Sizing Your Electrical System & Load Calculations

One question we are asked all the time is “how many batteries do I need for my van’s power system? Or, how many solar panels do I need to run my AC?”

This is a very simple question with a stupidly complex answer. I mean, look at how long this blog post is. Ugh.

First, solar panels are a charging source – typically one of many. Don’t conflate a “solar system” with what you actually need, which is a complete power system. We’ll dive into that more later but, for now, let’s start by rephrasing the question to “how much battery capacity do I need to be off-grid in my van”.

Water Analogy

One simple way to think about this is to use a slightly more familiar analogy: how much water do you need in your van? Which, of course, depends on how much water you’ll be using. Do you shower every day? Do you never shower (don’t be like that)? Are we talking about a long luxurious shower or a quick rinse? How much water do you drink? How often do you do dishes? You get the idea. Everyone’s answer is going to be different.

So, let’s start by thinking about batteries as tanks of water – each containing a specific amount of water measured in gallons since many of our readers/customers are ‘Mericans.

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

In water systems, the amount of water you use depends on the flow of a tap/valve. If you turn the faucet on full blast your tank will empty much quicker than if you have it running at a trickle.

In electricity, your faucets are what we call “loads”. Some loads, such as LED lights or charging a phone, are small (a trickle of water). Other loads are large (a firehose of water) such as an air conditioner or an induction cooktop.

In power systems, the flow rate is called current (amps) and the pressure is called voltage (volts).

So, just as you have a finite and measurable amount of gallons in your water tanks, your batteries also have a finite and measurable amount of amp hours in them.

How to Monitor Consumption

Every electrical system should have a capable battery monitoring system such as the Victron Energy BMV-712 or SmartShunt. It gets wired into your electrical system like a water meter is installed onto your house. As you use power it will track your usage against how many amp hours you had to start with when the battery bank (tank) was full. With this information, it can tell you, with reasonable accuracy, how much power you have left in your batteries (and other stuff).

Load Calculation
Sucks, But Worth It

So, how many amp hours do you need in your tank? It’s going to take some math kids! You need to do what’s called a load calculation.

Fair warning, this is going to take an hour or two and isn’t particularly fun (it sucks) but it’s worth the effort.

To get you started, we have a free example load calculation Google Sheet. If you click on this link it will open up and prompt you to make a copy. Once you have your copy you can experiment with it and edit the values as you see fit. There are three example sheets that you can use as launching pads for your system:

  • The first sheet/tab (named “Ex. 1: 600 Watts Solar, Moderate Driving, 12v AC”) is a larger system – it has pretty typical loads but also adds a Mabru 12 volt air conditioner that is run for a few hours every day. It has three, standard charging methods (much more on that later). In a system like this, you’d probably want to use external BMS batteries to take advantage of the significant space savings they provide and this discounted product bundle would be a good starting point.
  • The second sheet/tab (named “Ex. 2: 400 Watts Solar, Moderate Driving, No AC “) is a more modest system that is similar in loads but has a smaller solar array and omits the air conditioner making the total consumption much lower. It’s a good candidate for 2-3x internal BMS batteries and this discounted product bundle.
  • The third sheet/tab (named “Ex. 3: No Solar, Driving w/ 2nd Alternator, 12v AC”) is the most powerful system that uses a dedicated secondary alternator for rapid charging and doesn’t have/need any solar panels at all. It has all the same loads but assumes the AC unit will run all night. In this van, you can imagine the owner having a nice roof deck or other ways of using that roof space. We have a discounted product bundle for secondary alternator power systems as well!

But, hang on. Resist the temptation to dive into those spreadsheets right now and start tweaking the numbers. We suggest you read on a bit further to know what they mean first.

The first step is to identify all your loads – everything that is going to be powered by your electrical system: lights, Maxxfan, refrigerator, microwave, blender, coffee maker, chargers, Christmas lights, flame throwers, all of it.

As you do this, keep two lists: one for things that run off 12 volt DC – directly off the battery. And another for things that require household power – 120 volt AC – things that have a plug that you’re used to using in your house – a normal outlet. To power 120 volt AC loads, you need something called an inverter. It is wired to your batteries and converts (inverts) the 12 volt DC (direct current) to 120 volt AC (alternating current).

OK, take a quick peep at your copy of the load center worksheet. In all of the examples, you can see that there are four sections/tables. Go ahead and list out your DC loads in the top table and the AC loads in the next table below – into column A. Don’t worry about the numbers yet but feel free to add “rows” if needed to accommodate your list of loads.

Now that they’re all listed you have to find out the power consumption of each. Going back to our water analogy – is this thing a drip or a firehouse? We want to put those numbers into this “amp draw” column (column B).

DC Loads

You can start with the DC stuff since it’s typically easier because often the sticker or info you find will already be listed in amps. That’s handy and will save you some math.

AC Loads

Now let’s do some of the AC loads. Quite often the power consumption of AC devices are listed in watts. To convert that into amps, we’ll use the following formula. I know, math. I agree. It’s simple tho. The formula is:
Amps = Watts / Volts

In this case the volts in your battery voltage. We talk about 12 volt batteries but that’s honestly a pretty deeply discharged battery. The “nominal” voltage on a battery you’re likely to use is 12.8.

So, here’s an example. An induction cooktop says it uses 1800 watts. However, that’s the maximum draw on super-duper-high. Typical consumption is closer to 1200 watts. Even so, it’s typically best to plan for the “worst case scenario” in these load calculations. So, our math to convert this into amps is 1800 divided by 12.8 which is 140 amps. Yipes! Compare that to our LED puck lights at 1.5 amps. Those are the drips and the cooktop is a true firehose.

So, go ahead and list out all your AC load values into column B of that middle table.

Time

Now we can get into the last variable – time. Going back to our water analogy, we know that long showers consume more water than short ones – even when the flow rate is the same. Of course, it’s the same with electricity.

Go through each of your DC and AC loads and add something into the “Estimated Hours Used Per Day” column (column C) for each. We can start with that cooktop which was a monster firehouse of consumption. However, if you only use it for 5 minutes it makes a big difference. This is why the answer to the original question is not universal. Some people cook a lot, some people don’t cook at all. So each electrical system and battery bank needs to be customized to your particular needs.

Everything should be entered in hours. So, in the case of the cooktop, we need to divide 5 minutes by 60 to arrive at .08 of one hour.

Let’s do the refrigerator next. This is tricky because there are a ton of factors that determine how often the refrigerator is going to be running. How hot is it in the van, how often are you opening the door, how much stuff is in there, etc? In this example, we’re estimating for an Isotherm Cruise 130. According to the manufacturer, the “average” draw of this model is 34 amps a day. If we divide that by 24 it comes out to be 1.5 amps per hour on average. So, that’s what we’ll use. It will be running all the time so we’ll use 24 in the hour column.

Now check this out. That firehose of electrons, the cooktop only consumes a bit over 12 amp hours per day in this estimate. Meanwhile, the fridge, which is a drip by comparison (1.5 amps), is predicted to use nearly three times as much energy at 36 amp hours. This illustrates how important run time is.

And, getting back to the original question: how many hours can I run my AC off my batteries? Let’s reverse the question for a minute like you’ve done with the other loads. How long do you want to run that AC every day and what is the flow rate or energy utilization of the AC?

And, well, it gets even more complicated. What KIND of AC unit are you going to have in your van? To keep this complex answer as simple as possible, we’ll narrow this down to two choices: 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.). If you dive into the specs of these options (check out our comparison sheet), the 120 volt AC models use right around TWICE the amount of power as a 12 volt model. But, the 12 volt options are also somewhere around TWICE the cost. Despite that, when you consider the cost of batteries, they tend to be a better value.

So, let’s consider a Mabru, 12,000 BTU 12 volt unit. It uses anywhere between 22 and 55 amps depending on the cooling mode and fan speed. Let’s plan for a hot day where we will be running it on high overnight. But, our van is insulated, we have window coverings and we’ve set the thermostat to a reasonable temp. So, it won’t be running constantly all night. How often will it cycle on/off? Of course, it’s impossible to know for sure but let’s say the compressor is running about half the time. So, we’ll put 55 amps into the amp draw and 4 hours into the time column (half an eight-hour night). That totals 220 amp hours in a single day.

There’s one version of the answer about air conditioners… But there is more!

Consumption Totals

Once you put in the time values for all your loads you’re halfway done. Ha ha, not really – you’re a bit more than halfway done. Told you this was a lot of work!

At this point you know how much power your system is going to use – you can see in the bottom table (totals table) in the sheet. Look for the row called “Total Estimated Daily Consumption (Amp Hours)”. You can use this number to estimate how many batteries you need. In the first example sheet (named “Ex. 1: 600 Watts Solar, Moderate Driving, 12v AC “) the total consumption (before any of your customizations) was 358 amp hours per day.

Recharging

However, before you rush out and buy 400 amp hours of battery, let’s talk about recharging. How do you refill your empty tank?

The next exercise is to model your charging sources. In a van, it’s very common to have 3x of these: solar, alternator charging of some kind when driving and shore power which is when you’re connected to the grid at a house or campground and can use that utility power to charge your batteries. Therefore, we’ve provided a row for each of these methods. We recommend that your van have all three.

So, let’s start with solar and make it as simple as possible by ignoring all the real-world complexity of solar systems that affect their performance such as time of day, time of year, how cloudy it is, angles, how dirty or clean or panels are, and so on. We’ll use a simple rule of thumb for solar charging: for every 100 watts of panels you have on your roof, we’ll assume 5 amps of charging output to the batteries. So, if you have 400 watts of solar, you can enter 20 amps for that charging source. Of course, you’re welcome to do a more nuanced calculation here if you happen to have a very good idea of the places and light conditions you’ll be traveling in and knowledge of the panels you’ll be using.

Just like loads, the amount of time you’re charging makes a huge difference! In this example, we’ll assume we’re parked in the full sun while we’re off at the beach all day and enter 8 hours which will produce an estimated 160 amp hours of power to charge the battery bank.

The second way most people charge is with their vehicle’s alternator. We have a bunch of example power systems on our blog that you can take a look at. The most common options are either using one or more DC-DC chargers. The Victron Energy Orion DC-DC chargers can charge at up to 30 amps and most of our customers use two of them in parallel for 60 amps of charging. Typically you don’t want to exceed 60 amps or 45% of your vehicle alternator’s rating. This is a substantial charge while not overtaxing your vehicle alternator. In fact, even if you use a single DC charger, that’s typically more charging current than having 400 watts of solar! In our example sheet, we’ll use two of these (60 amps) and expect to drive for 3 hours each day.

Finally, so-called shore power which is whenever you can plug your rig into utility power. Often this is at a house or a campground. Remember that inverter we talked about that converts 12 volt DC power from your battery into 120 volt AC power for household-style loads? Often these things have a charger component that does the opposite – takes the utility power and converts it into some flavor of 12 volt DC power to charge your batteries. These are called inverter/chargers and we definitely recommend your inverter has this feature. Our most popular inverter/charger is the Victron Energy MultiPlus 12/3000/120 which can recharge your batteries at up to 120 amps!

Notice how the charging capacity ramps up with each of these in this hypothetical example with solar being the sort of drip and shore power being more like the firehose.

However, most of our customers prefer to stay off grid (boondock). For this reason, our example sheets have 120 in the amps column but zero into the time column. But, if your adventures lead to places with hookups, shore power can be a tremendously powerful charging source – even if it’s only once in a while.

A Delicate Balance

Now that we know about our loads (water going out) and our charging sources (how we refill), we can see how they balance out!

In our first example sheet (again, before any changes were made), our expected daily utilization is about 62 amp hours less than our expected daily charging. That’s pretty good. If you see the opposite, where you’re using more power than you’re recharging, that is where your batteries come in.

You want to size the capacity of your battery bank so that it is larger then your daily loads. If you expect your charging sources to be more variable (some sunny days, some days with less driving) you may want to have a larger battery bank to compensate and allow for these flutuations.

Typically in a camper van power system a battery bank is comprised of two or more 12 volt batteries that are identical wired up in parallel. This method of creating the “bank” keeps the voltages the same but adds up the amp hour storage capacity. In the example described in the first sheet, we might use 2x or 3x of the Victron Smart 200 amp hour batteries to create a nominal 12 volt battery bank with 400 or 600 amp hour of storage capacity.

Another reason to use multiple batteries is to ensure that your “bank” is capable of running your heaviest loads. Lithium batteries have a “maximum continuous discharge” rating that is often in the 50 to 100 amps range. Combining batteries together into a bank allows you combine this specification as well. So, if you have 3x batteries, each capable of 100 amps of continuous discharge, the combined bank of batteries would enable a maximum continuous discharge of 300 amps. Consider a 3000 watt inverter – if you max that out, you’d be looking at around 235 amps. That means the same batteries combined into a bank of 2x batteries wouldn’t provide enough juice to run that continously but 3x batteries would.

While lithium batteries are a lot more resiliant to being discharged deeply compared to older lead acid/AGM batteries, most manufacturers recommend keeping your batteries above 20% state of charge to ensure the longest lifespan. So, if you have a battery that’s rated for 100 amp hours, you should consider only using 80 amp hours in your calculations.

Going back to the water analogy, you could simply add a bigger water tank so that you can go longer without refilling or, in this case, recharging. But, the more balanced your consumption is with your utilization, the less you’ll have to worry about power. One of the key things to take away is that simply adding batteries for additional capacity isn’t a sustainable option without recharging sources.

Now that you have a reasonable idea of what a day in the life of your power looks like, the last big question is how you’ll be using the rig over time. Are most days the same? Do you expect to drive a lot on certain days for additional alternator charging? Perhaps you expect to be at a campground or back at your stationary house every few days so you can use shore power to recharge your rig every night? There are so many unique scenarios. With the information you know from your load calculation, you can project how these scenarios would play out.

Ultimately, these scenarios give you the insight you need to size your battery bank. Obviously, it’s best to have a little margin so that if your projections are wrong you have some extra juice.

Another tip is to consider leaving space for one more of your chosen batteries. If you do find yourself with an energy deficit regularly, wiring in another battery is really simple if you have the space.

Dedicated Secondary Alternators for High Power Recharging

As battery banks get larger and larger an increasingly popular option is to add a secondary, dedicated charging alternator to your rig. So, instead of using DC-DC chargers that top out at 60 amps of charging to protect the vehicle alternator, you bolt on another high-current alternator from a company like Nations. These charge at anywhere between 120 to 200+ amps depending on your vehicle and the RPMs of the engine which means they can recharge large battery banks in a few hours. You can see the effect of this in the third sheet in our example load calculations (named “Ex. 3: No Solar, Driving w/ 2nd Alternator, 12v AC”).

Next Steps

If you made it this far and your head has not exploded, you rock and you’re a good candidate for installing your own DIY electrical system. We have a ton more information on our blog – many of which were linked to from this post.

If you stopped reading because your head was about to explode, we have YOU covered too. Check out our pre-wired, pre-configured, drop-in power systems for camper vans. You still have to have a sense of how many batteries you need but a professionally engineered system that arrives in crate and is ready to integrate into your van build is a great option for those intimidated by – or just don’t have time to deal with a DIY system.

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