Campervan Off-Grid Calculations

A deep dive on how we go about calculating campervan off-grid times


Campervan Off-Grid Calculations

Here at Vunked, we design our electrical systems to meet your needs when travelling off-grid. To do this accurately we need to figure out your campervan’s off-grid calculations. This is a topic of great debate in the van life community; getting it wrong can be costly.

No one wants to run out of power when they’re hours from home and need to start work in 15 minutes. 

So, today we’re going to discuss how we calculate the off-grid times for our electrical systems, along with the benefits and drawbacks of our approach.

When we were converting our first van to work remotely, we had no idea how long we’d be able to stay off-grid. After seemingly endless hours of research and making spreadsheets to perform the calculations, we were still clueless!

Since then, we’ve been able to build electrical systems and test them in the real world. After several iterations, we developed our off-grid calculator.

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Campervan Off-Grid Calculation Steps

Calculating how long you can stay off-grid involves four main steps.

  1. Daily Power Consumption – Calculate how much energy you’ll use daily when in your van.
  2. Daily Power Generation – Combining solar and DC-D charging, we can calculate how much energy will be generated daily.
  3. Battery Consumption Vs Charging – Depending on the size and type of battery we choose will affect how long you can stay off-grid.
  4. Off-Grid Calculation – Bringing it all together, we can calculate how long you can stay off-grid throughout the year.

To make it easy to use, we’ve had to make some assumptions about the appliances you’ll use in your campervan. We want to be completely transparent on how we do this so that you can feel confident that our systems suit you.

Step 1 - Daily Power Consumption

Every time you turn a light on or want a cup of tea, you’ll need to consume energy to make that happen. 

We’re lucky to live in a world where we don’t have to worry about these things, and if you get your campervan electrics right, you’ll also be able to sleep easy knowing you’ll never run out of power.

Below we go through each 12V and 230V appliance you may use in your campervan and estimate its daily power consumption.

Simple Campervan Energy Calculations

Quick maths lesson. A 12V battery capacity is measured in Amp Hours (Ah).

To make our off-grid calculations easy we’ll calculate how many Ah each appliances uses. To do this we’ll use the following equation two equations.

Current (A) = Power (W) / Voltage (V)

Sometimes expressed as,

I = P/V

Amp Hours (Ah) = Current (A) X Hours

So all we need to know is the voltage of our battery system (usually 12V) and the wattage (W) of the appliances we’re using. This is always stated on the applaince.

From here we can calculate the Current (A) using our I = P/V formula and then multiply this by time it’s used each day, measure in hours, to find the total amount of energy charge (Ah)

12V Campervan Appliance Usage

First off, we need to figure out what power you will draw from your batteries while off-grid.

See the list below for all our appliances, the typical power rating and how long we estimate they run for in a day. We’ll also include any assumptions we make here and explain our reasoning.

12V power consumption

12V Campervan Appliance Assumptions

Here we will explain the assumptions we have made for our most popular 12V campervan appliances. 

LED Lights (4W) – 

Everyone has LED lights in their campervan. In this case we estimate 4W per light with a total of 6 lights in each campervan. We also assume they will all be used 4 hours a day. 

USB Sockets (18W) –  

If you are charging you phone or camera the most efficient way to do this is to use a 12V USB connection. We assume you’ll charge two phones 100% each day for 3 hours.

50L Fridge (60W) –  

You’ve got to keep the beers cold somehow. A 12V fridge is power-hungry but often necessary to stay off-grid for longer. A 50L fridge will consume about 60W. However, the compressor will only run about a third of the time, as the fridge automatically starts cooling itself. Therefore,  we assume your fridge is only on for 8 hours daily (1/3 of 24 hours).

230V Campervan Appliance Usage

Next, we need to determine what power your 230V appliances will draw from your leisure battery

See the list below for all our 230V appliances, the typical power rating and how long we estimate they run for in a day. 

230V power consumption

230V Campervan Appliance Assumptions

Here, for our 230V appliances, we will call out the assumptions we make for our campervan off-grid calculations.

Laptop Charger (80W) –  

Laptop chargers can vary between 40 Watts and 120 Watts. Most standard laptops will sit around the 80W mark, whereas your new shiny Apple Macbook Air will be as low as 45 Watts and if you have a big hungry gaming laptop, that will be closer to the 100W end of the spectrum. We assume most people have standard laptops and charge them for two hours a day, which will see them through the working day. 

Microwave (800W) –  

Have you seen the Watts rating printed on the front of every microwave? Well, that is not how much power (W) it consumes. The rating on the front indicates the power (W) it generates to heat your soup on those cold days. The power it consumes (W) is higher than this.  A study on 203 microwaves showed that the average efficiency was 71.29%. Meaning a 600W-rated microwave would draw 842W. We have taken this assumption and rounded it up to 900W. This means you can use your microwave with a 1200VA Inverter. 

Hair Dryer (1200W) –  

A rule of thumb is that anything that spins or heats requires a lot of power (W). Hair Dyers do both, which is why most standard hairdryers consume over 1000W. If you want that flowing hair, you will want a large Inverter.  We assume that you will be using a lightweight or travel hair dryer. 

Step 2 - Daily Power Generation

Now that we have calculated the daily usage Ah. Next, we will calculate how much power we generate daily for our campervan off-grid calculations. 

Power Generation from Solar

Next up, we figure out how much juice your solar panels will provide daily. We have two basic scenarios we base these calculations on – summer and winter. The sunlight changes massively throughout the year, especially as you move further north. So we also have the weather to account for. As a Glasgow-based company, we base our calculations around our fair city.

For  our campervan off-grid calculations and solar, we make two main assumptions: 

  1. Hours of daylight
  2. Solar panel efficiency 

These are both explained below…

Hours of Daylight Assumptions

We assume an average of 4 hours of direct sunlight each day in the summer and 1 hour in the winter. This may sound low, but we’ve found this to be a reasonable average estimate based on Glasgow’s weather data. 

If you live somewhere sunnier (think south), our solar calculations will likely be a bit conservative. This isn’t a bad thing. After all, it’s better to be conservative than risk running out of juice!

Solar Panel Efficiency Assumptions

We also assume an efficiency of 70% for all our solar panels. This is because your solar panels will only produce their rated power when the sun is directly overhead. Not all the sunlight is absorbed when the sun hits the solar panels at an angle. When this happens, the solar panels generate slightly less than their rated power. Our efficiency factor considers this.

If you want to learn more about this, you can read up on our Deep Dive into Campervan Solar

We end up with another amp-hour value that is provided to your batteries each day from your solar panels. This is combined with the DC-DC charging capacity and offset against your daily energy consumption.

Power Generation from DC-DC Charging

The next part of the off-grid calculations jigsaw is how much power is generated through the DC-DC charger while driving. This is dependent on two things: 

  1. The size of your DC-DC charger (we currently stock 18A and 30A chargers from Victron)
  2. How long do you drive for

We assume the DC-DC chargers are 100% efficient and, therefore, constantly provide the 18A or 30A stated while you drive.

Driving Times

We don’t assume driving hours, instead, when you fill out our online form, we’ll ask how long you expect to drive for each day. We perform our calculations based on various driving scenarios and will give you a feel for how much battery capacity your DC-DC charger is actually providing. 


For example, each hour of driving will provide 18Ah of battery capacity using a Victron 18A DC-DC charger.

Step 3 - Battery Consumption Vs Charging

We’re finally getting to the good bit – finding out the net change in your battery capacity each day. We offset your solar and DC-DC charging against your starting battery capacity and daily energy consumption to determine how your daily net charge.  This can be done using the equation bellow…

(solar charge) + (DC-DC charge) – (energy consumption) = (net charge)

If you charge your batteries each day more than you consume, you can last as long as you like off-grid. If like most van users, you consume more than you charge, you drain your batteries daily until they run flat. We love when people use examples, so let’s finish this off using one. 

“We have a campervan electrical system that generates 10Ah from solar and 15Ah from a DC-DC charger. The appliances we use drain 50Ah of battery capacity daily (we have long showers!). Our campervan is fitted with one 100Ah lithium battery.”

Walking through this using the above equation…

For example, if you top-up 10Ah from solar and 15Ah from your DC-DC charger, and drain 50Ah of battery capacity each day from your appliances you will drain a 100Ah lithium battery in 4 days.

Walking through that using the above equation…

Battery Capacity Change:  10Ah + 15Ah – 50Ah = -25Ah

net charge = -25Ah

We can see that every day we use our van off-grid we lose 25Ah of energy. However, what does that mean? How long can I stay off-grid? 

Step 4 - Off-Grid Calculation

Using the same example before we can see that losing 25Ah of energy a day will result in four days off-grid. 

Battery Capacity (start of the day)  –  net charge =  Battery Capacity (end of the day) 

  • Day 1: 100Ah – 25Ah = 75Ah
  • Day 2: 75Ah – 25Ah = 50Ah
  • Day 3: 50Ah – 25Ah = 25Ah
  • Day 4: 25Ah – 25Ah = 0Ah

A quicker method is to divide the total battery capacity by the net charge. Using the same example as before… 

Days Off-Grid = Total Battery Capacity / Net Charge 

  • Days Off-Grid = 100Ah / 25Ah
  • Days Off Grid = 4 days

Here you have it, and following these four (almost) simple steps, you can calculate how long you can spend off-grid in your campervan.

Don’t want to worry about going through all the hassle? Our Campervan Electrics Builder Tool takes the guesswork away so you can focus on what’s essential – exploring in your van!

Battery Discharge Assumptions

When performing this last part of the calculation, we assume deep-cycle AGM batteries can be discharged by 80% of their maximum capacity before causing lasting damage. Likewise, we assume a lithium battery can be discharged to 100% of its capacity.

Want More Time Off-Grid?

If you don’t like the results of or off-grid calculator, there are three things you can do to get more time off-grid:

  1. Increase your maximum battery capacity
  2. Increase your charging capacity
  3. Reduce your daily energy consumption

Remember our off-grid calculator is free to use and even provides a wiring diagram and quote for the entire electrical system, including all the bits and pieces you need to install it! You can play around with your system design and see the different ways to achieve the time off-grid you’ve

Still a Bit Confused? Want the Perfect Electrical System?

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