How do I 500W Grid Tie Inverter wire it up for battery backup?

Bob,

That MW&S system--Basically it is an off grid system (the AIMS inverter) that runs from DC Battery bank to the AC off grid loads. Note the warnings:

Because grid power is not 100% reliable, we are unable to allow the connection of wind turbines directly to this control board. However you may use an additional charge controller of suitable size and with its own diversion load to connect wind turbines to the same battery bank. This will help fill the batteries even faster and can provide valuable power at night. Even though the grid tie inverter is connected and prewired to the control board, it is not UL1741 listed. A common question we're asked is "Can I connect the grid tie inverter output to the regular battery inverter outlet/receptacle or visa versa," the answer is NO as this would most likely cause failure of the system in part or in full (i.e. irreparable damage). This controller board is simple to install, but, if in doubt, hire an electrician to help you.

Then there is MW&S's dump controller that is connected to a GT Inverter (120 VAC @ 500 Watt for that system)... When the battery is full, the dump controller turns on and feeds power back to the AC Grid (not connected to the AIMS/Off Grid Loads that I can see).

If you wanted to have a "protected AC Sub Panel" that would feed your AC loads with Grid Power OR Battery Backed Up Inverter power--You would need to do something else. Basically you would need a Transfer Switch, or an Off Grid Inverter with an internal AC transfer switch, or a "hybrid" AC inverter that can do both GT and OG mode.

Note that while the GT inverter used here does function it is "Not UL 1741 approved device."

The MW&S video and the video on GT inverter in the other Youtube video--They are not UL 1741 Listed devices and not legal to connect to your home's AC wiring. If you want to use either system, you do this at your own risk and liability (i.e., your insurance may not pay if there is fire--related or not to your installation, and if there is injury/damage to other's property, you have have serious legal consequences).

I am more than happy to discuss the details of the systems... And if they "can work" or not, possible weaknesses in the systems, etc.

However, do not take my discussion as a "how to install" set of instructions. There is no "legal" way to connect this type of GT system to a home/utility grid in North America in most jurisdictions (there are some/many rural locations that do have inspection requirements).

Will it work as shown in the video--Yeah. We have had a few posters here that have tried other brands of non-UL compliant GT inverters and they tended to be unreliable and run hot. How will this one run--No idea.

-Bill

How large of system do you need? What do you want to power?

In general, there are not a lot of smaller hybrid inverters (grid tie + off grid capable).... If you want a smaller off grid system, generally you just use an off grid inverter + battery bank + solar panels + solar charge controller + (your option) a backup AC charge controller + genset. (or an off grid inverter-charger unit).

True hybrid inverters backfeeding the grid are generally larger (3kWatt inverter or larger). If nothing else, the costs of permits, installation, inspections, power company approval, possibly a new utility meter, etc. generally mean that smaller GT type systems are just not worth the trouble.

Does this mean that you will "lose money" because unused power from the solar panels has nowhere to go--Yep, it does.

-Bill

Bob--The system you have listed will not really meet any of your needs. Way to little of solar panels. Way too large of AC inverter, and the 12 volt battery bank is way too large for the solar array and way too small for the AC inverter (to run at rated load).

Plus, 3,000 Watt AC inverters @ 12 volts usually are not going to work well and be expensive to wire. The current needed to run one would be:

• 3,000 Watts * 1/0.85 inverter eff * 1/10.5 volts cutoff = 336 Amps at 3kWatt AC load (huge copper cables)

3x 45 Watt solar arrays would supply on a sunny day around (and the Harbor Freight 45 Watt panels don't even usually output anywhere near their 45 watt rating, and usually do not last very long):

• 3 * 45 watt array * 0.77 panel+controller derating * 5 hours of sun per day = 520 Watt*Hours of charging current on a reasonably sunny day
• 520 Watt*Hours per day of energy * 1/(3,000 watt load * 1/0.85 inverter eff) = 0.147 hours of energy per day = 8.9 minutes of 3,000 Watt load per day

And the DC battery bank if it ~500 AH @ 12 volts would support around a 1,250 Watt Maximum AC inverter using (using the 400 AH per 1,000 Watts of inverter capacity on a flooded cell lead acid battery bank).

Plus the batteries (at least the Interstate batteries) are not true deep cycle batteries, but Marine batteries. If you pull them down to 50% state of charge, they usually do not last very long (months to year, not 3-5 years for a typical deep cycle golf cart battery).

The mix of hardware (as I understand it) does not play well together at all.

Of grid solar power is not cheap--And if you are looking for seasonal/weekend cabin power usage--A generator + fuel is usually a cost effective alternative to solar panels+batteries+electronics--Unless you are going to be off grid for 9+ months a year. And if you you still want solar power for running Heat Pumps--The system design needs to start over from scratch.

For example, say you need 1,000 Watts * 12 hours per day for your two AC systems total (assuming very well insulated cabin and you run the two heat pumps at ~500 Watts each--low power).. A typical system may look like:

• 1,000 Watts * 12 hours per day * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 max discharge * 1/48 volt battery bank = 1,176 AH @ 48 volt battery bank (about 8x larger than yours)
• 1,176 AH battery bank * 58 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 8,858 Watt solar array nominal (based on battery bank size)
• 1,000 Watts * 12 hours per day * 1/0.52 end to end system eff for off grid solar * 1/4 hours of sun per day = 5,769 Watt solar array (based on minimum of 4 hours of sun per day)

Without knowing anything more about your loads, the above is my first guess (you would throw in a 4 kWatt @ 48 volts off grid AC inverter too). Plus charge controller, plus backup ~10 kWatt AC genset (if you need one).

As we say--You start with your loads (amps, watts, hours per day), look at conservation (most efficient loads you can get, use insulation, LED lighting, etc. to get your heating/cooling loads a low as possible). Then start the design for you solar power system. Any other way, is most likely going to be costly and frustrating for you.

-Bill

Do you have a link to the Watt meter?

-Bill

Sizing your off grid power system--That is the $20,000 question.

We really need to know/understand your loads & electrical needs. If you already have AC Mains, then your utility is going to be your cheapest source of power--Unless you want to do Grid Tied Solar (solar panels -> GT inverter -> Main House Panel).... In some cases, GT Solar power can actually save you money (but it does depend on where you live, your utility's rate plans, etc.).

Off grid solar power--Is pretty much always going to cost you more money than utility power... Typically around $1.00 to $2.00+ per kWH--Although a few people here have gotten down into the $0.50 per kWH (shopping for good prices, designing a system that pretty much exactly meets their power needs, etc.).

If you are looking to save money, focus on energy conservation first. Start with insulation, double pane windows, more insulation, LED lamps, Energy Star Appliances, turning stuff off when not in use, etc.

If you are looking for backup AC power (2 weeks or less)--Look at a genset and stored fuel (or natural gas if available). Generators can be cost competitive with off grid solar power--But, again, you really should design your generator system for your power needs. If your loads are small, then an 8kWatt genset that may use $1-$2 per hour in fuel may be way way more than you need if a 1.6kWatt genset will keep you in lights and run your refrigerator for a few weeks (run the genset for 1/2 the day, on a $5 dollars of fuel per day).

If you are looking for 1-2+ months per year of backup power, a smaller solar power system + battery bank + off grid AC inverter may be helpful. But sizing the system to run all of your AC loads (including air conditioning, etc.)--Then it may simply be too expensive to be worth it for emergency backup power (see genset instead). An off grid system that can run even a smaller single AC system (one room), will probably cost you many tens of thousands of dollars. Plus you need to replace the batteries every 5-8 years or so, new AC inverter+charge controller/electronics every 10+ years, etc.

-Bill