What Size Inverter Do I Need? A Complete Guide for Off-Grid Power 

 

When people ask "what size inverter do I need", they usually want a simple answer. But the truth is - it is very dependent on what you want to power, how you’ll provide battery power to it and whether your devices are prone to very large start-up surges.  In this guide, we’ll walk you through how to choose the right sized inverter, explain common appliance requirements, and share real-life examples from off-grid setups. Our goal is to make sure you have enough power for everything you need - without over-spending on gear (and installation materials) you don't need.  

 

QUICK ANSWER

  • Continuous Wattage: Add up the continuous wattage of all appliances you want to “Run at the same time”. For high current appliances like coffee machine, kettle, induction etc, this is generally going to be one appliance at a time. .
  • Start-up Surge:Check the start-up surge requirements for any items with motors, pumps, or compressors - these can be 4–10x higher than their continuous running wattage. If you aren’t sure contact us for more information.
  • Allow for a Bit Extra: Choose an inverter with a continuous rating at least 20–30% higher than your total watts, and a peak rating that covers the highest surge load.
  • Formula: Required Inverter Size (W) = (Total Continuous Watts × 1.3) + Surge Allowance.

 

How to Work Out What Size Inverter You Need 

When choosing an inverter, you’ll need to calculate the continuous power in watts for all appliances you plan to run at the same time. Then, you must account for peak (surge) power - the short burst of electricity needed when certain appliances start up. 

Appliances with motors, pumps, or compressors (like fridges, coffee machines, and air conditioners) can require 4–10× their running power for a few seconds during start-up. If your inverter can’t handle this, it will shut down. 

Avoid overloads by: 

  • Not turning multiple high-demand devices on at the same time 

  • Staggering the start-up of appliances 

  • Choosing inverter models with a higher peak capacity 

 

Example: Running a Coffee Machine 

A Nespresso pod machine might need 1300–1500W whilst warming up and brewing coffee, but then dropping to almost nothing in between brews.  If you use it on it's own, an AllSpark Pro-series 2000W inverter will do this nicely.  However, if you want to make coffee at the same time as also charging cordless tool batteries and charging your laptop, you’ll need a larger inverter - for example, the AllSpark Pro-Series 2500W to handle all loads. 

 

Can I Run My Caravan Air Conditioner on an Inverter? 

The short answer: yes, so long as it is sized correctly, but for most people, it's impractical. 

Why it’s tricky: 

  • Standard air conditioners have high surge currents (up to 7.6kW).  Models with soft start will be fine. 

  • Your battery bank needs to be able to handle large start-up currents, and be sufficiently sized to continue running for your desired runtime. 

  • You’ll need a huge solar setup (around 1200-1500W) to replenish your batteries for more than a couple of hours running. 

Easier option (but still tricky): 

  • Choose an inverter-style air conditioner, like the Dometic Harrier.  These can often run on a 2000W inverter as they have a soft start to avoid huge start up surges. 

Why it's still tricky?  Even an inverter-style air conditioner will chew through about 70-90 amps per hour from a 12v battery bank, meaning big capacity required and lots of solar to recharge it.  

For more information on running an air conditioner head over here.  

 

Common Appliances and their Inverter Size Requirements 

Appliance 

Running Watts 

Start-up Surge 

Recommended Inverter 

Nespresso Coffee Machine 

1300–1500W 

1800–2000W 

AllSpark Pro-series 2000W 

Toaster 

800–1500W 

N/A 

AllSpark Pro-series 2000W 

Caravan Air Conditioner (Inverter Style) 

1300-1600W 

Low Surge 

AllSpark Pro-series 2500W 

Standard Air Conditioner 

1400–1800W 

7600W 

AllSpark Pro-series 2500W or consider a 24v system with a AllSpark Pro-series 3000W or 24V Hybrid 4000W

12V Domestic Fridge 

50–150W 

400–800W 

AllSpark Pro-series 1000W 

 

STEP-BY-STEP: HOW TO CHOOSE THE RIGHT INVERTER SIZE

  • List your appliances and note their running wattage
  • Identify surge loads (motors, pumps, compressors)
  • Add up total running watts for items you’ll use at the same time
  • Multiply by 1.3 for safety margin
  • Check inverter peak rating is above your highest surge
  • Ensure your battery bank can supply the required amps and continuous output for the full duration of your required runtime.

Do I need a 12V 3000W or 4000W Inverter? 

We have people asking us why we don't have a 3000W 12V inverter, and the answer is two fold.

Firstly, this size inverter is rarely needed. The AllSpark 2500W Inverter can seamlessly run multiple 230V devices simultaneously, but if you want to run two high current devices at a time, even 3000w isn’t sufficient and 4000w is the required size.  Doing this regularly would require a 24v system and a 4000w inverter.  People often believe that "bigger is better", but in the case of an inverter, unnecessarily bigger is can cost you, big time.  This cost is not in the price difference between a 2000W/2500W inverter and a 3000W model, it is in the installation costs of cables, lugs and circuit protection. 

To install a 12v 2000W inverter, you will generally need 0B&S cable (50mm2) and this comes in at around $32/m and 50mm2 lugs at around $3 each.  

For a 12v 2500W inverter, this goes up to 00B&S cable (70mm2) at around $43/m for cable and $4 each for lugs.  

To go to a 12v 3000W inverter you will need a minimum of 000B&S (95mm2) cable at $55/m and 95mm2 lugs at around $11 each.  

Even if you require just 1m of this cable (in black and red), this is $110 for a 3000W which in comparison to a 2000W inverter (which can run the same appliances but only one at a time) is only $60 for cable.  If you need cables longer than 1m, not only is this double the cost, but you will also need to step up a size again to reduce voltage drop increasing costs again. 
 
The second reasoning behind not going to a 3000W 12V inverter is the maximum current draw is over 300 amps and this starts to get to very dangerous currents if not properly sized, installed and protected - it can certainly be done right, but so many DIY’ers (and even inexperienced auto electricians) get it wrong and this can create a huge fire risk. Upgrading to a 24V is both safer and more efficient. 

 

FAQs 

Q: What inverter size do I need for camping? 

A: For basic camping (drones, laptops, camera and cordless tool charging), an AllSpark Pro-series 1000W inverter is usually enough. Size up to a 2000W inverter if running a coffee machine or other high-draw appliances. 

Q: Can a 2000W inverter run an air conditioner? 

A: Only if it’s an inverter-style AC with a low surge. Standard AC units will overload most 2000W inverters. 

Q: How do I size my battery for the inverter? 

A: Work out how many amps hours your appliances will use in a 24 hour period (watts/volts x % of hourly runtime = amps hours used).  Bear in mind that there are power conversion losses from converting 12v DC battery power to 230v AC mains power, so an inverter uses about 10% more power than the actual appliance draws. See our article on "How Much Power Does My Inverter Use" for more information.