Yup, We were just talking about this on another thread. They use a battery driven inverter to 'simulate' the grid and fake out the grid-tied inverters.
@JeepHammer is probably the forum expert on this.
The OP seems to be talking about finding an inverter that will put out the AC without batteries and without the grid..... An inverter that does that would be an unusual beast.
Not really practical for small hobby or 'Backup' systems, it's when you need an extra 3kW and up ADDED (expansion) on your system.
Since grid intertie starts at about 3kWh and goes up from there,
And since it's a peak sun hours thing only, it's a daytime consumption thing, more like for a business that consumes a lot in the daytime,
(Or to support a big air conditioner if you are home all day).
Produce & Consume immedately, it's power that doesn't have to pass through batteries with those losses.
While the 'Average' battery based system is going to lose 35% on charge controllers & batteries,
Grid tied is 98-99% efficient, but it has to be used IMMEDATELY.
You *Can* use it 'Immedately' by charging batteries, but know you will still have battery losses via power supply/battery charger & batteries.
What I'm finding is LFP (LiFe) will take charge MUCH FASTER than the smaller charge controllers will convert panel power to battery charge current, batteries that can take a rapid charge are limited by charge controller.
We are talking a LOT of battery, so it can absorb more energy faster than the little charge controllers produce.
The 'Rule' is generally a battery powered inverter (off grid) that produces pure sine wave,
AND is around 1.25% larger than the grid tied inverter.
An example is a 3,000 watt grid tied inverter would need a 3,750 Watt battery powered inverter to set the sine wave form so the grid tied inverter will 'Unlock' and add power to the common AC Lines.
Keep in mind the off grid inverter will use very little battery power,
And will draw power from the grid tie inverter production to charge the battery bank during peak sun hours in a combined charger/inverter unit.
With extra battery strings as backup, an AC battery charger will charge your extra battery strings when the grid tied inverter is producing, an AC powered regulated power supply for my LFP batteries,
A common AC golf cart battery charger for the last string of Lead Acid batteries.
To make an automated disconnect on the AC lines so battery chargers don't draw from batteries to charge batteries (charge loop with it's losses),
I started with a timer to switch the AC relay/line to the chargers, but this didn't compensate for raining/overcast days.
Now I use a single lower (older) panel to activate the relay in full sun, switches the AC chargers on in full sun,
Switches them off in shade, dark, overcast days.
Everyone days too much wiring...
How much wiring & switches do you think are between your home and power plant?
What I'm doing is pretty simple when you consider that.
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Last week I got a message saying SMA/Sunny Boy inverters weren't pure sine wave because they change frequency, which is NOT a spike in the sine wave that comes from square wave (digital switching) or modified sine wave (clipped switching with voltage spike).
This particular brand & model of grid tied inverter simply changes the frequency of the sine wave, it's still a pure sine wave form.
Since the grid can OFTEN be between 55 to 65 MHz, the shift in frequency of about 1.5 MHz to 'Throttle' the output of inverters to match demand isn't significant at all.
How they work,
Demand decreases, frequency shifts slightly higher on the lines, inverter reduces output no matter what the panels are producing.
The opposite is also true,
Demand increases, frequency reduces slightly, the inverter passes more current until voltage limit (240Vac in case) and frequancy shifts again to maintain output at 240Vac.
It's all VERY fast, automatic, and the kinks have been hammered out over the 20 years of grid tied inverters.
Requires the inverters to be in PARALLEL AC lines (easy wiring since it's common home wiring),
And it's relatively inexpensive since there are dirt cheap AC grid tied inverters all over the internet.
One battery powered inverter (off grid) where all the batteries have to do is power overnight and keep the 'Timing' for the pure sine wave, the big power 'Unlocks' through grid tied inverts when the sun comes up for daytime consumption.
While you have both DC/Battery inverter and AC grid tied inverters (more complicated) you automatically have more power on those long sun hot days for air conditioning, and what ever else you need to operate in the daytime.
At night it's cooler, air conditioning isn't running, your business is down for the day, consumption drops way off when the panels aren't producing anything and you are entirely on batteries.
It's not a 'Hobby' system, it's not 'Emergency' (limited production/consumption) system.
It's a full on, live off the system 24/7/365 with enough batteries it's a full on power plant you own.
It's just not going to be for everyone that's going to have 1-2 kWh of battery and that's all they want/need.
When you live off grid entirely with all modern appliances, it's an entirely different ball game, and systems can easily run well over $50,000.
If you are handy, and you can do 90% of the mechanical work, mounts, racks, panel, wiring runs with conduit, inverter install mounting, then it's just a matter of an electrician hooking things up.
When you go AC coupled, it's 3 wires in parallel on most of that system.
All the mechanical work takes a lot of time, time is money.
YOU already have insurance on your place, YOU don't have to provide insurance, benefits, pay taxes on YOUR work... A hired contractor does...
AND,
That hired contractor wants to make a profit out of all this, so you aren't paying just for the workers, you are paying for someone else's 'Lifestyle'...
Learn in how to run that little $150 a day trencher or little backhoe might save you $10,000.
Buying your own conduit, wires, fittings means you beat the 100%+ markup on component pieces a lot of contractors have.
You also get to use the surplus/aftermarket parts at well below retail cost for an investment of your time.
When you get with other folks interested in the same thing, they often show up with tools and do work.
I do this A LOT, we call it a MAG (Mutual Aid Group) and we trade work/tools/experience.
I have the big industral crimpers, I make wire/cable connections (since I can't work on roofs/ladders anymore) and bring experience to the table...
This forum *Could* be a good place for MAGs to form, several heads are better than one, and don't forget many hands!
Throw dogs & burgers on the grill, fill the cooler with sodas (no beer till the job is done), turn on the radio and you will be surprised who will show up...
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Do your research, have a plan!
Make sure everyone knows the plan so you don't have to try and herd people that don't know what's next.
Have the component parts handy so you don't have to stop for 'Widgets'...
Its a joy to watch someone work that knows exactly what they are doing, TAKE NOTES!
You will be 'That Guy' on the next project.
