Run appliances during the day, and switch to grid at night?

[twilliamsen]

I am wanting to power my fridge along with a chest freezer in my basement. My thought is to have these devices run on the solar power during the day and at night or when the sun goes down, on the normal grid power.

I do not want to tie into the grid, but just something that automatically switches when voltage drops or non existent.

I looked at transfer switches, but need the opposite on where the panels are zero not the grid and then switch power sources.

Has anyone done anything like this?

[Amy@altE]

Your appliances are AC, the solar panels are DC. So you will need to use the solar to charge a battery bank and an inverter to convert the DC to AC for the appliances. There is no off-grid inverter that will convert to AC without batteries. There are inverters available that will automatically do what you want to do, but as I said, they need a battery bank. So while it is a doable solution, it's called self-consumption, it is an expensive solution. If your idea is to do this to save money, it won't work unless you have outrageously expensive grid electricity.

[twilliamsen]

Thank you, but you are missing the point... They will be running off an inverter with a battery bank already, but I wanted to reduce the amount of usage of the battery bank and take advantage of the sun during the day. Then when the sun goes down, switch it back to the grid.

[Amy@altE]

OK, good, we get asked about doing this without the right equipment all the time. Yes, you can do this with the Schneider SW inverter, as well as their XW+ and Outback Power's Radian and FXR inverters.

[twilliamsen]

Thanks, can this be done without hard wiring into the grid?

[twilliamsen]

Wow, a bit over kill for what I am looking for...Kind of nullifies my ROI lol
Maybe I should just plug it into the array before I leave for work and plug it back into the socket when I get home

[sensij]

There is no ROI running on batteries, so you might need to reconsider your premise. You can't just plug into the PV array without a charge controller, but if you add that, your manual transfer approach is feasible. Is your inverter also a charger, getting the batteries to float by the end of the night? If you want ROI, and you have net metering available, forget the batteries and just grid tie.

[DanKegel]

Firstly: use grid-tied power. Seriously. Using the utility as an infinite-sized battery is currently a lot cheaper than having real batteries, at least where I live.

But in case you really have to go off-grid for some reason: go read up on off-grid appliances. In general, people who go off-grid have to work pretty hard at it, it's not easy.

Step one might be to grab a Kill-a-watt and measure usage. You might find as I did that one of your fridges is using a lot more energy than current models. Replacing an inefficient fridge - or ditching the second fridge - might be way more cost effective than going off-grid solar for it and/or might extend your battery life.

One fine point: Looking around on the web, I see defrost heaters for fridges as being 100 to 700 watts, and people worrying about how autodefrost hurts their battery life. I don't know how real an issue this is. You might want to look into it.
If it is a real issue:
Do your fridge / freezer have switches to turn off autodefrost? (If so, the fridge/freezer is probably very old, and needs replacing with a more efficient model )
Alternately, if your fridge/freezer does lack a switch to disable autodefrost, and you're experienced at electrical work, maybe you could add one the old-fashioned way.

(Finally -- in the blue sky category of fun ideas -- for fridges that can be remote controlled, like some recent GE models -- tell them to only run their defrost cycle when the panels are producing surplus power. That's not available off the shelf, but could potentially be hacked in using GE's API. Similarly, the target temperature of the unit could be set slightly higher when power is scarce, and slightly lower when power is cheap, thereby using the thermal inertia of the fridge as a kind of energy storage device. A few LG fridges in a few countries with terrible power have a feature called Evercool which is just a layer of phase-change material to add several hours of thermal energy storage. They're not sold in the US, but an enterprising hobbyist could probably throw something similar together if he was willing to experiment.)

[twilliamsen]

There is no ROI running on batteries, so you might need to reconsider your premise. You can't just plug into the PV array without a charge controller, but if you add that, your manual transfer approach is feasible. Is your inverter also a charger, getting the batteries to float by the end of the night? If you want ROI, and you have net metering available, forget the batteries and just grid tie.

Seriously, either we are all narrowly focused, or I am not communicating correctly. I already have panels, charge controller, batteries and inverter. All I want to do is to be able to switch between solar power to A/C and vice versa, either on a timer or based off another value, like a voltage drop or lack of voltage from the panels.

[Amy@altE]

Seriously, either we are all narrowly focused, or I am not communicating correctly. I already have panels, charge controller, batteries and inverter. All I want to do is to be able to switch between solar power to A/C and vice versa, either on a timer or based off another value, like a voltage drop or lack of voltage from the panels.

You never said you had equipment already, that's important information to get useful advice. What inverter do you have? Maybe the easiest solution is a Samlex STS-30 transfer switch and possibly a timer.

[sensij]

It still isn't clear what you are trying to do. Are you using the grid to recharge the battery at night, or just operate the appliances?

[jflorey2]

Seriously, either we are all narrowly focused, or I am not communicating correctly. I already have panels, charge controller, batteries and inverter. All I want to do is to be able to switch between solar power to A/C and vice versa, either on a timer or based off another value, like a voltage drop or lack of voltage from the panels.

Outback inverters have a feature called HBX (high battery transfer) that will do this. The inverter will run on batteries as long as it can, then when the batteries drop below a certain level, the inverter will automatically switch back to grid. When the batteries are recharged the next day it will switch back to battery/solar operation. This is VERY hard on batteries since you are always draining them as low as the inverter allows, then "storing" them discharged all night.

Other ways to do it -

A simple PV panel (a 5 watt one from Radio Shack) driving a 12VDC DPDT relay. You will get a power glitch but it shouldn't be too bad.

A simple AC timer driving a 120VAC DPDT relay. Same as above.

I looked at transfer switches, but need the opposite on where the panels are zero not the grid and then switch power sources.

Then just use the opposite terminals.

[SunEagle]

It still isn't clear what you are trying to do. Are you using the grid to recharge the battery at night, or just operate the appliances?

I think it is real clear what the OP wants.

He has equipment for a solar/battery system and wants to run his appliances on it during the day. He then wants to run those same appliances on the grid during the night or when the sun isn't shining.

Now what the OP doesn't understand is that the cost of electricity to run his appliances off the solar/battery system is probably 5 to 10 times the cost of running them off the grid during the daytime.

It is nothing more than what most people have been led to believe but really do no understand what the costs of an "off grid" system is. They think because the sun energy is "free" it will be less expensive to run their loads from it instead of the grid. That may be true for a "grid tie" system but is not true if you use a battery or "off grid" system.

My thoughts for the OP. If you do not care what it costs you to run your appliances from the solar/battery system during the daytime then you need to find a safe and electrical approved equipment to totally isolate your appliances from the grid when they are running from a battery power inverter. If you use something that is not UL listed you can run into issues with back feeding the grid or causing an electrical issue.

[sensij]

I think it is real clear what the OP wants.

He has equipment for a solar/battery system and wants to run his appliances on it during the day. He then wants to run those same appliances on the grid during the night or when the sun isn't shining.

<agree with the rest of what was written in the quoted post>

Yeah, but is the PV system sized large enough to run the appliances and take care of the battery on its own, or does the battery maintenance need a grid assist? The relay solutions are viable, but if the battery isn't getting properly maintained, it takes what is already a costly approach and makes it worse. That is why I was cautious about throwing that idea out there... just switching the source of power to the appliances is easy, but doing so in a way that also makes a best effort at prolonging the battery life takes a little bit more thought. If the inverter isn't capable of charging the battery from the grid, the transfer point should not just be whether there is voltage from the array, but probably based somehow on the condition of the battery. Keeping the battery from getting overdischarged has to be the priority above everything else.

[SunEagle]

Yeah, but is the PV system sized large enough to run the appliances and take care of the battery on its own, or does the battery maintenance need a grid assist? The relay solutions are viable, but if the battery isn't getting properly maintained, it takes what is already a costly approach and makes it worse. That is why I was cautious about throwing that idea out there... just switching the source of power to the appliances is easy, but doing so in a way that also makes a best effort at prolonging the battery life takes a little bit more thought. If the inverter isn't capable of charging the battery from the grid, the transfer point should not just be whether there is voltage from the array, but probably based somehow on the condition of the battery. Keeping the battery from getting overdischarged has to be the priority above everything else.

I agree with you. An "off grid" system needs to be properly designed or the batteries will have an early death.

At this point the OP doesn't seem to care what the cost is so trying to redirect him to not using it or making sure it is properly designed is IMO a mute point. He just wants to use his solar equipment to run his appliances during the day. The problem is the batteries will probably soon fail and he will hopefully learn from his mistakes.