In previous articles here, here, and here, we’ve been discussing our off-grid solar power system we used during Hurricane Matthew. We also gave an overview of solar panel and lead-acid battery principles. While there is much more to discuss regarding batteries, including alternative battery technologies, let’s skip ahead to the inverter, which is the tip of the off-grid power spear. However, some features of inverters are often misunderstood, or obscured by clever marketing claims.
You will recall from the charger article that our interior setup is as shown below:
The inverter we’re going to focus on in this article is the 24 volt MicroSolar 1000 W pure sine inverter shown between the batteries and the charger. Our experiences with this inverter will be a nice tour of the important features of any inverter.
The first important feature of any inverter is the input voltage. This is almost always a fixed value, although a given brand may come in a variety of options. Although 12 volt inverters are almost always cheaper than their 24 volt cousins, the price difference isn’t usually enough to overcome the technical advantages of using a 24 volt array. However, moving up to 48 volts usually incurs a steep price hit on the inverter side. If you are designing a powerful off-grid system, 48 volts is the way to go, or if you have a specific low-power system in mind then 12 volts might be appropriate. But, for our off-grid backup power intentions with this system, 24 volts is the better choice, and gives plenty of room to grow. The MicroSolar inverter we selected was a 24 volt model.
Output Voltage and Frequency
Most readers will want to make sure they select a 110 volt, 60 Hz US power inverter, although European 220 volt, 50 Hz options are available. Also available, but rare in the 24 volt range, are split-phase options. For our off grid purposes, a single 110 volt circuit will be fine. The MicroSolar inverter we selected was a 110 volt, 60 Hz model. We measured its output at 119 volts, which is still well within the 120v/125v that most US appliances are designed to, and enough to easily overcome losses in extension cords.
This is one of the more deceptive features of an inverter, and usually is mentioned as continuous power and surge power. Most non-resistive loads provide an extra power surge at startup, and the surge rating of the inverter needs to handle this. Otherwise, the inverter will trip a safety and turn off. Similarly, the continuous power rating is how much power the inverter can continuously supply without overheating or other damage.
Unfortunately, it is impossible to predict whether this rating is realistic. In the case of the MicroSolar 1000 watt inverter, the most we were able to load it to without tripping was 750 watts, and we had to be very careful in creeping up to that with small or resistive loads. Even so, when a 150 watt refrigerator would start its compressor, the inverter would trip. Assuming a worst-case of 6x surge power when starting, this should have only added about 900 watts to the remaining 600 watts, adding to a total surge power of 1500 watts, well within the nameplate 2000 surge watts for this 1000 watt model.
We also had trouble with the MicroSolar inverter when simultaneously starting the 70-80 watt freezer and two 150 watt refrigerators with no additional loads. We had to start the freezer first, let it run for a while to stabilize, then start one refrigerator, let it run, and finally start a third. In this configuration (just under 400 watts) with no other loads, the inverter did not trip, but it is conceivable that it would if all three appliances restarted their compressors at the same time.
If you care about running motors, such as the compressors in a freezer or refrigerator, this is the single most important feature, and the most often deceptively labeled or omitted. The premium output waveform is known as a pure sine output, which means that it looks just like the public utility waveform. The alternative is the ancient square wave (hardly ever found anymore) or what is known as a modified sine output, which isn’t sine wave at all, hence the deception. However, generating a pure sine output is relatively expensive, about 3x to 4x the cost of generating a modified sine wave.
Modified sine has its uses, however. If you don’t care about motors, then a modified sine wave may be the right choice. Most 12 volt automotive accessory inverters are modified sine, and do a great job with many non-inductive loads such as lights, hair-driers (which have an inductive component but are predominately a resistive heater), battery chargers for tools, or computers. Most computer UPSs are modified sine, and now that big cathode ray displays are no longer in use, it would be hard to tell whether a computer is running off of the UPS or not (other than the beeping, which can often be turned off).
However, with an important appliance such as a freezer or a refrigerator, it is essential that a pure sine wave inverter be used. If a freezer were run from a modified sine inverter, it may work for a while, but the motor windings would be damaged, and eventually burn out.
When it comes to a pure sine output, this is where the MicroSolar inverter really shines. We put a scope on the output, and even at its highest 750 watt load, it was difficult to see any distortion at all, only a little bit of shaving at the peaks but this was very minor. At the 400 watt level, the power might as well have been coming out of the power grid.
Standby power, sometimes called no-load power, refers to the power that the inverter takes to run itself before it supplies the first watt to a load. The more awesome an inverter is, the more standby power it will probably use, but this is typically still only a negligible fraction. In the MicroSolar case, we measured 12 watts of standby power, or about a half-amp from the 24 volt array.
Most inverters will also have a separate five-volt USB charger output. While these are nice-to-have, they aren’t essential since you can easily tack on a car charger adapter (in fact, this is most likely the exact circuit internal to the inverter). Don’t disqualify an otherwise great inverter if it doesn’t support USB charging. The MicroSolar we selected has a 1000 milliamp charger output, and we had no problems using it.
Unlike chargers, which have to be specially designed to work in parallel to charge the same battery array, it is simple to string many inverters downstream of the battery array for different circuits, assuming the batteries and cabling are sufficient to the task, of course. This allows management of the loads to optimize costs by using several inexpensive modified sine inverters for the loads that don’t need pure sine, and saving the more expensive pure sine inverters for those loads that do.
One thing you do not want to do, however tempting it may be, is to power cheap 12 volt inverters off each half of a 24 volt array. Unless you are extremely diligent in manual charge management, this will cause the array to become unbalanced, and result in damage to the battery array due to over- or under-charging. The headaches aren’t worth it, and you can easily destroy a thousand dollars worth of batteries to save a hundred on an inverter.
If you want to use the less-expensive 12 volt modified inverters, consider fielding a separate 12 volt array, all the way from the panels, through the combiner and charger. This is not necessarily a wasteful approach to supplement your 24 volt capacity, particularly if you have a separate building for which 12 volts would be perfect. Keep in mind, however, that the charger immediately becomes half as effective, and power losses increase due to higher currents.
Based on our experience with the 24 volt MicroSolar 1000 W pure sine inverter, we can recommend it for use in a small, off-grid freezer/refrigerator system, as long as you derate its power capacity to about one third of the labeled 1000 W continuous / 2000 W surge. There is also a 3000 W continuous / 6000 W surge model available. Based on our experience with the 1000 watt version, we would expect the 3000 watt model to be good for about 1000 watts.
Even with derating, the price is in line with other 300 to 400 watt versions (assuming those aren’t similarly derated as well), and the larger model appears to be well-priced also. At the time we bought our 1000 watt unit, we paid $189.99 on Amazon, and are happy with it at that price. As of this writing, the 1000 watt model is available for $159.99. The 3000 watt unit is available at $299.99. Expect these prices to change as Amazon vendors often hunt around for the most effective price.
To us, the most important feature of this inverter is the exceptionally high-quality sine wave output we observed on ours with the freezer and refrigerator loads. We are willing to pay more for a higher-quality sine wave, and are willing to be flexible on the maximum load to get that.
Be sure to also read the reviews on Amazon, and in a future article we’ll be comparing the MicroSolar to a competing 4000 watt model we bought since the hurricane.