A 2,000W inverter might sound like plenty for a caravan, until the kettle, coffee machine or microwave starts at the same time as the battery charger. Knowing how to calculate inverter load before buying equipment helps prevent nuisance shutdowns, flat batteries and an inverter that is oversized for the job.
For caravans, motorhomes, 4WD canopies and off-grid camps, the right answer is not simply the largest inverter you can fit. It is a matter of matching the inverter’s continuous rating, surge capability, battery bank, cabling and charging capacity to the appliances you genuinely intend to run.
How to calculate inverter load accurately
Start by making a realistic list of every 240V appliance you may use from the inverter. Look at the appliance rating label, power pack or instruction manual and write down its power draw in watts (W). Do not use a vague estimate where the appliance has a stated rating.
The basic calculation is:
Total inverter load (W) = the combined watts of appliances running at the same time
For example, a caravan traveller may want to run a 900W microwave and a 100W TV at the same time. The running load is 1,000W. If a 65W laptop charger is also connected, the load becomes 1,065W.
That figure tells you the minimum continuous output the inverter needs to provide. It does not yet account for inverter losses, startup surges or the fact that an inverter running at its absolute limit has little room for real-world variation.
A sensible rule is to select an inverter with a continuous rating at least 20 to 30 per cent above your expected maximum running load. For the 1,065W example, a quality 1,500W pure sine wave inverter is usually a more practical choice than a 1,000W unit. The 1,000W inverter is already too small, while a 1,200W unit may still be working harder than you would like.
Use the appliance’s input power, not its output claim
Some appliances create confusion because their useful output is not the same as the electricity they consume. A microwave may be described as an 800W microwave, referring to cooking output, while its electrical input can be 1,200W or more. Check the compliance label for input watts or input VA.
The same applies to coffee machines, induction cooktops, air fryers and battery chargers. These are common high-draw items in mobile setups. If the label only shows amps, use this calculation for Australian 240V appliances:
Watts = volts × amps
At nominal 230V supply, a 4A appliance draws about 920W. Appliance ratings can vary, so use the manufacturer’s stated voltage where available rather than assuming every item is exactly 240V.
Running load and startup surge are different
Many appliances draw a brief but much higher current when they start. This is called surge, inrush or peak demand. It matters most for loads with motors, compressors or pumps, such as fridges, freezers, power tools and some air conditioners.
A fridge may run at 120W but briefly demand several times that amount as its compressor starts. A 1,000W inverter could theoretically handle the fridge’s running load, yet still cut out during compressor startup if its surge rating is too low.
Check two ratings on the inverter: continuous power and peak or surge power. A reputable inverter may supply double its continuous rating for a short period, but the duration and actual performance vary by model. Do not assume two inverters with the same wattage rating will handle difficult loads equally well.
This is one reason pure sine wave inverters are generally the better choice for caravans and off-grid systems. They provide cleaner power for sensitive electronics and are more suitable for appliances with motors, compressors, battery chargers and electronic controls. Modified sine wave models can be cheaper, but may cause extra heat, noise or unreliable operation with certain equipment.
A practical caravan load example
Consider a typical morning setup with the following appliances:
| Appliance | Running load |
| --- | ---: |
| Coffee machine | 1,450W |
| TV | 80W |
| Satellite receiver | 25W |
| Laptop charger | 65W |
If the coffee machine, TV, receiver and laptop are all operating, the total is 1,620W. Add 25 per cent headroom and the required continuous inverter capacity is roughly 2,025W.
In this situation, a 2,000W inverter may work on paper but offers very little reserve. A quality 2,500W inverter provides a more comfortable operating margin, provided the battery bank, cables and fusing are designed for it. If the coffee machine is used on its own while the TV and chargers are off, a smaller inverter could be suitable. Your normal usage pattern is what matters.
Calculate the battery draw as well
An inverter converts low-voltage DC battery power into 230V AC power. This conversion is not 100 per cent efficient, so the battery must supply more power than the appliance is using.
Use this calculation:
Battery current (A) = AC load (W) ÷ battery voltage (V) ÷ inverter efficiency
Most good inverters operate around 85 to 95 per cent efficiency depending on load. For a conservative estimate, use 90 per cent, or 0.90.
A 1,500W appliance on a 12V battery system draws approximately:
1,500 ÷ 12 ÷ 0.90 = 139A
That is a serious current draw. At 1,500W, voltage drop from undersized cable or loose connections can quickly cause a low-voltage alarm or inverter shutdown. It also explains why a modest appliance load can flatten a battery faster than expected.
On a 24V battery system, the same 1,500W load draws roughly 69A. Higher-voltage systems reduce current and can make larger installations easier to cable correctly, although they are not always practical for an existing 12V caravan setup.
Allow for battery capacity and usable energy
Battery capacity is usually expressed in amp-hours (Ah), but watt-hours (Wh) are more useful when comparing it with inverter loads.
Battery watt-hours = battery voltage × amp-hours
A 12.8V 200Ah lithium battery holds about 2,560Wh of stored energy. In real use, allow for inverter efficiency and avoid planning to use every last watt-hour. At a 1,000W AC load, that battery may run the load for a little over two hours under favourable conditions, not two and a half hours.
Lithium batteries are generally better suited to high inverter loads than lead-acid batteries because they hold voltage more effectively and offer more usable capacity. Even so, check the battery’s continuous discharge rating. A 100Ah lithium battery with a 100A battery management system cannot continuously supply the 140A needed for a 1,500W load on a 12V system.
Check the entire system, not just the inverter
An inverter is only as capable as the equipment supporting it. High-current DC wiring needs the correct cable size, short cable runs where possible, properly crimped lugs, a suitable fuse or circuit breaker close to the battery, and a solid battery isolator. These are safety requirements as well as performance considerations.
Also consider how the battery will be recharged. A large inverter load used daily can consume far more energy than a small solar panel can replace, particularly during cloudy weather or short winter days. Solar input, DC-DC charging while driving, mains charging and generator use all affect whether the system is sustainable away from powered sites.
For larger loads such as air conditioners, induction cooking or electric heating, it often makes more sense to change the usage plan than to keep increasing inverter size. Electric heating is particularly demanding on batteries. Gas cooking, diesel heating or running high-draw appliances only when connected to mains power can be the more practical option.
Common mistakes when sizing an inverter
The most frequent mistake is adding up every appliance in the caravan, even though they will never all run together. The opposite mistake is sizing for only one appliance and forgetting the fridge compressor, TV, satellite system, chargers or other background loads.
It is also easy to overlook standby consumption. An inverter left switched on all day can use meaningful battery power even when no 240V appliance is connected. Check the idle draw, especially if you spend extended periods off-grid.
Finally, do not confuse an inverter’s wattage with a guarantee that your caravan’s existing wiring can carry that power. A correctly installed inverter circuit should be designed for its intended use, with appropriate protection and compliance work carried out by a qualified person where required.
Get the numbers right before you buy
Write down the appliances you will actually use together, include a safety margin, then calculate the battery current at your system voltage. That simple process will quickly show whether you need a compact inverter for TV and charging, or a properly supported 2,000W-plus system for kitchen appliances. If the figures are close to the limit, get practical advice before fitting the gear - the right inverter package should suit how you travel, not just the biggest number on the box.