“How many batteries do I need for my van’s power system?” Or, how many solar panels do I need to power my air conditioner?”

This is a simple question with a ridiculously complex answer. Look at the length of this blog entry. Ugh.

To begin, solar panels are a charging source – often one of several. Don’t mix up a “solar system” with what you really need, which is a full power system. We’ll go over it in more detail later, but for now, let’s rephrase the issue to “how much battery capacity do I need to be off-grid in my van?”

Water Symbolism
One straightforward approach to consider this is to use a somewhat more common analogy: how much water do you need in your van? Which, of course, is dependent on how much water you intend to use. Do you take a shower every day? Do you never shower (please don’t be that way)? Is this a long, luxurious shower or a short rinse? How much water do you consume? How frequently do you do your dishes? You get the picture. Everyone’s response will be unique.

So, because many of our readers/customers are ‘Mericans, let’s think of batteries as tanks of water, each containing a specified amount of water measured in gallons.

We’re measuring stored energy in batteries, just like we’re measuring stored water in a tank, except instead of gallons, the metric is amp hours or watt-hours.

The amount of water you use in a water system is determined by the flow of a tap/valve. If you turn the faucet on full blast, your tank will empty considerably faster than if it is set to a trickle.

Your faucets are what we term “loads” in the electrical world. Some loads are modest (a trickle of water), such as LED lights or phone charging. Other significant loads (such as a firehose of water) include an air conditioner or an induction cooktop.

In power systems, the flow rate is referred to as current (amps), while the pressure is referred to as voltage (volts).

So, just as your water tanks have a finite and measured number of gallons, your batteries have a finite and measurable number of amp hours.

How to Keep Track of Consumption
A capable battery monitoring system, such as the Victron Energy BMV-712 or SmartShunt, should be installed in every electrical system. It is linked into your electrical system in the same way that a water meter is installed on your property. As you use power, it will compare your usage to the number of amp hours you had when the battery bank (tank) was full. It can tell you how much power you have left in your batteries (and other things) using this information.

It’s a pain, but it’s well worth it.
So, how many amp hours does your tank require? It’ll require some math students! You must perform what is known as a load calculation.

Fair warning: this will take an hour or two and isn’t really enjoyable (it sucks), but it’s well worth the effort.

We provide a free example load calculation Google Sheet to get you started. If you click on this link, a new window will open, prompting you to create a copy. Once you’ve made a copy, you can play about with it and change the values as you see appropriate. You can use the following example sheets as launching pads for your system:

The first sheet/tab (titled “Ex. 1: 600 Watts Solar, Moderate Driving, 12v AC”) is a larger system that includes a Mabru 12 volt air conditioner that runs for a few hours per day. It has three standard charging methods (see below). You’d presumably want to utilize external BMS batteries in a system like this to take advantage of the huge space savings they provide, and this discounted product set would be a nice starting place.
The second sheet/tab (titled “Ex. 2: 400 Watts Solar, Moderate Driving, No AC”) is a more modest system with identical loads but a smaller solar array and no air conditioner, resulting in a substantially lower overall consumption. It’s a suitable option for 2-3x internal BMS batteries and this product bundle at a discount.
The third sheet/tab (labeled “Ex. 3: No Solar, Driving w/ 2nd Alternator, 12v AC”) is the most powerful system, which relies on a dedicated secondary alternator for speedy charging and does not use/require any solar panels at all. It has the same loads as before, but it thinks the AC unit will operate all night. In this van, the owner might have a wonderful roof deck or use the roof space in various ways. We now offer a discounted product package for secondary alternator power systems!
But wait a minute. Resist the want to open those spreadsheets and start fiddling with the numbers right now. We recommend that you read on to find out what they signify first.

The first step is to identify all of your loads, or anything that will be powered by your electrical system: lights, Maxxfan, refrigerator, microwave, blender, coffee maker, chargers, Christmas lights, flame throwers, etc.

Keep two lists as you go: one for things that run on 12 volt DC directly from the battery, and one for everything else. And another for products that need home power – 120 volt AC – and have a plug that you’re used to using in your house – a standard outlet. An inverter is required to supply 120 volt alternating current loads. It connects to your batteries and transforms (12 volt direct current) to 120 volt alternating current (AC).

OK, take a quick look at your load center worksheet. There are four sections/tables visible in all of the instances. List your DC loads in the top table and your AC loads in the table below – into column A. Don’t worry about the numbers for now, but feel free to add “rows” if necessary to fit your load list.

Now that they’re all listed, you need to figure out how much electricity each one uses. Returning to our water analogy, is this a drip or a firehouse? We’ll enter those figures into the “amp draw” column (column B).

You can begin with the DC items because it is usually easier because the label or information you find is frequently already specified in amps. That’s useful and will save you some time with math.

Let’s start with some AC loads. The power consumption of AC devices is frequently indicated in watts. We’ll use the following formula to convert that to amps. I understand, math. I concur. But it’s easy. The formula is as follows:
Watts / Volts = Amps

In this scenario, the voltage in your battery. We’re talking about 12 volt batteries, but that’s a rather depleted battery. A typical battery’s “nominal” voltage is 12.8 volts.

As an example, consider the following. An induction cooktop claims to consume 1800 watts. That is, however, the maximum draw on super-duper-high. The typical consumption is around 1200 watts. Nonetheless, it is usually advisable to plan for the “worst case scenario” in these load estimations. So, to translate this to amps, we divide 1800 by 12.8 and get 140 amps. Yipes! In comparison, our LED puck lights draw 1.5 amps. Those are drips, and the cooktop is a veritable firehose.

So, enter all of your AC load values into column B of that middle table.

Time
We can now go on to the final variable, time. Returning to our water analogy, we know that long showers use more water than short showers, even when the flow rate is the same. Of course, the same is true with electricity.

Add anything to the “Estimated Hours Used Per Day” column (column C) for each of your DC and AC loads. We can start with that cooktop, which was a monster consumption firehouse. Even if you simply use it for 5 minutes, it makes a significant difference. This is why the original question’s solution is not universal. Some people cook frequently, while others do not cook at all. As a result, each electrical system and battery bank must be tailored to your specific requirements.

Everything should be entered within a few hours. In the instance of the cooktop, we must divide 5 minutes by 60 to get.08 of an hour.

Let’s go on to the refrigerator. This is difficult since a variety of factors influence how frequently the refrigerator runs. How hot is it in the van, how frequently do you open the door, how much stuff is in there, and so on? We’re estimating for an Isotherm Cruise 130 in this example. The manufacturer claims that the “average” draw of this model is 34 amps per day. If we split that by 24, we get 1.5 amps per hour on average. That’s what we’ll do. We’ll choose 24 in the hour column because it’ll be running all the time.

Take a look at this. According to this estimation, the cooktop consumes slightly more than 12 amp hours every day. Meanwhile, the fridge, which draws only 1.5 amps, is expected to consume nearly three times as much energy at 36 amp hours. This demonstrates the significance of run time.

And, to return to the original question, how many hours can I run my air conditioner on my batteries? Let’s flip the question for a second, just like you did with the other loads. How long do you want the AC to operate every day, and what is its flow rate or energy utilization?

And then things get even more difficult. What KIND of AC unit will you have in your van? To simplify this complex response, we’ll limit it to two options: a traditional, RV-style, 120 volt AC rooftop unit (think Coleman, Dometic, etc.) or a newer, 12 volt DC rooftop unit (think Mabru, Dometic RTX 2000, B-Cool, etc.). When you look at the characteristics of these options (check out our comparison chart), you’ll notice that the 120 volt AC units utilize around TWICE the amount of electricity as the 12 volt models. However, the 12 volt options are around TWICE the price. Despite this, when the cost of batteries is considered, they tend to be a better bargain.

So, have a look at a Mabru 12,000 BTU 12 volt unit. Depending on the cooling mode and fan speed, it consumes between 22 and 55 amps. Let’s schedule it for a hot day and leave it on high overnight. But our vehicle is insulated, we have window coverings, and the thermostat is set to a comfortable temperature. As a result, it will not be operating continuously all night. How frequently will it turn on and off? Of course, it’s impossible to know for certain, but let’s say the compressor runs around half the time. So we’ll enter 55 amps in the amp draw column and 4 hours in the time column (half an eight-hour night). In a single day, that amounts to 220 amp hours.

There is one solution concerning air conditioners… but there are many!

Total Consumption
You’re halfway done once you’ve entered the time values for all of your loads. Not at all – you’re already more than halfway through. I warned you that was a lot of work!

You now know how much electricity your system will need, as shown in the sheet’s bottom table (totals table). Look for the “Total Estimated Daily Consumption (Amp Hours)” row. This value can be used to estimate how many batteries you will require. The overall usage (before any of your customizations) in the first example sheet (called “Ex. 1: 600 Watts Solar, Moderate Driving, 12v AC”) was 358 amp hours per day.

Recharging
But, before you go out and buy a 400 amp-hour battery, let’s speak about charging. How do you refuel your depleted tank?

The next step is to simulate your charging sources. Solar, alternator charging of some form while driving, and shore power are all prevalent in vans. Shore power is when you’re linked to the grid at a house or campsite and may utilize that utility power to charge your batteries. As a result, we’ve included a row for each of these ways. We recommend that you have all three in your van.

So, let’s start with solar and keep it as basic as possible by ignoring all of the real-world complexities that effect solar system performance, such as time of day, time of year, how foggy it is, angles, how dirty or clean the panels are, and so on. For solar charging, we’ll apply a simple rule of thumb: for every 100 watts of panels on your roof, we’ll estimate 5 amps of charging output to the batteries. So, if your solar panel output is 400 watts, you can enter 20 amps for that charging source. Of course, if you have a very good concept of the places and light conditions you’ll be traveling in, as well as knowledge of the panels you’ll be utilizing, you’re welcome to conduct a more detailed estimate here.

The amount of time you charge, like loads, makes a tremendous effect! In this example, we’ll assume we’re parked in full sun while at the beach all day and enter 8 hours to generate an estimated 160 amp hours of power to charge the battery bank.

The alternator in your vehicle is the second most common way to charge. On our blog, we offer a number of examples of power systems that you can look at. One or more DC-DC chargers are the most typical solutions. The Victron Energy Orion DC-DC chargers have a charging capacity of up to 30 amps, and most of our clients utilize two of them in parallel for a charging capacity of 60 amps. Generally, you should not exceed 60 amps, or 45% of your vehicle’s alternator’s rating. This is a significant charge that will not overtax your vehicle’s alternator. In fact, even a single DC charger often provides more charging current than 400 watts of solar! In our example, we’ll utilize two of these (60 amps) and anticipate driving for three hours every day.

Finally, shore power refers to any time you can plug your rig into utility power. This is frequently done at a house or a campground. Remember the inverter we discussed before, which transforms 12 volt DC electricity from your batteries to 120 volt AC power for household-style loads? Often, these devices have a charger component that transforms utility electricity into some form of 12 volt DC power to charge your batteries. These are known as inverter/chargers, and we strongly recommend that your inverter have this feature. The Victron Energy MultiPlus 12/3000/120 inverter/charger is our most popular, and it can recharge your batteries at up to 120 amps!

In this hypothetical example, notice how the charging capacity ramps up with each of these, with solar acting like a drip and shore power acting more like a firehose.

However, the majority of our customers prefer to live off the grid (boondock). As a result, in our example sheets, the amps column has 120 but the time column has zero. However, if your activities take you to regions with hookups, shore power can be a wonderfully powerful charging source – even if you only use it occasionally.

A Tender Balance
We can observe how our loads (water going out) and charge sources (how we refill) balance out now that we know what they are!

Our estimated daily use is approximately 62 amp hours less than our expected daily charges in our first example sheet (again, before any modifications were made). That’s quite good. If you see that you’re using more power than you’re recharging, here is where your batteries come in.

You should size your battery bank such that it is larger than your daily loads. If you anticipate that your charging sources will be more varied (some sunny days, other days with less driving), a larger battery bank may be necessary to compensate for and accommodate for these fluctuations.

A battery bank in a camper van power system is typically made up of two or more identical 12 volt batteries linked in parallel. This way of constructing the “bank” keeps the voltages constant while increasing the amp hour storage capacity. In the first sheet’s example, we could utilize two or three Victron Smart 200 amp hour batteries to produce a nominal 12 volt battery bank with 400 or 600 amp hours of storage capacity.

Another reason to use numerous batteries is to ensure that your “bank” can handle your most demanding loads. Lithium batteries often have a “maximum continuous discharge” rating of 50 to 100 amps. When you combine batteries into a bank, you can also combine this specification. So, if you have three batteries, each capable of 100 amps of continuous discharge, the total capacity of the battery bank is 300 amps. Consider a 3000 watt inverter; at full power, it will draw roughly 235 amps. That means that combining the identical batteries into a bank of two batteries would not give enough juice to run that continuously, but three batteries would.

While lithium batteries are much more resistant to severe discharge than older lead acid/AGM batteries, most manufacturers recommend keeping your batteries above 20% state of charge for the maximum longevity. As a result, if your battery is rated for 100 amp hours, you should only use 80 amp hours in your calculations.

Returning to the water analogy, you could simply add a larger water tank to go longer without refilling or, in this case, recharging. However, the more you balance your consumption and utilization, the less you’ll have to worry about power. One of the most important takeaways is that merely adding batteries for more capacity isn’t a viable solution in the absence of recharging options.

Now that you have a good sense of what a typical day in the life of your power looks like, the last major question is how you intend to use the rig over time. Is every day the same? Do you plan to travel a lot on specific days to charge your alternator? Perhaps you intend to be at a campground or back at your stationary house every few days so you can recharge your rig every night using shore power? There are numerous unique instances. You may project how these scenarios would play out using the information from your load calculation.

Finally, these situations provide you with the information you need to size your battery bank. Obviously, a small margin is preferable so that if your forecasts are incorrect, you have some breathing room.

Another suggestion is to leave room for one additional of your selected batteries. assuming you have a regular energy shortfall, wiring in another battery is very straightforward assuming you have the space.

Secondary Alternators with High Power Recharging Capabilities
As battery banks grow in size, adding a secondary, dedicated charging alternator to your rig is becoming a more popular alternative. So, instead of using DC-DC chargers with a charging capacity of 60 amps to safeguard the vehicle alternator, you install a high-current alternator from a business like Nations. These charge at rates ranging from 120 to 200 amps, depending on your vehicle and engine RPMs, and can replenish big battery banks in a matter of hours. The effect of this can be seen in the third page of our example load estimates (labeled “Ex. 3: No Solar, Driving w/ 2nd Alternator, 12v AC”).