Pros: Small and light, good build quality, high surge rating
Cons: Voltage is pre-set and not adjustable
A solar system usually needs an AC inverter to be useful in the home. Cheap modified sine inverters are easy to come by and work with most appliances but I discovered that using a mod-sine inverter caused problems with my TV and hi-fi, causing them to make a buzzing noise. This is because they do not produce a smooth current, but one made up of rough square waves and lots of radio frequency noise.
Cotek are an established maker of pure sine wave inverters that use high frequency conversion to produce small, lightweight inverters that will power the most sensitive loads. The other inverters that I looked at were much more expensive and had lower surge ratings of just 1.5x the continuous rating. This inverter can deliver 2.0x the continuous rating for starting up difficult loads (like my big hi-fi amplifier, which has a massive power supply).
The Cotek SK range are also quite efficient for pure sine inverters. The 48VDC model is up to 95% efficient at full load.
Pure sine inverters are less efficient than mod-sine inverters and this can be a problem with solar systems that have precious little juice to spare. Cotek have updated the S range of inverters with the SK range that include programmable power saving. A set of DIP switches on the front panel allows you to set a threshold load power, below which the inverter will go to sleep and use half of it's normal "idle" power - the energy it consumes when no AC load is being powered (1.2A for the 12V model and about 600mA for the 24V model). It "pings" the AC line once per second and measures any load it sees. If the load exceeds the set level, the inverter wakes up and powers the load. You can set the limit to as low as 20W - enough to make the unit go to sleep if the last light in your house is turned off.
In practice, I found that the function sometimes caused very low power compact fluorescent lights to flash without turning on. This was because I use a lot of 8W or 11W lights and turning one on wasn't quite enough to force the inverter to wake up but it was enough to make the bulb flash.
I also discovered that if you try to use a RCD breaker with the power save mode on, the RCD interferes with the load sensing because the RCD has a relay that activates when power is first applied to it. If the inverter sleeps then the RCD breaker turns off and then the mechanical switch in it "pings" as well. This would wear it out quickly. So if you are using a RCD, you can't use the power save mode. A pity.
Another feature that is very useful on this inverter is the digital remote control port. You can buy two types of remote control. One is a simple on/off button and the other is a full remote display panel that allows you to turn on/off the inverter plus it shows the inverter status (battery volts, power output %, power save status and error conditions like over/under DC voltage and temperature warnings). I bought the full remote panel and it comes with 15m of flat telephone-type cable. The inverter has a front panel 3-way switch that is centre off, and then either "on" or "remote". So you can over-ride the remote control if it is some distance away and you are working on the inverter or battery bank.
The front panel of the inverter itself has some status LEDs that give you a rough indication to the power output and battery voltage as well as warning of critical errors. They do this by various states of flashing and colour changing from green to yellow to red.
The unit is almost silent in operation up to 200W output, just making a slight "whispering" noise as it is a high frequency converter. Above about 200W, the cooling fan comes on and this is quite noisy but not objectionably so.
I have owned the 12VDC and 24VDC model of the same inverter as I started out with a small number of solar panels charging a 12V battery bank but when I reached more than 260W of solar panels, I had to change the battery bank to 24V to accept 600W of solar input.
The 12V and 24V versions are physically identical.
Using a 24V system also meant that the inverter cables could be smaller for the same power, although I had already over-sized the cables at 35mmsq for the 12V system.
The inverter has internal fuses but requires an external in-line fuse as well. The internal ones are just for reverse polarity protection. It has comprensive fault handling as it is fully microprocessor controlled and will shut down gracefully if overloaded or get too hot or the input voltage gets too high (15.3V / 30.6V / 61.2V) or too low (10.5V / 21V / 42V).
It is unconditionally stable into all power factor loads and can start capacitive and reactive loads.
There is an additional DIP switch for setting whether the output is 60 or 50Hz, although I don't think it is terribly useful as you can't set the output voltage. Mine is preset to 230V and I don't know of anywhere that uses 230V 60Hz, so it seems pointless to be able to set it. If you could set it to 115V, 60Hz then I would have seen the usefulness of it.
Despite being on all the time when plumbed into the battery, the unit consumes next to no power. The microprocessor control and remote panel are designed to self-start and kill. This means they use no power when off but can be commanded to start by a logic button (not a heavy duty power switch). This is noticeable by the short delay (about 1 second) between pressing the start button and the power coming on. The unit beeps once to let you know it is starting (the remote panel also has a beeper so you can hear it) and then beeps again when the power is output. This also ensures that the inverter output is geared up and stable before the socket is turned on - enabling the inverter to start heavy loads from "cold" and eliminating the chance of a start-up spike or other anomaly.
The usefulness of the ability to start the inverter by a logic signal was that I was able to easily modify the Cotek remote control to incorporate a wireless remote switch (fashioned from a battery operated wireless doorbell, no less) to enable the inverter to be started from bell push buttons strategically placed round the house.
Physically, the inverter is a solid anodised aluminium box with mounting lugs and vents at both ends (for the fan). The DC input lugs are very solid and are shielded to prevent short circuits or the accidental touching of the DC terminals (remember this same inverter is available as a 48V model). The output is via a regionally selected domestic power socket (so a US one has a US socket, and a UK one has a UK socket). The inverter is supplied with four DC power rings that you can crimp or solder on to cable up to 35mmsq. Only two are required for the inverter end. Heavy duty stainless steel bolts are included as well.
An AC Earth terminal is provided for connection of the AC Earth to your house Earth. This is connected to the case of the inverter and the AC Earth of the output. The DC input is fully isolated from the output and Earth, so can be used with negative or positive DC grounded PV systems. On the 230V range models, the Neutral is left floating but can be tied to Earth, if required (by wiring in the output plug). On the 115V range models, the Neutral is internally connected to the Earth.
So, all in all, the Cotek inverter is a cost effective and flexible standalone inverter that should serve well in any off-grid system.