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DC to AC inverter

How to Choose the Right DC to AC 240V Power Inverter

What Is a Power Inverter

First of all, let's start with the definition. What is a power inverter? 

A power inverter is a device which converts battery power into the mains power, i.e. it transforms 12V direct current (DC) into 240V alternating current (AC).

240V AC is the power supplied to our houses by utility companies, and this is the power required by most normal household appliances such as lights, TV, fridge etc.

There are 2 main types of inverters:
- Off-grid, which work as a standalone system and require batteries (typically used in caravans, cars, boats or locations without access to mains power)
- On-grid or grid tie inverters, which require mains power to work (typically used in household solar systems and take input directly from solar panels)

In this article we are going focus on off-grid inverters only - i.e. inverters which can be used to build an off-grid 240V solar system.  

Which 240v Power Inverter to Choose

So what off-grid inverter should you choose for your battery / battery bank in a caravan, motorhome, boat or for a standalone off-grid solar system?

In order to answer this question, you will need to consider three key points:

  • Type of inverter: Modified sine wave / Pure sine wave
  • Power rating (wattage) of the inverter
  • Input voltage (specifically in the case of a standalone solar system)
Type of Inverter - Modified Sine Wave vs. Pure Sine Wave Inverters

Deciding on the type of power inverter you require depends on the type of power output you need. Pure sine wave inverters provide an output which harmonically follows a sine wave. This is similar to the utility-supplied grid power. These inverters switch polarity ("+" and "-") between power cables smoothly, gradually reducing or increasing the voltage as required (in case you were wondering why inverters have to switch polarity: this is the essence of alternating current, polarity changes happen approximately 50-60 times per second).

In contrast, modified sine wave is a simulation of the pure sine wave output when the inverter sharply drops or increases voltage to switch polarity. As a result, the output form closely matches pure sine wave but still has much greater distortions.

As you can see from this example, though the modified and pure sine wave produce the same levels of output, the pure sine wave inverter produces a much smoother and less erratic output.

Modified sine wave output vs pure sine wave output of inverters

Ok, this seems more like science, but what impact will your choice of inverter have in practice?

  1. Modified Sine Wave output is not suitable for certain appliances, particularly those with capacitive and electromagnetic devices such as: a fridge, microwave oven and most kinds of motors. This also covers some types of printers as well as capacitive fluorescent lights etc
  2. Typically modified sine wave inverters work at lower efficiency than pure sine wave inverters
  3. Modified sine wave inverters are normally significantly cheaper than pure sine wave inverters

To sum up, even though modified sine wave inverters can work well with many appliances including light bulbs, mobile phone chargers and office equipment, if your budget allows it, we would always recommend buying a pure sine wave inverter.

Determining the Power Rating of Your Inverter

If you need to power small appliances like a mobile phone charger or energy efficient light bulbs, you don't really need to buy a 2000W power inverter because it will consume significantly more power even in standby mode and work very inefficiently with small appliances. On the other hand, if you connect a coffee machine to a 150W inverter you will quickly blow a fuse (if not the inverter itself).

Therefore the power rating of the inverter should be chosen based on the power consumption of your load. The following table contains the typical power consumption of some standard household appliances and the recommended inverter rating.

Equipment Power consumption Continuous power rating of inverter
150W 300W 600W 1000W 1500W 2000W 3000W 4000W
---  Audio and video appliances ---                  
 12'' colour LCD TV  20W Ok Ok Ok Ok Ok Ok Ok Ok
 Satellite TV receiver 30W Ok Ok Ok Ok Ok Ok Ok Ok
 HiFi stereo with CD changer 50W Ok Ok Ok Ok Ok Ok Ok Ok
 20'' colour LCD TV 100W Ok Ok Ok Ok Ok Ok Ok Ok
 42'' colour LCD TV 200W - Ok Ok Ok Ok Ok Ok Ok
 Stereo amplifier 250W - Ok Ok Ok Ok Ok Ok Ok
 Home theatre system 500W - - Ok Ok Ok Ok Ok Ok
    Continuous power rating of inverter
--- Home and kitchen appliances ---    150W 300W 600W 1000W 1500W 2000W 3000W 4000W
 Table fan 50W Ok Ok Ok Ok Ok  Ok Ok Ok
 Sewing machine* 100W - Ok Ok Ok Ok Ok Ok Ok
 Halogen light* 100W - Ok Ok Ok Ok Ok Ok Ok
 Pedestal fan* 100W - Ok Ok Ok Ok Ok Ok Ok
 Blender* 350W -  - - Ok Ok Ok Ok Ok
 Small fridge* 200W - - Ok Ok Ok Ok Ok Ok
 Large fridge* 500W - - - - Ok Ok Ok Ok
 Washing machine* (no heating) 700W - - - - - Ok Ok Ok
 Microwave oven 900W - - - Ok Ok Ok Ok Ok
 Coffee maker* 1200W - - -  - - Ok Ok Ok
 Iron 1200W - - -  - Ok Ok Ok Ok
 Hair dryer* 1200W - - -  - - Ok Ok Ok
 Toaster 1200W - - - - Ok Ok Ok Ok
 Vacuum cleaner* 1500W - - - - - Ok Ok Ok
 Dishwasher (hot dry) 1500W - - - - - Ok Ok Ok
 Hob 1500W - - - - - Ok Ok Ok
 Washing machine* (with heating) 2000W - - - - - - Ok Ok
 Air conditioner* 3000W - - - - - - Ok Ok
 Electric oven 5000W - - - - - - - -
    Continuous power rating of inverter
  --- Computers and home office ---   150W 300W 600W 1000W 1500W 2000W 3000W  4000W
 Mobile phone charger 5W Ok Ok Ok Ok Ok Ok Ok Ok
 Inkjet printer 40W Ok Ok Ok Ok Ok Ok Ok Ok
 Laptop 50W Ok Ok Ok Ok Ok Ok Ok Ok
 Fax machine 50W Ok Ok Ok Ok Ok Ok Ok Ok
 Desk lamp 50W Ok Ok Ok Ok Ok Ok Ok Ok
 Desktop computer with 17'' monitor 400W - - Ok Ok Ok Ok Ok Ok
 Large laser printer* 800W - - - - Ok Ok Ok Ok
    Continuous power rating of inverter        
--- Power tools and other equipment ---   150W 300W 600W 1000W 1500W 2000W 3000W 4000W
 Drill* 800W - - - -  - Ok Ok Ok
 Electric chainsaw* 1000W - - - -  - Ok Ok Ok
 Electric lawn mover* 1000W - - - - - - Ok Ok
 Pressure washer* 1000W - - - - - - Ok Ok
 Gridner* 2000W - - - - - Ok Ok Ok
 Air compressor* 2000W - - - - - - Ok Ok

*Appliances marked with a star are likely to have their starting power consumption several times greater than their normal working power (typically this is caused by electric motors or capacitors in such appliances). This should be taken into account when choosing the right size of inverter. For example, even though electric lawn mowers have normal working power 1000W, their starting power is higher than 4000W, so inverters with continuous power 2000W are not suitable because their peak power is limited by 4000W. Always take into account starting power requirements of your equipment, especially devices with electric motors, when choosing the right inverter.

Please also note: this table is provided for general guidance only. The actual power consumption of your appliances, as well as starting power requirements, may vary considerably. In all cases you need to check specifications of your equipment for continuous and starting power requirements.

If more than one device is connected to the inverter, the total power consumption should be a sum of all appliances used. This will determine the total inverter power required.

Determining the Input Voltage of Your Inverter: 12V or 24V

IIf you are looking for an inverter for your 12V leisure battery in a motorhome, caravan or boat, there's not much choice in terms of the input voltage - you will need a 12V to 240V inverter, because your battery is 12V.

However, if you are building a standalone off-grid solar system (e.g. in a house, garden, shed or farm), then you can choose the voltage of your battery bank and the input voltage of an inverter accordingly.

Instead of a 12V battery bank you may choose to build a 24V battery bank (2x12V batteries wired in series will produce 24V). In this case you can use a 24V DC to 240V AC inverter, rather than 12V DC to 240V AC. This set up will give you some important benefits:
- The input current for the inverter will be 2 times smaller for the 24V than the 12V making the inverter and the entire system safer and more reliable
- Cables between your battery and the inverter do not have to be as thick as those used for a 12V battery bank and inverter.

This is particularly important for large standalone solar systems with 2000W inverters and for such power levels our recommendation is to opt for a 24V battery bank and an inverter with 24V input voltage.

© Photonic Universe Ltd, 2012