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In an off grid system, the inverter relies on a battery bank to run appliances. But does an inverter draw power even if there is no load? It is an important question especially if you are doing everything possible to save energy and dollars.
An inverter will draw power even without a load. This is known as a no load current although the energy drawn is only 2 to 10 watts n hour.
How to Calculate Inverter No Load Current Draw
The no load current is listed on the inverter specifications sheet. It will be either no load current draw (amps) or no load power (watts), they mean the same thing.
To find out how much power an inverter draws without any load, multiply the battery voltage by the inverter no load current draw.
A 1000 watt 24V inverter with a 0.4 no load current has a power consumption of 9.6 watts.
24V x 0.4 = 9.6 watts
If you want to figure out the no load current in amps, divide the watts consumption by the battery voltage.
9.6 / 24 = 0.4 amps
This computation applies to any inverter size. The only difference will be the voltage which is usually 12V or 24V. The higher the voltage the greater the no load current / power consumed.
Why no load current matters. Given that the power consumption is quite small, why is it important that you learn about it? It matters for two reasons.
The first is that if you only run small appliances, the inverter no load current might consume more power. Imagine if you install an inverter and it draws 10 watts without a load and you only run a 5 watt radio. It would be a waste of money and energy.
The other reason is that the no current load will accumulate. Suppose your inverter is left on 24/7. With a 10 watt no power load draw, that is almost 70 watts wasted in a week. Leave the inverter on for several months and the cost will add up.
Can You Turn Off Inverter No Load Current?
With an older inverter there is no way to control the no load current feature. With newer models however there are a couple of ways you can reduce or even eliminate no load currents.
Some inverters have an on/off switch. By turning off the system it will stop drawing current. Think of the no load current as similar to a TV standby mode. When your inverter is on, it continues to draw a small amount of power. By turning off the switch you stop the current flow.
It is a convenient feature found in many large solar power systems. The drawback is you have to turn the inverter back on and wait for the appliances to load again. For some, turning the inverter on and off can be an annoyance.
Other inverters have a “load sense” feature. Basically it monitors the system and only turns on the system when there is a load. This means the inverter is still on, but this mode only consumes a single watt.
Not all inverters have this feature, so do some research first. You can check the inverter specs online before you buy, so you will know how many watts / amps will be used. With modern inverters the amount will be very small, and with an on/off switch it won’t even be an issue.
How Much Power Does an Inverter Waste?
The inverter no load current should not be confused with inverter efficiency, which determines how much power is converted by the system.
In an off grid system, the inverter transforms DC into AC power so that it is compatible with home appliances. Some of the power will be lost during the conversion, though the amount varies.
The energy preserved during the process determines the inverter efficiency rating. An inverter with an 85% efficiency rating means 15% of the converted energy cannot be used.
Newer inverters have a 90% to 95% efficiency, but there is no 100% efficient inverter yet. This means that a 3000 watt inverter with a 90% rating has about 2700 usable watts available for use.
The lower the efficiency rating, the more power is wasted. If you add this to the no load current, it can be quite a bit. This is why you should always get the most efficient inverter possible.
Pure sine wave inverters are the most efficient available. Most are 90% efficient such as the Yueqing WZRELB 3800W or even higher. They are also optimized to run modern appliances, electronics, motors and other electronics. Modified sine inverters lose more power during conversion, but they are more affordable.
Do Inverters Drain the Battery if Not in Use?
Yes, but the amount drained depends on the inverter size and design. The more modern the inverter, the more power you save.
A 90% efficient inverter means it requires 10% more power than what its load requires. If you run a 300 watt load for instance, the inverter will need 330 watts. With larger inverters the drain could be up to 2 amps even a load.
If you leave an inverter connected to a battery without load, the battery will be completely drained over time. A 200ah battery hooked up to a large inverter will be totally drained in a week or two. Even without load, the inverter will keep drawing amps.
Batteries drain faster the more it is used . So the longer you let the inverter pull amps from the battery, the quicker it will lose power. At some point you will hear a beeping sound. It means the battery can no longer provide the power required by the inverter.
If you have an off grid system you probably have the inverter on 24/7 anyway. For many people it is too much of a hassle to turn the system on and off every night just to save a few watts.
The power consumed by inverters from no load current cannot be avoided if the system is on. But if you run a large load it becomes negligible. If you have a 5000 watt inverter and run it at almost full load, that 0.4 no load current can be ignored. The system probably loses more power during the DC to AC conversion procedure.
5 Tips to Prevent Inverters From Wasting Power
The best way to prevent power wastage is to buy an inverter with a very low no load current draw. If the system comes with an on/off switch button that is even better. If you are willing to invest in a good inverter, pure sine is worth getting. Here are some other suggestions.
Use the right inverter system. For RVs, a 12V inverter should be enough. For an off the grid mobile home or cabin, a 24V inverter is more suited. 48V systems are also available for high powered appliances.
Inverter watt rating vs. power consumption. The inverter watt rating is not indicative of how much power it will use. You have to add a percentage to the power used by a load according to the inverter efficiency. For example, a 90% inverter with a 200 watt load will draw 230 watts, or 200 watts plus 10% to make up for the inefficiency.
Higher volts means lower amps. If you have a 230 watt load on a 12V inverter, the inverter draws 19.1 amps (230 / 12 = 19.1). With a 24V system the draw will drop to 9.5 amps (230 / 24 = 9.5). For deep cycle batteries you have to divide the draw in half due to their discharge rate.
Check the System Efficiency. The inverter is only a part of the system. You also have to consider the solar panel, batteries and charge controller. Each of these components must must run smoothly to produce the best results.
Inverters are getting better all the time, both in terms of functionality and energy efficiency. These days it is commonplace to see inverters with 90% to 95% efficiency and the no load current draw is getting better as well.