Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

Say you get an average of 4.5 hours of sun over the year and pay $0.12 per kWH for power:

• 3x.055 kWatt panels * 4.5 hours of sun * 365 days per year * 0.77 GT efficiency * $0.12 per kWH = $25 per year worth of electricity

Your building department may charge you between $45 and $900+ for the building permits and inspection. And the utility may charge you $277 for a new meter, or charge you $10-$25 per month for billing, etc... And your utility may only pay you $0.05 per kWH (they may also pay you more--you have to check).

And you will have to purchase a micro inverter (and possibly a power line interface to monitor the output power from the 1x micro inverter).

My own two cents--I would suggest that a 3kW array is probably the smallest "cost effective" array for many people based on the above issues... Of course, some areas may cost less for an install/utility charges.

Your choice.

-Bill

Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

To keep it simple--The NEC is not a magic safety document--but if various people follow the basic rules (different electricians over the decades in the life of a home/building). Then the overall installation should be/remain relatively safe.

The NEC is pretty much setup to have "one power source"--the GRID--and multiple distributed loads. In some cases, such as with backup generators, it is a simple transfer switch that moves the loads from one power source to another.

And with just those assumptions--it can be amazing how complex/confusing a large/complex/"special" installation can become (ground loops, back feeds, etc.).

So--when you add multiple power sources (like Grid and a Grid Tied Inverter) to a power system--we now have multiple sources of current/energy into the building wiring.

In the simplest example--You have a 15 amp branch circuit that is designed to carry 80% of rated current, or 12 amp max continuous current.

Now, you add 165 watts of solar panels--which could add:

• 165 watts / 120 VAC = 1.4 amps

Adding 1.4 amps to a 15 amp branch circuit does not really hurt much (14 AWG wiring is actually OK to 20 amps)...

But what happens when somebody adds 1,650 watts of solar panels to a 15 amp branch circuit:

• 1,650 watts /120 VAC = 14 amps

Now, if you had ~10 of those micro-inverter setups, you have 29 amps available in that circuit. And since heating of wiring (resistance heating/voltage drop) is:

• power = I^2 * R
• 29 amps^ / 15 amps^2 = 3.7x more heating available in wiring

So--if somebody just kept plugging in additional solar (or wind, etc.) GT micro inverters--until they popped a breaker, then backed of one... They have seriously way too much current available in the upstairs bedroom circuit. And if somebody plugs in an alarm clock or vacuum cleaner, or the wiring up to the roof, with an electrical short--the chances of starting a fire are much higher.

The only "safe" way to connect a Grid Tied Inverter is to hardwire a dedicated branch circuit to the GT inverter... And engineer the Main Panel (starts at 20% of main breaker bus bar rating) and inspected by the building department to insure that the wiring system still meets requirements.

Add the fact that (by code) you probably need to bring the solar power in through the roof/wall with a metal conduit, some locations may have hurricane tie down requirements, others safety grounding the frames against shorts circuits and lightning... etc...

It becomes near impossible to make a generic "plug in" GT inverter that will be safely installed by the "average" person. (there are other issues too--but these are enough to kill "homeowner" plug-in installs to existing wiring).

Remember, UL was originally created by Insurance Companies to have some standard way of insuring that various "stuff" used in building met some minimum level of safety requirements. Started back in 1894 testing a non-combustible insulation material.

I could see some day that, perhaps, there will be a series of "plugs on the roof" pre-wired into new homes/businesses for solar arrays that could reduce the cost and complexity of installation. But I doubt that many people would pay to "pre-wire" solar and never install it on the assumption that someday, somebody else may.

Anyway, my two cents worth.

-Bill