Inverters: AC From DC
Free camping locations generally have limited facilities and most do not provide any mains electrical power, which we often want to run various appliances in our RVs like TVs, laptops or air conditioners.
Unlike generators, inverters can be used anywhere at any time, since they produce no noise or air pollution. Instead of fuel, inverters use the energy stored in the RV’s house batteries to directly convert 12VDC into 240VAC power via the magic of electronics.
All inverters produce AC power from DC battery power but not all inverters are the same.
Inverters whose output closely follows the mains waveform are termed pure sine wave inverters because that’s the shape of the waveform that the AC mains follows (see Inverter Output diagram). As you would expect, pure sine wave inverters can be used to power any type of AC appliance, including sensitive computing, audio/visual and medical equipment.
Inverters whose output only approximates the shape of a sine wave are called modified sine wave inverters. These inverters are only recommended for appliances that can tolerate the rather noisy output that they produce. Appliances that fall into this category include light bulbs, heaters, electric cookers and some motorised appliances like fixed speed electric drills.
While a lot of other appliances might work from a modified sine wave inverter, they typically make more noise, run hotter or produce interference on visual display devices.
So why would anyone buy a modified sine wave inverter? The reason is usually cost, as a modified sine wave inverter can cost as little as one-third that of a pure sine wave inverter.
Inverters are generally specified according to how much electrical power (in Watts) that they can continuously produce. However they can usually produce significantly more power than this for a short time, which is called peak power. For example, the peak power of many inverters can be twice their rated output.
Inverters use a lot of battery power. As a rule of thumb, divide the AC power (in Watts) by a factor of 10 to determine the current drawn from the battery. Using a divisor of 10, rather than 12, accounts for the voltage drop in the cables to the inverter and its electrical losses.
Let’s assume that you want to run a typical 1800W electric toaster from an inverter to make toast for breakfast. According to our rule of thumb, this toaster will draw 180A from the battery. Running this toaster for just 10 minutes will consume at least 30 amp-hours of precious battery energy.
While the toaster’s current draw is easy to calculate, other AC appliances use more battery power than expected due to a phenomenon known as reactance. Reactance causes the AC current and voltage waveforms to shift relative to each other, with the result that the apparent power used is more that the rated power. The ratio of real power (Watts) to apparent power (Volt-Amps) is termed the power factor.
For example, a washing machine rated at 550W with a power factor of 0.5pf has an apparent power of 550W÷0.5pf = 1100VA. The current drawn from the battery to run this machine would be 110A.
Also some appliances, like AC induction motors, take a lot more power to start up than they do when running. The difference between startup and normal running current can be as much as a factor of 7.
The combination of power factor and startup current often means that the capacity of the inverter needs to be a lot higher than first thought.
WHAT SIZE INVERTER?
Answering this question is always difficult because it depends upon a number of factors, including what appliances you want to run, their power factor, startup current and how long do you want to run them before recharging the battery.
If you are going to limit yourself to running low power devices like laptops, LED TVs and camera battery chargers, an inverter between 300~600W and a house battery of 80~120Ah is normally sufficient as these devices use very little power.
However, if you want the inverter to run high power consumption devices like washing machines, power tools, hair dryers and microwave ovens, be prepared to accept the fact that you will need a very large inverter (2000~3000W) and an accompanying large battery system (200~400Ah). The most cost effective way to run these high power devices is usually via a generator.
Also, buying an inverter substantially bigger than is needed is not recommended. Inverters are in the most efficient around 80~90% of rated load. This means that running a high capacity inverter at low loads will waste a lot of energy.
The purchase price of pure sine wave inverters these days is between $0.60 and $1 per Watt of rated output, with modified sine wave inverters costing one-half to one-third of this. As a general rule, inverters with higher output ratings have a lower cost per Watt.
It is essential that inverters be mounted in a location where they can receive an adequate flow of cooling air. Otherwise they will tend to overheat and shutdown for self-protection.
The cables between the battery and the inverter need to be a short as practicable in order to keep the voltage drop to a minimum. For example, it only takes a resistance of one-hundredth of an ohm to generate a voltage drop of 1V when carrying a current of 100A. For best performance, aim for a combined voltage drop of less than 0.25V at rated output and use the same gauge cable for both the power and ground circuits.
The inverter cables need to be protected from excessive current by a fuse or circuit breaker mounted as close as practicable to the battery. Without this protection, a short circuit across these heavy cables could easy cause a major fire.
Some inverters only produce 240Vac at their outputs if they sense an electrical load present. They often do this by periodically switching the output on for a brief period and checking for current flow. This is why there is often a delay between turning an appliance on and it powering up. Also be aware that some inverters will only work when loaded sufficiently and will cycle on and off if the load is too small. Typically larger inverters will have larger minimum loads.
ENERGY SAVING TIP
Even under no load, inverters can easily use as much as 1A of DC current when switched on, which amounts to 24Ah of energy loss per day. To save this wasted energy, either mount the inverter where you can easily get to its on/off switch or install an easily accessible remote switch. However, you must always remember to switch the inverter off when not in use.