How to Choose to right Size Off-Grid Power Inverter ?? August 04 2020

Q: Firstly, what is an inverter?

A:  A device which changes direct current (DC) into alternating current (AC) Power.
Example: It may turn (convert) 12v, or 24v or 48v DC into 230V AC power (so you can run standard AC appliances that you would normally plug into the wall).

Q: What size of inverter should I use?
A: The size you choose will depend on the watts of the appliance or tool you want to run (find the power consumption by referring to the specification plate on the appliance or tool). We recommend you to buy a larger model than you think you’ll need (at least 10% to 30% more than your largest load).
Example: You want to power a computer with 20” LED screen and a lamp. Computer and Screen: 300 Watts. Light Wattage: 75 Watts.
Total Needed: 375 Watts.  For this application, you would need a minimum of a 450 Watt inverter but we would recommend a 600w inverter which is the MINIMUM wattage we would supply.

You should always consider a larger unit than your minimum requirements, as there will likely be a time when you wish you would have bought a higher capacity model. If the wattage of the appliance or tool is not listed explicitly on the specification, you may use the following formula. Multiply: AMPS x VOLTAGE = Watts.

Example: On the microwave shows 5.22 AMPS for continuous use. The
Wattage is 5.22 AMPS x 230 V = 1200.6 Watt.

Q: Why can the 1500 Watt inverter not run my 1200 Watt microwave?
A: The power commonly advertised for microwave ovens are the cooking power (the power delivered to the food) not the power actually consumed by microwave oven. The microwave ovens might consume 40% to 100% more than its advertised cooking power. The recommended inverter to run 1200 Watt microwave is 2500 Watt or larger.  Often the Microwave manufacturers only show the "constant" wattage rather than the peak wattage that their unit will use.  Many microwaves cycle the power (on and off quickly) to give a lower constant wattage shown but in reality they are still using the higher peak wattage (from the inverter) even if it's only for a short time. 

The best microwaves for Off-Grid usage are the Panasonic INVERTER Microwaves as you have the ability on these units to run them at half power and rather than cycle the power (with high peaks) the power input / output of these units is able to keep constant at a lower rate, or which ever power level you set it to.  A 1200w Panasonic Inverter Microwave will typically run happily from our 2.2kW Inverters while many other cheaper 1200w microwaves may not.

Q: What is meant by the terms “continuous wattage” and “peak surge wattage” on the inverters?
A: The “continuous wattage” is the wattage that the inverter can supply 24/7 as long as the DC input power supply is in good condition (usually, the DC power supply is a car battery) The “Peak surge wattage” is the maximum wattage that the inverter can supply for very short period of time (a split of a second or sometimes 1 second or a half second etc)

Note: Induction motors such as air conditioners, refrigerators, freezers, microwave ovens, and pumps may have a start up surge of 3 to 7 times the continuous rating. Heat generating appliances such as hair dryers and water heaters will have high peak surges as well.

Example: If the microwave oven has peak surge which is 4 times the continuous wattage, the 1200 Watt continuous microwave ovens will have 4 x 1200 Watts = 4800 Watts peak surge. The recommended inverter to run 1200 Watt continuous and 4800 Watt peak surge microwave is 2200w constant / 4800 Watt Peak Inverter or larger. In general, induction motors require an initial surge of power to start up (“starting load” or “peak load”). Once started, the tool or appliance requires less power to continue to operate (“continuous load”).

Q: What is meant by inverter efficiency?
A: Inverter efficiency tells you how much power (as a percentage) is lost in the conversion.
Example an inverter with a 90% efficiency rating will have a 10% loss.  This means to run a 1000w load / appliance you are actually using 1100w of real DC power / battery power etc.

Q: What is inverter overhead power consumption?
A: When an inverter is producing ANY AC output (even just to run a 10w LED light bulb for example) it will have an overhead power consumption (per hour).  Typically for many larger inverters eg 3kW - 5kW in size this may be around 50-70w (per hour) and in some inverters cases this could be as high as 120-140w (per hour).  With smaller inverters (eg 2000-2200w size) you are often looking around 30w per hour overhead power consumption, so that usually needs to be added into your 24 hour power consumption calculation to get a more accurate power consumption

Q: What is idle power consumption?
A: An inverter when it has NO load (0w of load) running can sometimes enter into an IDLE power saver mode, and sometimes that may bring the overhead consumption of the inverter down from 30w to say 15w for example (saving you some power per hour).  The only problem with this is that in order to kick the inverter OUT of power saving mode back to delivering a nice constant stream of power often requires running at least 120w worth of loads (eg switching on a kettle or a TV screen / computer screen etc), however if you were just switching on 1-2 x 10w LED light bulbs then often with just a 10-40w load the inverter would NOT have enough load present to KICK it out of Power Saving Mode and as such you get flickering of lights or other weird problems and issues.  As such we usually recommend for MOST clients to NOT enable the Inverter Idle Power Saver Feature as usually for most people it will cause many issues and only result in headaches rather than any real world power saving.  For all of our power calculations we always factor in for an inverter running 24/7 in normal output mode with it's normal overhead power consumption and we recommend you calculate in this manner also.

For this reason the default setting for most (actually almost all) inverters is with the power saving mode turned OFF as the default setting.  Some AC appliances will refuse to run correctly when it's turned on hence we in 99% of cases we don't recommend enabling it.

Q: What is Pure Sign Wave and what is Modified Sign Wave / Square Wave Inverter?
A: Inverters can be built to 2-3 different main AC output power standards and these are typically split into Pure Sign Wave Output (a nice rolling AC Sign wave) which is compatible with 99.99% of normal AC appliances, and the other type is the cheaper Modified Sign Wave (otherwise known as a Square Wave) AC Output.  We ONLY recommend and supply the Pure Sign Wave Inverters as these run happily with normal AC appliances without causing issues.  In turn inverters that have a modified sign wave / square wave can sometimes shorten the life of some AC appliances or cause issues / problems because of the more squared edges and less smooth power (for the AC Output curve) of the AC power that they supply.

This is now the end of this posting regarding Small / Mobile Off-Grid Power inverters, we hope that you were able to learn alot of useful information. If you need info regarding larger off-grid Inverters eg to run 5kW or 10kW or 15kW worth of loads, or regarding on-grid or hybrid inverters, or if there's a question you have or something that you are confused about or something we've not covered that you want to know about please feel free to email us or fill out the contact us page, or give us a call so we can best assist you further.