Choosing an Inverter
We have become dependent on gadgets that impact every aspect of life. From refrigeration to surveillance cameras and CPAP machines, we need these devices even when the power goes out. Frequently, the electricity goes out because of weather, accidents, riots or rolling blackouts and last only a few hours, depending on the location and the cause of the outage. Most outages are usually an inconvenience but a power outage could last much longer during civil unrest or after a CME, EMP or terror attack. A power inverter can be a lifesaver or at least get you through a few days without grid power.
What is a power inverter? A power inverter converts electricity stored in batteries into power similar to electricity supplied via the power grid. There are thousands of inverters available, some good and some junk. Stay away from the junk inverters and let somebody else buy those. Inverters hold true to the old saying: Buy once and cry once. Good inverters start out at about $50 and go up so don't despair. There are two primary cost drivers for inverters to keep in mind: power quality and power quantity. Quality inverters mimic clean household grid supplied sine wave electric power. The other cost driver is how much power (watts) the inverter can convert from a battery to AC electricity at a moment in time. Obviously, higher quantity is better but more expensive.
So what does quality household grid power look like? It looks like a sine wave, alternates at 60 hertz, measures around 120 volts and does not change wave form based on the load plugged into the circuit. It runs vacuum cleaners, electronics and CPAP machines without concern of damage or malfunction. These are the same characteristics to look for in a power inverter.
Grid Power Sine Wave
Know what to look for and what to avoid. There are two types of power inverters -pure sine wave and modified sine wave (MSW) inverters. Pure sine wave inverters produce a close approximation of the household grid power we take for granted. It has a smooth wave form that most devices are designed for and run on without problems. The wave smoothly reverses polarity at 60Hz and is suitable for both inductive loads (motors) and resistive loads (space heaters). Appliances like refrigerators, microwave and newer TVs are much happier with pure sine wave than modified sine wave power. On the other hand, a modified sine wave doesn't really look like a wave at all. It is a repeating pattern of voltage "steps" up then down as the polarity is also switched back and forth at the same 60Hz frequency. It works well for resistive loads but may not be suitable for many inductive loads or electronics.
Bestek 300W Pure Sine Wave
Black & Decker 100W Inverter
Pure sine wave inverters are marketed as such in order to justify the higher price for any given power level. A 300 watt pure sine wave inverter is more expensive than a MSW inverter but is worth the additional cost. These inverters will have "pure sine wave" plastered all over the website and retail box it comes in. Assume its a MSW inverter when no sign of "pure sine wave" in the description. Though less expensive, MSW inverters do have drawbacks. Some appliances may run slower and motors may run hotter. After a long outage, refrigerator compressors frequently shutdown due to overheating on a MSW inverter. Box fans often run slower and have an unsettling buzz as well. The cost of pure sine wave inverters is coming down and are often less expensive than a comparable MSW inverter of just a few years ago. It probably best to go with a pure sine wave inverter that can run almost anything within its power rating unless all expected loads are compatible with a modified sine wave.
What size inverter is needed? How many watts? This is simple. Add up the watts from all the devices that will run at the same time and multiply by at least two. The reason for this is motors need a lot more watts to get started than they do once running. Inverters have a surge rating but the continuous rating is the the one that matters. Continuous watts is the number to double after adding up all the loads. When calculating a 1400W load, double that number to 2800W and go for the next model up...probably a 3000 watt continuous inverter.
Don't forget the power source. In some cases a simple deep cycle battery will be sufficient but consider investing in robust battery bank with a grid or solar charging system to power the inverter. Some inverters can be tied into the grid, will charge the battery bank and draw from the batteries when grid power is down. Most often inverters boost voltage from 12-24 volt DC to 120 volt AC. Remember, inverters convert direct current to alternating current but do not "create" power. As mentioned, power is measured in watts and we get watts(P) by multiplying volts(V) x current(A) so P=V*A. For example, a 1500W space heater draws 12.5A at 120V (1500W=120V*12.5A). Therefore, 1500 watts is what the battery must supply. When the battery only supplies 12 volts, it must provide enough current to the inverter to make 1500 watts so in this case 1500W=12V*125A (assuming 100% efficiency -which it is not). The battery bank must provide 125 amps to drive the 1500 watt inverter load and that is a lot of batteries, chargers and solar panels. The inverter will be the least expensive component running the space heater. There are more cost effective ways to heat a room.
There are a number of factors to consider when purchasing a power inverter. Inverters make sense in areas where noise and space are a concern such as an apartment complex. While inverters can be inexpensive, the investment in batteries and charging equipment quickly add up. With careful planning, a power inverter can be great investment for primary or supplemental back up power.