Hello Jeff,

John

John,

I have had several articles describing how to size and install a small solar power system including the article on a solar cabin in the current issue that is on the newsstand in a week or two.

Although it should not take much to power a small unit like you are talking about with solar, I will give you a word of caution. Most DC powered telephone answering machines, and some computer network equipment like routers utilize either a “floating” ground or a reversed positive ground system. You would not normally notice this because this equipment typically uses one of those plug-in AC transformers that convert the 120 VAC from the wall outlet to 5, 9, or 12 volts DC as required by the small electronic devices. However, we have had major problems with “hum” or communication line noise when we tried to power this DC equipment directly from a DC source without the external plug-in power supply, since the ground was either reversed or the equipment had isolation filtering provided by the transformer.

Although your solar power system charging a DC battery might easily have the capacity to power this equipment, you need to verify that the equipment will work when powered directly, or you will be forced to add a 120 VAC inverter to supply the power and that involves added battery losses and much higher cost. It’s best to keep it simple, but some equipment just will not work without the isolation function of the plug in power supply. Check first.

Good Luck,

Jeff Yago

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Jeff,

I’ve been looking at different generators after reading some of your articles. My goal is to build a home that uses generator power while I slowly shift to solar as it becomes financially effective. That being said I’m looking at using a propane generator so I can have a propane stove, and a propane water heater for radiant floor heating. Using this much propane I figured that a propane generator just made sense in order to stick with one fuel source.

Dean Ash

Dean,

Up until around 1995 what you are trying to do was very common as most DC to AC inverters were not as reliable and could only produce a modified wave form output which was not very good to power any type of battery charger. There were several manufacturers making a generator that consisted of a gas or propane engine driving a heavy duty truck DC alternator for direct charging large batteries. There are a few still being made, but since todays higher quality inverters now have perfect grid quality (or better!) sinewave output and very high capacity battery chargers built in that are designed to operate from either grid power or a generator, we now design everything around normal 120 or 120/240 home wiring systems.

Since these new inverters are so efficient, the system losses are almost the same when using a high quality inverter to charge a battery bank with a generator to power the inverter, as it would be to use an engine driven alternator to charge the same battery bank. Also, since your generator would be supplying 120/240 volt AC, you can supply larger AC equipment and power tools directly that you could not do with a DC output generator. Unless you are planning a very small battery system and are trying to avoid the cost of an inverter, I would go for a high quality generator like the Kohler which we prefer since it uses a very simple 2-wire start/stop circuit. This makes it very easy to remote control from almost any inverter, while many other generator brands require complex 3 or 5 wire start circuits which sometimes require buying a separate control box to allow remote start/stop control with an inverter.

Good luck,

Jeff Yago

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Jeff,

Allyn:

Funny you ask. I just had an article on this subject in a solar trade magazine but it is for those in the business and I doubt you can find it at a book store.

Yes, all grid-tie systems shut down when the grid fails but this is not a utility scheme, it is required by the design of all grid-tie inverters. The only way the grid-tie inverter can sell solar power back into the utility grid is to match the grid’s sine wave and voltage and cycles perfectly. If any of these do not match, there will be no flow out to the grid. This means the inverter is constantly taking these measurements which it uses to make this perfect match and cause the flow from the solar power back out to the grid.

However, almost all battery-based inverters designed to also sell power back to the grid first put the solar energy into the battery bank, then this stored battery energy is converted to grid power and sent out on the grid. In other words, battery based inverters have no problem providing power to your emergency loads when the grid fails, but this means you will need a large battery bank, a room for these batteries, and a higher cost inverter.

I have found that many of the newer solar dealers just entering the solar business have never installed a battery based solar system as they are harder to design, harder to install, and cost much more so they have very few people interested in buying this type system.

Those of us who have been installing solar systems for years have no problem installing battery systems because at one time NONE of these solar systems sold power to the utility grid and they all had batteries. I recently had a client tell me another solar dealer told them the manufacturer would void the warranty if they installed a battery based system. Keep in mind both systems use the same size and type solar modules. The only difference is a grid tie system will wire 10 to 12 modules in series and use a high voltage DC inverter which allows using smaller wire to connect the array. A battery based system will have the solar array wired for 24, 48, or 60 volts DC to match the battery bank voltage. This requires a different wiring arrangement but can use the exact same module.

Hope this helps. Time to find another installer.

Jeff Yago

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Hi Jeff,

Mark:

You are comparing inverters for totally different types of solar systems (comparing apples and oranges).

An inverter that indicates it has a solar power input of 150 to 450 volts DC or in that general range is called a “grid-tie” inverter and is not designed to be installed in a battery-based system. The output is 240 VAC and connects directly into your home’s main circuit breaker panel and “turns the meter backwards” when operating. The high voltage solar input allows using much smaller wire since the current is low.

Inverters designed to provide backup power when the grid fails or in an “off-grid” application with no utility grid are usually designed for a 12, 24, or 48 volt battery bank. Usually a 12 volt battery inverter would be under 1000 watts output, while 48 volt DC inverters are usually in the 3,600 to 4000 watt AC output range.

Lower cost solar charge controllers are usually designed to have the same voltage solar array as the battery. For example, a 48 volt solar array connected to a 48 volt battery. However, when the solar array is a long distance from the inverter and battery bank, there is a large voltage drop due to the long wire. One way to off-set is to use a “power-point” charge controller, which allows a mis-match between the solar array and the battery and inverter. For example, we may design the solar array to have a 72 volt DC output less a few volts loss going into the charge controller, with a 48 volt output going to the battery and inverter.

Hope this helps,

Jeff Yago

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