# Looking for small hybrid inverter offgrid/gridtie

Registered Users Posts: 20 ✭✭
I'm building my second solar set up. First one was 1.2kw gridtie with micro inverters. Now for my second in another location. I'm building a cabin, its a little isolated but there is a utility pole about 300ft away. The place will see little usage, mostly weekend use only. I really want to postpone tying into the utility company and paying fees for the privilege for a few more years.   I already have a nice and quiet honda 5400 genny, but prefer things more quiet with less fuel. My cabin is dried in but not wired yet. Last weekend I purchased 6x 123watt 12volt sharp panels for \$75 each. Now its time to get started. Loads will be mostly led lighting, small 12volt water pump to power my cistern, phone chargers and small electronics, maybe a ceiling fan or two. Seems I could go down and dirty with a cheap inverter and a couple of batteries, and depend on the generator when I need more. I recall there are hybrid inverters, that can do both gridtie, and battery like a ups and some might even have a built in charger, but I'm having a difficult time finding anything small. If I did hybrid this would allow me to start with off grid and later grid tie with a little backup thinking power outages might be more common in the rural area. Any suggestions?

edited November 2015 #2
Ok--A little math first to get a rough size of the system and its capability.

Personally, I would recommend a conservative rate of charge of 10%. For a weekend cabin, you can get a way with 5% (2x larger AH battery bank)... But lets try to keep the bank smaller (save some money). Using 10% rate of charge (5% to 13% rate of charge for solar is typical):
• 6x123 Watt * 0.85 panel+controller derating * 1/14.5 volts charging * 1/0.10 rate of charge = 433 AH @ 12 volt battery bank
A set of 4x 6 volt @ ~220 AH golf cart batteries would work nice (either as 2 series x 2 parallel, or as 4 series for 24 volt bank).

Next, practical power output from such a battery bank:
• 12 volts * 440 AH * 0.85 inverter eff * C/20 = 224 Watts (average, 5 hours per night, two nights of storage)
• 12 volts * 440 AH * 0.85 inverter eff * C/8 = 561 Watts peak continuous
• 12 volts * 440 AH * 0.85 inverter eff * C/5 =898 Watts peak (minutes to hour)
• 12 volts * 440 AH * 0.85 inverter eff * C/2.5 =1,795 Watt peak surge (seconds)
So--The above bank would suggest that an AC inverter with a maximum rating of ~600 to 900 Watts would be supported by this lead acid battery bank.

If you keep your usage low--I would highly suggest the 300 Watt MorningStar 12 volt TSW inverter (600 watts for 10 minutes).

And if the place is near Austin Tx:

### AustinAverage Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 60° angle (from horizontal:
(For best year-round performance)  Jan Feb Mar Apr May Jun 3.82 4.18 4.86 5.27 5.19 5.65 Jul Aug Sep Oct Nov Dec 5.90 5.72 5.31 4.87 4.07 3.67
Toss the bottom 3 months (use generator as backup), February estimated power harvest for an AC off grid power system:
• 6x 123 Watt * 0.52 typical off grid system eff * 4.18 hours of average sun Feb = 1,604 WH per day (Feb)
Use your genset for larger loads (power tools, possibly a well pump to cistern), etc. The Honda eu2000i (or similar) inverter-generator would be a good choice for almost any 120 VAC standard device (smaller power tools, etc.).

Don't bother worrying about a Hybrid inverter/GT solar... It is not going to be worth it to "design for the future"--You can keep your "old" off grid system for emergency backup (if needed).

Do look at the charges for making a connect to the utility power... Costs are only going up and could be "much more" a few years int the future--Or, if there is nobody else using the utility circuit nearby--It is possible that the utility could remove the line--Leaving you with no nearby power, or even a higher charge to bring the power in.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20 ✭✭
Thanks Bill thats the info I was looking for. Any suggestion for a charge controller?  Off hand it should be at least 65 amp rated and preferred to be mppt.
Our host has a wide variety of MPPT controllers... Midnite, MorningStar, Outback, and Schneider are all good options to look at.
http://www.solar-electric.com/inverters-controllers-accessories/chco.html

If the solar array stays "small" at 6x 123 Watts, you could get away with a ~40 amp maximum controller pretty nicely (on a 12 volt battery bank).

## Morningstar TriStar 45 amp MPPT Solar Charge Controller

If the array is reasonably close to the charge controller+battery bank (few 10's of feet)--You might get away with a minimum of 45 amp PWM controller too (much less expensive).

It depends on your needs (remote monitoring, other options).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Oh, and I see I forgot the link for calculating the hours of sun per day by month (that table I had in my earlier post)--here it is:

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20 ✭✭
Thanks again! One more question. Keeping my family and friends down to 300watt might be hard, (my wife requires a fan24/7), could I add another morningstar inverter if needed? Are they 'stackable'? Does it matter if they aren't? I realize there is a potential of 220vac across them if they end up phased far apart, but seems unlikely to become an issue.
edited November 2015 #7
The MorningStar inverter cannot be "stacked" for 240 VAC output... There is a 230 VAC 50 Hz version of the inverter--But probably not what you want.

You can certainly put two (or more) AC inverters on one battery bank. Just keep the AC circuits separate (you could make a common/ground bonded neutral for the two inverters if needed).

There are some small/efficient DC fans that may work. And we have talked about DC or even efficient AC ceiling fans that may work.

The issue to remember is that small systems work best if you conserve your energy usage. Fans and water pumps move "stuff", and this can use quite a bit of power--Especially if running 24x7.

For example, if you plan on using 1,600 WH per day--Then a 24x7 load would be:
• 1,600 WH * 0.85 inverter eff * 1/24 hours = 57 Watt average AC load
That is one or a couple AC ceiling fans... And you have used up pretty much all of your energy (plus, this is an average February harvest--Actual daily harvest will vary).

If need these type of power levels, then a larger system, or AC grid connection will probably be needed.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20 ✭✭
That answers my question. No need for 240v, just want to be able to add a little more inverter wattage if needed. So a second inverter that could be turned off until needed would be the plan. I'm building the house/cabin as efficient as possible, 300watts should suffice until more family and guests get into the mix.