Trying to get the most for your money is always a challenge, and building a solar power system is no exception. The terminology can be confusing, and when we try to compare specifications we can easily be fooled. Manufacturer's literature can be optimistic or pure fantasy but there are ways to protect yourself if you do a little research.
Product reviews these days can be written by professional reviewers who have never seen, touched, or used the devices they are reviewing. It's unfortunate, but that's the way it is. Even negative reviews can be fiction, as vendors seek to get an edge over a competitive product. When I was growing up my father taught me not to believe anything I read and only half of what I see. Now I'm not saying things are that bad, but we all need to be careful about what we accept as fact.
We are all susceptible to deception. One of the reasons is that we often WANT to believe what is not true. This makes it easy to fall prey to those who tell us what we want to hear. So be careful and make sure that you make a decision for the right reasons. If you want quality, the price is a secondary issue and even free might not be a good deal in the long run.
Another major difficulty we can fall into is ego. If we want 'nothing but the best' we often go overboard, and while the results can be impressive you often waste a lot of money. You could end up spending your whole budget and have only half of the system you need. You also risk making the project excessively complex to implement and maintain. It's better to keep things as simple as you sensibly can and maximize value for your dollar.
Here are some common pitfalls and strategies that you use to avoid them:
1. Poor Inverter Reliability
If you've decided to solar power your home it's amazing how inverters pop out of every discount catalog. You see them everywhere. The prices vary considerably for a given power output. A lot of this equipment is manufactured in China for a highly competitive consumer market here in the US. The primary motivation is to sell a lot units at the lowest possible price. They think we want it cheap, so they build it cheap.
For some people this is ok, especially if their expectations or needs are not that high. However when you need to be able to count on the reliability of your equipment year in and year out, these inexpensive units are simply not a good value. Three companies that produce good quality and extremely reliable inverters are; Outback Power Systems, Magnum Energy, and Xantrex.
The greater the complexity of a system the more unreliable it can become. For this reason I advise using the Xantrex Trace Series TR3624 120-60 24v inverter. These are commonly available for less than 1,000.00 and are very reliable. It is not a sine wave inverter but is extremely reliable and has excellent overload capabilities for starting pumps and other tricky loads. I have used them personally for years. I'm going to feel bad when they're not available any more. The Xantrex website no longer supports this unit, but they are still widely available.
2. Sine-Wave vs Modified Sine-Wave Inverters
Pure sinewave inverters have come a long way. They can be very reliable and can offer power output quality better than the grid. However they are twice as expensive as modified sinewave inverters. I have only two devices in my home that have any difficulty at all with my modified sinewave inverter, but I came up with 'work arounds' so there are no problems for me. If you are not on a tight budget go pure sinewave, but the alternative is good too and can save you money you might need for someting else..
3. Insufficient Power
One of the things that has gotten better over the years is the ability of new inverters to handle short term loads well in excess of the wattage rating. Pumps and other motors like a fridge or freezer can take a lot of power to start. Once started, power requirements settle to a manageable level. The ability to deal with motor starting issues is an important consideration.
Another benefit of commercial-grade inverters is that they have ratings you can count on. The cheapies are often rated improperly, and simply cannot be counted on to deliver the power indicated by the manufacturer.
We can run our house in a single 3600w Xantrex inverter. We have an electric fridge and a small electric 10 cu/ft freezer. I use my desktop computer all day long and my wife is often on her desktop computer too. We have a well pump that runs periodically, a big microwave, lights are on all over the house and so our home is pretty normal for a family of four. We have a 24v system and we use about 1,000 watts in average power consumption. Power consumption overnight drops to about 225 watts. 1KW is almost exactly the proper power output for maximum efficiency for this model Xantrex. Our peak power requirements are higher than the rating, but the whole system works well, and has done so for years. Note that we do not have a furnace, which would increase average power requirements.
I cite my personal experience so that you can see that a 4KW inverter which may not seem like a very big unit can reliably provide enough power to run a home.
You can always add another inverter to work in tandem if you need more power than I do. Be sure the unit can be paralleled or stacked if you need more power.
4. What about 240v?
In addition to 120vac output, Magnum Energy and others provide split single phase inverters (120v-0-120v) with half power available across each winding. The 3600w Xantrex inverter can be 'stacked' with a special adapter to provide 7200 watts in the same split single phase format.
Outback Power Systems, and Magnum Power offer the same capabilities with a variety of products. When you purchase an inverter system, you need to think ahead and make sure you aren't painted into a corner and that a migration path exists to increase system capacity in the future.
You can also use an autoformer to convert 120v to 240 volts which is what we use to power our submersible water pump in our well. We also use another autoformer for getting power from our generator at 240 volts which is easier on it and converting the voltage down to 120v for our inverter/charger.
5. Why not use 12v? What voltage is best?
12v is really great for a camper or motor home, and for power requirements less than 3000 watts. The reason requires a little explanation, but the issues are wire size and power losses.
The power formula is P=IE where P is power in watts, I is current in amperes, and E is voltage. Notice that if I drop current in half and double the voltage that the power is the same.
To understand power losses in wire you add in ohm's law and you can modify the power formula to be P=I^2*R where P=power in watts, I is amperes, and R is resistance in ohms.
The biggest factor in wire losses is the current. For a given value of resistance, the power losses in the wire increase by the square of the current. If current through the wire doubles, the power losses go up 4 times!
At 12 volts, the power lost in the cables is 4 times what it is with the same cables in a 24 volt system. In order to supply the same amount of power it takes twice the current at 12v as it does at 24v. That's why really high powered solar installation (over 8KW) use 48 volts or even 96 volts.
I think it's sensible to keep battery voltage as low as you can even though you can see that higher battery voltages reduce losses. Here's why; DC arcing is not self quenching. This means that a DC arc, once established doesn't want to stop!
To illustrate the advantage of using 24 volts, let's use the example of building a 24vdc light for working on a system with the inverter shut down. I would like to use a properly fused DC circuit that included a regular light switch that I buy from the hardware store. I see on the switch that it has a 32v DC rating, as well as a 120v AC rating. It should work great.
If I have a high voltage (48v to 96v) battery system I would have trouble finding a switch. If I used a standard light switch I could end up with a fire hazard, (or at least very short lived switch) since the switch might not be able to stop the arc once established.
Two 12v batteries in series give you 24v. It takes 4 batteries to get to 48v and 8 batteries to get to 96v. This means it is much more expensive to upgrade a battery bank as voltages get higher. More batteries, more wiring, connectors, and etc. are needed. A 24 volt system can more easily be upgraded if greater capacity is needed. A high voltage DC system increases cost and is less flexible.
The decision to use 24v has been a good one for me here. Higher voltages are good but leave you with fewer options, and a more expensive battery system.
6. Physical Installation Issues
Inverters are usually designed for wall mounting, or mounting within a manufacturer' designed enclosure or mounting system. I really like the OutBack multi inverter installation system, but all the companies have ways to nicely install against a wall somewhere. A concrete basement wall would be ideal. The room should be reasonably open, cool, and well ventilated. Make sure that proper fusing and a quality battery disconnect is in place.
It's good to keep wire runs short and use large 0000 (pronounced 4-ought) welding wire which is high quality, easy to purchase at a welder supply house. It's flexible enough that it's easy to route which is important. They also will have the terminals, crimping tools and other things you might need to complete your system wiring. Conduit hangers can be used to secure the wire to the wall.
Restating The Point
When all is said and done you need a reliable and worry-free power system that is robust, efficient and easy to maintain. Try not to buy junk, make sure you can handle high short term transitory loads and make sure that you can expand your system if you need to.
Verify that service centers are available in case you need parts or repairs. This is a good way to verify that the company stands behind their products.
To the best of your ability stay objective and keep your future needs in mind when you're making choices. A deal on something that isn't what you need is not a deal.
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