Electronics > Power/Renewable Energy/EV's

Batteries and inverters going off grid with solar.

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Simon:
I've always considered a home solar system. Due to the large expense of a professional system and political instabilities in solar I have shied away from a grid tied system where I sell back the power. The latest political move of our Conservative government to not only remove subsidies from renewables but to tax ecologically generated power has hardened my resolve to go off grid and do it my way rather than get tied in to the politics of the moment and what some company is offering me.

Panels are not a problem. My main thing is that is batteries and inverters. Most items in my house would probably work of DC so a modified sine wave inverter would not be a problem things like my computers and LED lights. I also seem to have separate upstairs and downstairs circuits for both lights and sockets so I can easily distribute the load over smaller inverters. The only thing I can think of for things like my fridge and perhaps microwave is to use a dedicated pure sine wave inverter. That way I spend the money just where it's needed and get a cheaper inverter for other stuff.

The next thing is batteries, these will probably be the first thing I need to buy and I am uncertain as to which voltage to work with. Below are the voltage the more batteries in parallel I will need or the larger single batteries will need to be. If I can work from say 100 V then even 110 amp leisure batteries would easily give me a 5 kW storage capacity.

I also need to give some consideration to wiring up, I cannot go completely off grid as I doubt I would be able to generate enough so I would need it ability to switch over. It ability to switch over automatically would be good unfortunately that would require running wires from the back of my house to the front and modifying my distribution box to put in switchover relays and perhaps devised some sort of under voltage mechanism for the batteries so that the inverters cut out before the batteries are damaged. I expect that to be relatively easy as all I need do is detect the voltage on the batteries and turn inverters off. If the inverters or the battery management are powering a relay relay could hold the system on and once released remain on the grid until I manually switch back over.

I think inverters are my main concern as they will drive the rest of the system but I need to forward plan as I will need to bite the batteries first.

I don't want to spend a fortune on inverters but I don't want to buy cheap rubbish.

bookaboo:
I suggest getting a comfy chair and a marathon of Martin Lortons videos, he covers a lot of what you have asked. Good inverters will cost and I'd stick to pure sine for everything, I would however suggest sharing loads across a few different units, that way when one goes bang you still have some functionality.
I have a similar project in mind for a few years time, I'm going to research the feasibility of having a DC bus for lower loads... I think having an inverter drive a bunch of switchmode powersupplies (for example LED lights, TV, PC, Laptop, Chargers etc) is probably a bit wasteful and adds two levels of complexity. The efficiency of such will of course depend on distance from the battery array.

I wonder if the Tesla powerwall will be a feasible solution any time soon.

Zero999:
Is it really worth it from an economic perspective?

How long will it take for you to see a return on your investment?

What battery life do you expect?

If the batteries need to be replaced before you've got the money you've spent on them back plus inflation, then it's not worth it and you would've been better  off putting the money in the bank. Of course thse calculation isn't easy, since you don't know what the price of energy will be in the future.

From an environmental perspective, you need to take into account the impact of the batteries being manufactured and recycled which is often not done in an environmentally friendly manner.

johansen:
as far as solar goes.. i can buy a pallet of solar panels for about 75 cents a watt.. i do have to pay for shipping however, which means i would need to buy 4 to 10 grand worth of panels before its really worth it.

the problem being where i live, i would probably need 8 kw of panels to provide 8-16 kwh a day of electricity. not worth it at 10 cents a kilowatt hour.
if the panels last 27 years, they would pay for themselves.

in reality its probably worth it when i look at the electricity rates rising faster than inflation.

lead acid batteries will cost you a minimum of 25 to 50 cents per kilowatt hour. --so you'd better have a good sized hot water heater, and a heat pump connected to a 500 gallon tank of water as a thermal buffer.

the jury is out with regard to lithium iron phosphate being cheaper than lead acid.. it may very well be.


as far as inverter cost goes.. its not negligible but its close to zero as far as efficiency goes.
what is not zero is the no load cost. if your 6KW inverter sucks up 100 watts parasitic load thats probably unacceptable.
there are ways to get it well below that.

check the fieldlines.com forum for the recent thread on building your own inverter from relatively cheap power jack boards and winding your own toroidal transformer.

TheElectricChicken:
All solar is brilliant, just go for it. Start with one panel. Start saving right away. By the time you move along from one panel you'll learn a lot and be expert at big systems.

You don't need batteries in the daytime. That is that part of your bill taking a hit. You can eventually set things up so that there are different tariffs on your own home grid. So that storage gets filled, lights get priority and off peak things like a stone polishing tumbler or http://articles.latimes.com/2011/mar/10/food/la-fo-grinding-grain-20110310 get done later when there is too much sunlight.

Don't worry about generators yet unless you have a factory near your house where they throw out wood.  Then you can get one of these or and also can drive but it helps you when there is cloudy weather for a week or you want to have a welding party.

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