Since the last time I mentioned my inverter research, I’ve been digging even deeper to try to find an ideal solution for the vintage bus. I’ve updated my earlier post with a smidgen of additional detail as well. If you’re not into motorhome systems or technical terms, then you might wan to just skip this post. I’m writing all this stuff down, just to get it out of my head and to help others who might be considering a similar project.
First, I’ve learned some terminology that can be useful when comparing inverters:
Inverter: A device that transforms Direct Current (DC) battery power to Alternating Current (AC) 120 volt (V) power. The simplest form of an inverter would be the kind you can plug into the cigarette lighter port in your car (its source of 12V DC battery power) and delivers a single 120V AC household electrical outlet. The simple inverter shown above might be fine to power your laptop, but just won’t cut it when you want to run air conditioners, microwave ovens and other power-sucking appliances. An inverter can deliver a limited amount of power. The power output of an inverter is usually measured in watts. The inverter shown above is a 100 watt inverter, which is perfectly suited to supply power to my laptop computer which draws 85 watts. If you inspect most electrical devices, you’ll find the power draw listed. It will usually be found near where the power plug enters the device, or on any brick-style power adaptor. On my laptop power adaptor, it lists “85 watts” in tiny text, while I find 1.4Kw listed on my microwave oven. A Kilowatt (Kw) is the same as 1000 watts, just like a centimeter is the same as 10 millimeters. A typical household outlet is able to deliver between 1,800 and 2400 watts depending on if it’s wired to a 15amp or 20 amp circuit breaker. Don’t let all those numbers and terms mess with your head too much. Things become pretty simple once you understand a few concepts which I’ll try to explain in a later post. For now just know that an inverter gives you household electrical power using the energy supplied in a bank of batteries.
Combi: A device that combines the features of an inverter (to produce 120V AC power from 12 or 24V DC battery power) and battery charger into a single unit. This is the only kind of inverter I want since it is generally required to get the most convenient features mentioned below.
Power Boost: The ability to supplement incoming AC power by synchronizing the inverter’s output with shore or generator power to prevent the shore power breaker from tripping. For example, when you’re plugged into your neighbor’s garage outlet that also has a small refrigerator and a few other appliances running on it. Let’s say it has a 15 amp circuit breaker and the other appliances in the garage are sucking up to 4 amps. With Power Boost, you can dial in that you want to draw a maximum of 11 amps (4+11=15) so that you won’t trip the 15 amp circuit breaker. Then, let’s say you run your refrigerator which draws 2 amps, a few lights and then try to use your microwave which draws 13 amps. If all those loads total maybe 16 amps, you’d usually trip the 15 amp breaker in your friend’s garage (your 16 amps plus the 4 amps already being drawn equals 20 amps). With Power Boost, your inverter would be set to draw a maximum of 11 amps and the inverter would supply the additional 5 amps from battery power. Manufacturers don’t always use the term Power Boost to describe this feature, sometimes calling it Cogeneration or a Synchronizing Inverter.
Load Shedding: A method for limiting shore power draw by temporarily disabling various appliances when the total amp draw would cause the shore power breaker to trip. This feature is used in two ways. In an inverter, you can have two AC outputs, one that is always receiving power and the second that only receives power when you’re plugged into shore power or running your generator. By putting loads that you’d never want to run via battery power on the second leg, you can make sure that you never accidentally use things like your electric water heater or electric toe kick heaters while running solely on battery power.
The other way to implement Load Shedding is in a device that is separate from your inverter that is sometimes referred to as an Energy Management System or EMS. In that case, you choose the size of your shore power circuit breaker and the device monitors your power draw. When you exceed the available shore power, the Load Shedding device will cut the power to a device such as an air conditioner, if you continue to increase your power draw, then it might cut power to a second air conditioner, or to your electric water heater to insure that you don’t suck too much power from shore power. This separate device is what you commonly see in RVs that don’t have Power Boost capability. I’m not a fan of this setup for multiple reasons:
- The load shedding is done by defining a preset list of devices in a specific order that cannot easily be changed, for example: air conditioner #1, air conditioner #2, water heater, etc. This list is limited to four devices on most systems.
- You are usually limited to choosing from three preset maximum amp draws of 20, 30 or 50 amps and would not be able to dial in 11 amps as would be needed when plugged into a 15 amp circuit that already has other loads drawing power (as in the friend’s garage example I mentioned earlier).
- The shed-able loads are usually housed in a separate electrical panel.
- You can easily trip the shore power breaker by using too a few high drawing appliances that are not part of the load shedding system such as the microwave and a hair dryer or coffee maker.
Power Share: The ability to vary the amount of shore power used for charging the house battery bank. When an inverter has this feature, it will first supply the AC power needs to the coach from shore power and if that amount is less than the available power, then it will use any left over power to charge the batteries. If the amount of shore power available is less than the power your coach is demanding, then it stops charging the batteries all together and starts using the Power Boost feature to supplement shore power. Some inverters will have Power Share, but not Power Boost, which means that it can lessen the amount of power used to charge the batteries, but cannot supplement the shore power. When that’s the case, you’d want to have a Load Shedding EMS to make sure you don’t trip the shore power breaker if you demand too much power. My current bus does not have this feature. My inverter just has an on/off switch and draws as much power as it thinks is necessary without care for what the power needs are for the coach systems. It will start by pulling around 12 amps and slowly lessen its draw as the batteries reach a full charge. That doesn’t leave much power to use in the coach when I’m plugged into a 15 or 20 amp circuit. My next inverter must have the ability to Power Share!
Paralleling: The ability to to connect multiple inverters together to deliver more power than a single unit is capable of delivering. The highest power inverter I’ve found that offers the features I want is a 4000 watt inverter. 4000 watts is just over 33 amps of 120V power. If I think I want to use more power than that when running off battery power, then I’d want to parallel two inverters to get 8000 watts of power. That can only be done if the inverters are smart enough to talk to each other and synchronize their wave forms and only a few inverters are capable of that.
GFCI: GFCI (also referred to as GFI) stands for Ground Fault Circuit Interrupter. A GFCI is like a mini circuit breaker built into an outlet that is designed to trip if a condition is present that could electrocute a human. A GFCI outlet is required by code for all exterior outlets and outlets that are installed near water (like in a bathroom or kitchen). This term might not seem related to inverters, but they are the most common type of outlet you’ll encounter when plugging into an outlet at a friend’s house. My current coach will trip a GFCI the moment it is plugged into it, which forces me to run long extension cords through a door or window into an outlet on the inside of someone’s house. I’ll give you an idea of why this happens, how the outlets work and how to get choose an inverter that won’t trip one in a future post.
Now that I’ve gotten all those stupid terms out of my head, I feel that I’m just about ready to describe the difference between the various inverters I’m considering