Anyone using solar?

Is anyone out there using solar on a grid tied, battery back up system? Im looking at a system that can do up to 30% of my needs and Id like to talk to someone who has practical experience with current system. My main goal apart from the savings and hard asset investment is redundancy in the event of an emergency that involves no grid power. Thanks!

I do, but I'm also likely little help. It's not an off the shelf system. I look at it as a two part system.

Part A: conventional grid tied portion. PV array on the roof. SMA grid tie inverter. There is an automatic transfer switch so if the grid power goes down the PV power feed to the grid is also disconnected to prevent feeding power into the dead grid where it could electrocute an unsuspecting worker.

Part B: my non conventional backup system. This is based on a 24 volt battery bank using AGM batteries and an Outback inverter meant for off grid use. 

The house electrical system was divided into two groups when wired. Section One has circuits to essential services like; refrigerator, freezer, gas forced air furnace, and some lighting scattered through the main areas of the house, two outside lights. Section Two covers everything else; A/C, other lighting and outlets inside and outside the house, workshop power, and so on.

If the grid goes down the system disconnects from the grid. Then after a thirty second delay the battery backup inverter turns on and power is supplied to essential circuits. The auto transfer switch that disconnects the PV output from the grid also trips a switch to transfer the PV array to the Outback inverter to charge the batteries.

If the outage lasts long enough to drain the batteries to a preprogrammed point, a propane fueled generator will auto start and recharge the batteries; also after a 30 second delay.

Part A was installed back when my utility company did not permit battery backup with a grid tie. Later, after inspections and being authorized to connect to the grid, the battery system was added, hence it appears to be an afterthought.

The house wiring was setup as a main panel and a subpanel. The original electrician and inspectors thought I was nuts but it all was code compliant so was passed with only a passing reference to the nutty homeowner.

Today there are approved grid tie inverters that also include battery backup, but I don't know too much about them.

How big of a system covers the essentials that you have? Im basically looking for a system to cover my fridge, freezer, and my geothermal unit. I would then have the option of turning of the geothermal and turning on my stove and/or oven/micro. My peak during the day according to the power company is 6-8 KWH, however that is rare, and the solar companies are telling me that 100% of my power would be covered by a 12KW system, isnt that overkill?

If I understand how this works....If I have a 3KW system that is grid tied with battery backup, then as long as there is no need for the solar system it will all go right to the battery and fill up. Once the battery is filled up, where does that excess go? For example, at night we use less than 1KWH on average, does that still go to grid or my solar system? Of course once the power goes off from the grid, the im all solar as long as the sun is shining. How long do those battery packs last? Enough for 2 days without sun or longer/shorter?

Finally, they are quoting about $25,000 for a 30% coverage system. Does that seem in line? Thanks!

Seems to me that IF this system is primarily a grid tie system the peak daily use does not matter. In grid tie mode, with grid active, the house draws power from the PV array to meet needs. If you hit a peak that is higher than the PV array can supply the system seemlessly draws from the grid to make up the balance. The more important figure is the average monthly use. If the average month uses 500 kWh you can calculate the array size needed to cover 100% use, or any fraction of that. A tool like the one available HERE can be used. They also have a manual use chart down the page.

There are sample grid tie kits listed too. The problem with pricing the kits is they may not include everything. Some folks will need additional components because the roof does not point in an acceptable direction, or they may need to pole mount some PV modules. Most likely there will be additional hardware. Those grid tie kits also do not include an inverter that can be used grid tied with battery backup. That will be extra as will batteries.  There is also no labor costs included and that can be thousands of dollars. If you are capable try to find an installer you can work with. Some will permit home owner labor and some will not.


As for how many batteries are needed, that requires some more calculating. That company also has an off grid calculator, that could be helpful with that. But ytou can also total up the watt hours required for an average 24 hour period manually. Only you will know what you needs are.

Is this to be sized to emergency use if and when nobody is present. That's why I separated circuits and have the generator as backup to the batteries. We are absent days or a week at a time. So the batteries need to be sized to cover a few days of cloudy, poor sun and grid absence. In the past 10 years our longest grid outage was only 24 hours, but our place in the mountains has had no power for up to 4 days at a time according to grid tied friends.


As I said previously I have not familiarized myself with the present selection of grid tie with battery backup inverters, BUT they should work something like this. But they may not.  When the sun shines and the grid is powered the batteries and home needs should be supplied first. Excess goes to the grid.  When batteries are full, the house needs are first priority and excess goes to the grid. If the sun goes away then batteries or house needs are supplied from the grid. The battery charging from the grid should be programmable so if grid power costs less at certain times the batteries only get charged then.  Equipment features mat vary so all that needs to be verified.

Batteries; we have enough to last for three days of no grid, no sun. The battery bank needs to be sized to your needs and conditions.

Hope that helps some.

I guess I understand the usage bit, but I don’t accept it as true…..For example, I took my coldest day of the year and looked at my KWH in 30 min increments. If I had a 3KW system, then that means that anything under 3KW I don’t pay for. There was only about 4 hours about of 24 hours that I went above 3KW, in that case that is the only time I would pay the power company? Based on my amp readings, my geothermal, fridge and freezer can operate at 3KW or below depending on whether the compressor is running or not….with that logic, won’t a 3KW system cover all of those appliances 24 hours a day as long as there was enough sun/battery? I guess my objective for having the system is a little different than most people.

Rob S. aka Blitz wrote:
…..For example, I took my coldest day of the year and looked at my KWH in 30 min increments. If I had a 3KW system, then that means that anything under 3KW I don’t pay for.

That is only valid when the sun is shining enough to provide the full 3 Kw. That means early in the day and late in the day and at night you pay for everything. On cloudy days you may pay for some portion of the 3 kW base.  If the PV array is 2, 3 or 4 times larger than 3 kW base then when the sun shines you send excess power to the grid. IF the utility is using true net metering that means the meter turns backwards during that time. You are placing watts into the 'bank'. When there is no sun or insufficient sun you withdraw watts from the 'bank'.  That only works if the utility offers true watt for watt net metering and depends on the PV array providing enough excess to have power to 'bank' or in effect sell to the utility, so you can buy it back later.  Not all power companies do that.  Mine does and goes one better; they pay me higher rates for our excess than we pay them for most of the power we buy from them. That is subject to change at contract renewal time.

My power company does net metering. And I understand where you are coming from when you say its only when the sun shines, but isnt that what a battery is for? I should have battery left over for the evening in most cases except for middle of winter when the sun might not come out for days. Right? Thanks for working through this with me.

Okay, then you're using the battery for more than backup, to my way of looking at things. Maybe its that we view that term differently. To me backup is for when the grid is down. At night our home draws power from the grid the way our system is setup. Our batteries only come into play if the grid goes down. Most of the time they do nothing except provide peace of mind.

There probably is a way to do what you want,  I've never thought that through. If you want to use the batteries as a source when the sun is gone, not draw from the grid, then you are looking at more of an off grid system than a grid tie system. The PV array then has to be large enough to replenish the batteries during the short number of good solar sunshine hours as well as supply the daytime power needs. How much larger an array than what I was envisioning will depend on the total "no sun" power needs. If you are presently grid tied that can be calculated from the total monthly use. Take the daily average use from that and calculate the required size of the batteries for whatever number of days you want covered. Then calculate the number of panels required to recharge the batteries in the average least amount of sun hours.




Our cabin is totally off grid. All the power comes from the sun and is routed through the batteries. That system is sized for three days of autonomous (no sun) use. There is also a generator is required but mostly it never is called on.

i think we are on the same page now. My point was that we tend to go over the 3kw mark during certain times like when we are cooking, heating and doing laundry. Most of the year because of our climate we run well below the 3KW mark, so Im hoping that we can use the excess during the day to charge the batteries for at night. And in real emergencies I can get away with the bare essentials by managing the breakers. I figured that the worse case KWH per day is 61, and of course batteries are in amp hours…grrr. 61,000 watts/240 V=254 Amp Hours? So if I figured it out correctly I would need a 254 amp hour battery for one day? That’s only one to 2 larger capacity batteries so maybe I figured it wrong.

Let's use 250 amp-hours. A Trojan L16 has an amp hour rating of 325 to 370 amp-hours at the 20 hour rate. However, the deeper the depth of discharge the shorter the battery life. If 250 amp-hours was drawn from the 325 battery that is about three quarters of its capacity. Expect a short, couple years, battery life.

Most systems should be designed to never go below 50% in one days use. I prefer 25% as the normal use aimpoint. Our cabin rarely even uses 15% in a single days use. I fully expect to get at least 7 years out of those batteries. To keep that 250 amp-hours to no more than a 25% discharge that single battery should become three.

But I've saved the worst for the last. That 250 amp-hours is at 6 volts; the Trojan L16REA is a three cell, 6 volt 325 amp-hour battery. We began with 250 (254) amp-hours at 240 volts.   240 / 6 = 40.  Forty batteries, not one; 120 batteries, not 3. 

And that does not address losses in charging the batteries and withdrawing power from the batteries as well as losses in inverting from DC to AC, or the loss in effective capacity when the batteries are stored at temperatures lower than 77 degrees F.  For a rough pessimistic figure add 20% to the required battery capacity. Also note that many batteries is totally impractical. The batteries for such as system would need to be larger and more expensive types.

All that is is one reason why most totally off grid systems avoid things like electric ranges and electric water heaters.

Well thats depressing!!!

BTW I think I figured it wrong, shouldnt it be 61,000 watts/12 v (DC battery) not 240 V AC? In that case I would need alot more batteries. Maybe 7-8 batteries with a 1000 amp hour rating?

So, my next question is...apart from reduncy is it really worth getting a system to cover 25-30% of my electricity? Sound like a lot of money and equipment, and apart from the PV panels, most of it goes bad within years.

The 61 kWh came from the power company meter I assume. That's AC volatge and power rating.  If 240 AC is assumed that would equal 61000 / 240 volts = 254 amps for one hour.  At 120 VAC, 61000 / 120 = 508 watts for one hour.... That part of the math is correct. 


What it boils down to is it is seldom economical to go off grid when the grid is available to connect to. For emergency backup power a generator that will auto start when power is needed may be a less expensive backup to keep the refrigerator and freezer going and the heat pump working.   

There is a home builder here that offers new homes that are essentially net zero consumers of energy. They start with highly efficient frames, well insulated. Then the PV array is grid tied. The homes cost more than a conventional tract built slap it together home to begin with. The PV system adds about $40K to the typical home. Worth it? Depends on what you might be doing with that money. If it has to be borrowed, maybe it's not a good return. If the money is burning a hole in your pocket it may bring a greater financial return to put it into PV than in the stock market.

Alternative energy does not come cheap.

IF the pwer needs can be pared down it becomes easier to build an economical system. Our cabin system cost just under $9K for system hardware, no labor. There was a 30% energy credit on our tax bill. It is only a one room, 16x30 cabin with propane fridge and range, wood or propane heat. The propane is a poor energy cost trade off, IMO, IF cheap grid power was available. In the case of the cabin it would have cost $55K to connect to the grid. (because of it's location)  Now that did not make sense to me.  Our main residence has the solar because the money was available and needed something to do. And I was intrigued by the PV possibilities. The SW, NM, has loads of sun power.

OK, well thanks again for working through this with me. I don't have money burning a hole in my pocket, but I do have savings for later on in life that I might tap into to for the investment, but if Im only covering 25-30% of my needs then financially it might not make sense. Ill let you know what I come up with ultimately.

Rob S. aka Blitz wrote:
OK, well thanks again for working through this with me. I don't have money burning a hole in my pocket, but I do have savings for later on in life that I might tap into to for the investment, but if Im only covering 25-30% of my needs then financially it might not make sense. Ill let you know what I come up with ultimately.

Have you looked at http://www.builditsolar.com/ ? They have a "half project" that is probably one of the best energy pre-projects I have seen. I have got some solar panels I am collecting to power my freezer (no grid connect at all) and maybe some emergency lighting. I figure I can find a way to do without anything else if power goes out. If I have paid for the panels I may as well take at least one thing off my power bill

By the way... when I first saw the title my first thought was "yeah, I use solar, it's 8C outside and the thermostat is set to 19C and the inside temp is 22C. I love solar."

Len, I love that link and will probably lose days of productivity looking at it!

Build it solar is  a great site the solar hot water heaters that Gary has work, I have built 2 of them. I also have a 2.25 kw grid tied/ no battery back up system that covers at least 25% of my yearly average use. This is the production site for the system http://enlighten.enphaseenergy.com/public/systems/rQq46398 .  The ted 5000 will do the most to help you figure out your power use and help you determine which size and type system will be most suited to your needs. Also contact the power company to determine if they have fees (monthly or one time connection) and if they have a minimum monthly charge for having a meter, this fee if $25 can cause you to pay more per kwh for reducing your use. Mississippi has several ways they reward excessive use though their billing practices it will cost the production of 3 panels just to cover their fees. You can check how your state is set up here http://solarpowerrocks.com. Please post your opinion while your there and forward it to your power company.

Thanks for the info peeps.

Otherpower.com has a good forum.
kent