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What Inverter Size Do I Need?

Can I run my 230V appliances and my air-conditioning?

We come across this question in varying forms about a dozen times a week on social media. To determine your best power options when you are off grid, you first need to consider a number of factors.

1. What do you want to run? 12 volt or 230 volt appliances (yes this is the correct voltage in Australia since 2003 – read about it here or here)
2.  Am I better to change over to gas? Maybe a gas kettle or the grill instead of a toaster? 
3. What is the amperage or wattage drawn from each appliance that you want to run? 
4. How many of those appliances do you want to run “at the same time”? 
5. How big do my batteries need to be to provide sufficient power? 
6. How much solar do I need to keep my batteries topped up? 
7. Am I better off with a generator for my high-powered appliances?

Everyone has their own idea of what free camping is. Whether you want all the mod cons of home or just enough to run your laptop or coffee machine, everyone is entitled to their own “version” of camping. We all share the love of getting away Offroad, be it out bush in a remote area or in a caravan park. Do whatever makes you happy and don’t let anyone tell you, you can’t!

If you want to run 230/240-volt appliances, you will either need a 12/230V Inverter or a 230V Generator. They are your only options. 

Let’s start with Inverters

When calculating an Inverter size, you will need to calculate the normal operating power (continuous power) in Watts. To do this you need to add up the total Watts of each of your appliances that you realistically want to run “AT THE SAME TIME”.

But most importantly, you need to understand what their initial start-up needs (peak) are.

Anything with a pump, motor or compressor can have start-up current draws of 4-10 times their continuous needs. Whilst most is only for a very short time (1-5 secs), your inverter still needs to be able to handle that peak power or it’s over current protection system will shut it down.

You can avoid mega peaks by not turning everything on at the same time.

As an example, let’s look at a Nespresso pod machine. When you first turn it on it needs around 1300-1800 watts or 5.65 – 7.82 amps @ 230 volts (~103-142 amps DC from your battery @ 12.6V) as the element and pump, heat and pressurise the system ready for your next brew. It then drops back to almost nothing in between brews. When you brew the next cup, it spikes back to full power draw again. In this example, a 2000W Inverter will be adequate as long as nothing else will run at the same time. If you want to make toast and coffee at the same time, you will need a larger unit, like the AllSpark 2500W Pro Series.

When we first started out, we had a simple set up for weekends, and used each appliance (including a Nespresso pod machine) separately with our 2000/4000W All Spark Inverter.

Now for one of the biggest questions I comes across “Can I run my caravan air conditioner of my inverter?”

Well technically the answer is yes in some scenarios, but reality for almost all caravan owners is no. Why? Let’s look at two examples, but first please note these are not recommendations or reviews. I have no experience with these and are random models selected online.

I will start with the Dometic Harrier “Inverter” Roof Top + ADB as an example of new technology. This unit will output 3.1kW of cooling capacity, but only uses 1500W continuous (~2:1) whilst in cooling mode (less in heating). This is a new “Inverter” type, designed specifically to avoid start-up spikes like most traditional AC’s. You can therefore run these on a 2kVA generator.

Dometic Harrier Inverter Air Conditioner

Alternatively, a non-inverter style Aircommand, Cormorant which outputs 3500W of cooling capacity whilst only using 1344W of input power (also ~2:1) but it doesn’t have the same inverter style soft start to take out the starting spikes which can be as high as 7.6kW (LRA). Some generators around the 2kVa may be able to handle this initial spike, but most won’t. Similarly, many 12/230V inverters sized for 2000W continuous power won’t either. They would need to handle peaks of 8000W to take the start-up current and have sufficient battery discharge capabilities as well.  

So, what does all this mean? Can I run my Aircon of my 12/230V Inverter? Well, sort of “Yes” but for most people “No”. Here’s why…

1. It would need to be an Inverter style aircon that has a much lower start-up current draw;
2. Your 12/230V Inverter must be sized appropriately for the aircon you have and can handle that start-up current; and
3. Your battery banks need to be specifically designed to handle both the initial start-up current through the Inverter (~120amps in the above example) and sufficient amp hour capacity to keep it running at that rate.

If you cannot do all three, then a generator is your answer (like 95% of most caravaners).

For the record to meet item 3 above, you would need around 1500W of solar to put that much power back into your batteries. Plus, your batteries would also need to be designed to handle a “continuous” discharge of 120amps from the battery bank. And that’s just for day time use. Night time is another matter altogether. Lots of very specific calculations to be done to get this right, not run out of power in 1 hour or kill your batteries.

Check out more info on running your aircon over here. 

Fridges, water pumps, etc all follow as similar cycle…massive start-up peaks, dropping back to normal running draw (or even to almost nothing in standby) and back to peak again each time the motor, pump or compressor kicks in.

Hope that helps. We're more than happy to go through your specific circumstances to ensure you have the right set up, so throw us a line if you want more info. 

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