
How to Size RV Battery Capacity?
In order to size your RV battery, you’ll need to know how much power you use on average or be able to estimate your average power usage. This can be done using a tool that will record the power consumed by estimating your usage based on electric device specs, or by just going camping and measuring the battery before and after camp.
So here are 3 ways to figure out how much battery capacity you need:
- Using tools to track usage.
- Estimating your usage based on your components.
- Estimating usage based on your current setup and battery.
Size Battery Capacity Using Tools (the easy way)
The easiest way to properly size your RV battery for off-grid use assumes you already have your RV and you’re using it. The method involves using a tool that records real-time electrical consumption.
There are devices that can tell you the power consumption of a device when it’s plugged into it. There are other devices that can tell you the total power consumption used over a period of time. These devices are very handy as they show your real usage, not just an estimation.
KILL-A-WATT Meter
A popular device to use that can tell you the consumption of a device is called the KILL-A-WATT meter.
This device can be used on microwaves, toasters, blenders, or other 110 VAC devices. The only downside is that it won’t capture your usage from using 12-volt devices such as your fans, lights, or water pump unless you can plug in your entire RV.
You’re going to plug your devices into this meter and then plug those into an outlet. The KILL-A-WATT meter will record amps, watts, and watt-hours of the device.
You’ll do this on all your devices that you plan to use when you’re camping. If you use a device several times a day it’s good to leave it connected to a single device for a 24 hour period to see the total consumption.
But you can also measure multiple devices too. The KILL-A-WATT meter can handle up to 15-amps or about 1500-watts. You can plug multiple devices into a power strip and let it record. But if your RV uses more than 1500-watt at one time there’s another device that can work with 30 and 50-amp loads.
Power Watchdog, 30 & 50-Amp Versions
The Power Watchdog not only shows power consumption but can also protect your RV from unstable power sources. It’s essentially a very smart surge protector that will sync with your phone and give you all sorts of information.
This is by far the easiest way to find your regular power consumption so you can size your battery. A few camping trips to an RV park or using your RV in your driveway connected to the house and you’ll get great usage information.
The Power Watchdog comes in two flavors:
Most smaller RVs, 30-ft travel trailers, Vans, small class b’s are going to be 30-amp. Larger RVs or buses will likely be 50-amp. Consult your RV dealer or manuals to figure out what one you’ll need.
Estimate Battery Capacity With Device Data
Not everyone has their RV ready to go camping. For those of you that don’t have your RV yet, or just don’t want to invest in another tool, you can estimate your power consumption and size your battery to this estimate.
This method of finding your battery capacity is a little less accurate so we recommend that you increase your final estimate by 20% just to be on the safe side. It’s not a pleasant experience to get new batteries and run out of power on your first few camping trips.
Estimating battery capacity based on what you might use is going to require you to think about all the devices that you want to use and either get electrical data off the device or try to find this information elsewhere. But first, you need to learn a formula where we can calculate watts.
The Basics of Wattage Estimation
Before we get started let us give you an overview of what needs to happen.
What we are going to accomplish is getting a watt-hour estimate of your power consumption. We’ll then use this estimate and divide it by 12-volts to get an amp-hour rating. If you use a 24-volt battery bank you’ll divide by 24-volts.
So if we estimate that we use 900-watts (watt-hours) on an average day, we’ll divide that by 12-volts and get 75-amps (amp-hours). 900 / 12 = 75
This means our battery capacity NEEDS to be 75 amp-hours or bigger. And to give us a little cushion I would add 20% to this for a total of 90 amp-hours.
Sizing Lead Acid Batteries
For a lead-acid battery, you’ll want to double this figure. You should only use around 50% of a lead-acid battery for the best life of the battery. That would be 180 amp-hours of battery capacity. This will give you a full day of power, and if you get into an emergency situation you’ll have 2-days but will do a bit of harm to the battery.
Sizing Lithium Type Batteries
For lithium-type batteries you can use 100% of the battery. It’s common to find lithium batteries rated at 50, 100, 200, and 300 amp-hours. At a minimum, you should consider a 100 amp-hour battery. If you have solar connected and sized right, you’re all set for off-grid. Alternatively, you can get a much larger bank and have days worth of power. With a 200 amp-hour battery at 75 amps a day you’ll have about 2.6 days’ worth of power before you need to recharge.
This is the basics of what we are going to do to find what battery capacity can work for you. Next, we need to find the total wattage.
Device Wattage Formula
In order to calculate how large your battery capacity needs to be we need to get an accurate estimation of power consumption from both 12-volt and 120-volt devices. We’re going to do this by recording watts from the device, or calculating watts from the device’s electrical sticker.
All devices are required to have the electrical characteristics printed on a sticker or stamped on the device. If the sticker isn’t on the device you should get a KILL-A-Watt meter and find it that way. However, not all stickers are going to display the same information. Some devices will show watts, which means the number of watts per hour. Or some will show amps and volts. We can calculate the device wattage from the amps and volts (amps is the number of amps per hour so watts is still per hour).
The formula you need to be familiar with is Watts = Amps x Volts.
Household or 120-Volt Devices
Most household devices or 120-volt devices will have watts displayed. This is the max watts that can be used.
It’s worth noting that not all devices will use the max wattage displayed. If you have a power setting you’ll get close to max on the highest setting.
Saller 12-Volt Devices
Many small devices will have amps and volts listed or if you’re lucky amps, volts and watts. If you have a device that only lists the amps and volts you can calculate watts using the formula above – Watts = Amps x Volts.
My computer plug has a sticker that lists 20V, 2.25A, and 45W. Luckily I can see the watts, but if I didn’t have this I would use 20 x 2.25 = 45.
My computer will consume 45-watts an hour at full max potential. That is if my battery needs a charge, the screen is on full power, and the CPU is working hard.
But in reality, my computer will consume less, but I would still estimate with 45-watts just to be safe.
Why do we Calculate Wattage?
Wattage, volts, and amps all have a relationship with each other. But wattage is a universal measurement that makes it easy to calculate how all your devices with different voltages and amperage will impact a lower voltage system.
Also, since there are 24 hours in a day it’s also handy that watts or watt-hours is the number of watts over an hour. When we calculate watts we can easily calculate total wattage for a day so we can size the battery and solar system just right.
An AMP is not ALWAYS the Same
Volts and amps have a relationship too, but if you think that ALL amps are the same, it’s a mistake. Amps have a proportional relationship with volts. The higher the volts the lower the amps, the lower the volts the higher the amps. It’s tied for each device and can’t be changed.
For instance, let’s say a device uses 1.1-amps at 120-volts. If we want to power this device off a battery we need to factor in the voltage of the battery system at 12-volts, the base power source. We do this so we can get a clear picture of how many amps a 120-volt device uses when powered off a 12-volt battery system.
We want to do our formula above, Watt=Amps x Volt. 1.1 x 120 = 132 (watts). We can then rewrite the formula to find amps, Amps = Watts/Volts, and sub in our two knowns. 132/12=11 (amps).
There’s a big difference between 1.1-amps and 11-amps. If you run this device for hours it can pull some serious power you didn’t expect. This is why we are going to convert all of our devices to watts and divide that number by our base voltage of the batteries of 12. If you have a 24-volt battery system you’d want to divide by 24.
Just for fun, if we use the same numbers above we can find that 132-watts off a 24-volt system is 5.5-amps, 132/24 = 5.5 (amps)
List All Your Device Info
Now that you know where to get the info you’ll need to start collecting it. I find it easy to just use a spreadsheet or scratch pad for this info. It’s going to look something like this:
| Amps | Volts | Watts | Hours Used | Total Watts |
|---|---|---|---|---|
| 2.2 (known) | 5 (known) | 11 (calculated) | 3 (estimated) | 33 (calculated) |
| 1.1 (known) | 120 (known) | 132 (calculated) | 1.5 (estimated) | 198 (calculated) |
| 50 (known) | 5 (known) | 250 (calculated) | ||
| 10 (known) | 110 (known) | 250 (calculated) | .3 (known) | 75 (calculated) |
| 556 (Total Watts) |
Your list will be much larger but your goal is to list out everything you can so you can start to understand how your devices will impact your battery system. It wise to estimate the time conservatively so you don’t short yourself.
A note on hours of use – If you have a device that you don’t use everyday but occasionally, it’s worth taking the time to estimate the usage for your daily average.
Calculate Amp-Hours Needed from Total Watts You Calculated
One you calculate the total watts that you will use on an average day you can then calculate what this means in battery capacity, Amp-Hours.
Lets say your camp trips are about 2 to 3-days long, we should simply use 3-days to make sure you have the power you need. You’ll want to take your daily average wattage and multiply that by 3-days.
If we use our wattage from above we get 1,668 watts, also can be 1.68 kilowatts.
We would then take this number and divide it by 12 for our 12-volt battery (base power system). We would take 1668/12 = 139 amp-hours.
If we do off-grid camping without another charging source we need a battery bank that will handle up to 139-amp-hours. To be safe we generally recommend adding 20% to this number, so lets estimate a battery bank that can handle up to 160 to 170 amp-hours.
If using any lead-acid battery you should now double this number, so we’re looking at 320-amp-hours. Read this to understand why we double the battery size with lead-acid.
If using a lithium type battery we might consider a 200-amp-hour battery bank, we don’t need to double it like with lead-acid battery technology.
Using Solar To Recharge Means a Smaller Battery Bank
If you’re doing some calculating you can quickly see that a large battery bank is expensive. And if you size your battery to your off-grid usage you have a limited amount of time you can camp before you have to recharge. But if you get a solar system installed that will charge the battery daily you can forget the large battery banks and the time restriction. You’ll be able to stay out and camp as long as you want.
This is where solar really shines. It gives you so much freedom to camp however long you want. It also allows you to use more devices and have a bit more comfort when caming. You won’t have to worry so much about your battery unless it’s super cloudy or you sized your system wrong.
We’ll link to another article that goes into solar sizing here. But for simplicity, you can take your total calculated daily watts and divide it by 6-hours (summer) or 3-hours (winter) to find how large of a solar system you’ll need to get. So if we use our 556-watts from above and divide it by 6 (for summer camping), we get 92. You could get a 100-watt solar system for your RV and not worry about running out of power.
Sizing Your Battery Bank Based on Usage
Not everyone gets an RV and jumps right into dry-camping. Most people will get their RV and do some camp trips at a campground. It’s convenient to have power, water, and sewer and it makes getting used to your RV a bit easier.
But eventually, the desire to camp wherever you want pulls on us all. So if you have an RV with a battery and primarily use it at a campground you might be able to size your off-grid battery bank by seeing how your current battery handles off-grid camping.
The strategy here is to understand that you’ll need a new battery bank to go off-grid camping because your current system is likely too small. It’s not wise to use a battery until it dies, but that’s what you’re going to do here. We’re going to see how long it takes for your current battery to give up and then calculate an off-grid battery bank to that size.
Careful With Dealership Recommendations
There is a good chance that you didn’t give much thought to your battery when you bought your RV. The dealer probably recommended a base battery and you said “no problem”.
Dealerships don’t put in the best batteries most of the time. There isn’t much money in selling batteries for them and their goal is to sell the RV and get you out of there. So battery knowledge isn’t great. They likely gave you a cheaper dual-purpose battery. You can read why we don’t like these here True Deep Cycle vs Dual Purpose Battery.
If you do get lucky with a knowledgable dealer they’ll have multiple options for batteries and likely go through a battery sizing exercise listing out all your usage as we instructed above. But the chance you have a weak battery for off-grid is likely.
Measuring Battery Voltage for Off-Grid Battery Sizing
The basic strategy here is to start with a full battery, camp for a day, and measure voltage to see how far down the battery is.
You can do this several days in a row, or repeat the process with a full battery several times.
Renogy has a great chart on battery state of charge based on battery types.
Keep in mind that these voltages are estimated. Each manufacturer is a little different.
To measure the battery properly you’re likely going to need to plug in your RV for several days to get it fully charged. But we don’t want to check the battery voltage as it’s charging or right after we disconnect it from the charger. We want to give the battery a good rest period before recording battery voltage, at least 6-hours.
Get a Voltmeter
In order to measure voltage, you’ll need a voltmeter. It’s a device that will measure the voltage of circuits or batteries.
One of my favorites is Klein, but any voltmeter can work. Read the instructions for how to record DC voltage – you’ll want it set to 20 intervals for a battery.
Take a recording at the start, your battery voltage should be near 12.8 VDC. Use your battery for a 24-hour period the check the voltage again. It’s best to check the battery voltage in the morning before you’ve turned anything on or powered anything from the battery. Using the battery can excite it and give you a false reading.
Once you have your two numbers you can find the percentage of charge on the chart. Let’s say you have a flooded battery that when full was 12.75VDC and after a day’s use is 12.20 VDC. From our chart, we can see that the battery capacity is likely 60% full.
If we have a 100-amp-hour battery, 60% of the battery is 60-amps-hours, and we’ve used 40-amps from the battery. If we do this a few more times and see similar results we can assume our 24-hour usage is about 40-amp-hours. We would want to size our battery accordingly. If we normally camped for 3-days we would estimate usage of 120-amp-hours. Again, based on this our lead-acid battery bank should be double or 220-amp-hours, and our lithium battery needs to be larger than 120-amp-hours to successfully be off-grid for 3-days.
Or you can size your solar system to deliver at least 40-amps a day. Or roughly an 80-watt solar system for a good summer sunny day. A larger system for cloudy areas.
Get Help if You Need it
So there is a ton of information packed into this article – and it can be confusing as it’s a very complicated and deep topic. If you have ways that you think simplified the topic we’re all ears – but we do write each article so you can understand the basics of why we need to do all this. We could kick out an article that just says get a device and plug it in, but that doesn’t really give you the insights as to the “why”.
And for this reason, if you’re still confused you can reach out to us. We can do an energy audit for you and calculate what size battery, type, and voltage might work for you. We can also size your solar system. We’ll need to hop on a call and get some info and ask some questions so we can get you rolling with some good recommendations.
For this consultation, we do have a fee of $49.95 per consult. Just reach out to us – info@backcountrysolar.com if interested.