Frequent Questions: Inverters

What size inverter should I get?
Inverter capacity is measured in Watts which should be higher than the total watts of all the devices that you plan on running simultaneously.  For example, if you have a 1500 watt microwave oven, a 300 watt refrigerator and maybe another 200 watts of lights, cell phone charging and misc., you will need at least 2000 watts of inverter capacity.  If you only have a 2000 watt inverter and you try to run all of that equipment plus a 1000 watt blender it will overload your system and your inverter will turn off.

The size of an inverter system has nothing to do with how many solar panels you have or how much battery capacity you have, it is only relevant to the size of your AC loads.

What is the difference between pure and modified sine?
A pure sine inverter is universal and works with all AC appliances.  A modified sine inverter is usually less expensive and does not work well on devices with moving parts.  The output of a pure sine inverter is smooth and constantly changing as its output goes from negative to positive.

What kind of efficiency would I get on an inverter?
A typical inverter used in RV applications would have an efficiency of around 90%.  In mathematical terms, this means Voltage out x Current out x 0.9 = Voltage in x Current in, or if an inverter is driving a 2000W load, you would see 120V x 17A x 0.9 = 13V x 141A.

In addition to power conversion inefficiency there is also an idle wattage consumption of around 7 watts, just for having your inverter turned on.  A consumption of 7 watts over a 24 hour period works out to 168 watt hours, which is about half the daily production of a single 100 watt solar panel.

What extra features might I want on my inverter?
Many inverters will have an option for a built-in DC charger for your 12V system when you are plugged into shore power or running from your onboard generator. Other inverters may have an option for a pass-through AC function that enables shore power (or generator) to bypass the inverter when you’re “plugged in.” Some inverters may even have both of the options described above. Of course, the price point changes with the addition of these options.

Is my inverter a good match for my panels?
A better question would be “Is my inverter capable of supporting my load?”.  The solar panels and charge controller determine how quickly the battery bank charges.  The inverter determines how quickly power can be drawn off the battery bank and converted to Alternating Current.  The two systems don’t have any contact or influence over each other.

What happens to the onboard converter/charger when I install an inverter/charger combination?
In our shop, we typically disconnect the onboard converter/charger from the circuit breaker in the AC distribution panel. We do this in order to prevent a feedback loop between the two chargers. Most often, we leave the onboard converter/charger in place, as a backup, in case the inverter/charger fails. For this contingency, you may want to leave the wiring intact, but label the circuit breaker to remain OFF at all times.