is this even possible?

phaidinho

Hey all, I'm new to the forum so a bit of relevant background info. I live in one of the sunniest parts of Greece (300+ days full sunlight) and I have a large south oriented roof.
I tried to get a 20kw grid tie-in deal with my utility a few years ago but they wanted the unanimous consent of all 16 apartment owners in the building so that was impossible. Now I'm looking at something much more small scale.
At this point, all I want is to save on my electricity bill in the daytime. I want to be able to run one AC unit, an energy saving but decent sized fridge and my pc (especially in the summer) off a pv system in the day time and switch over to the grid at night. My ambition is to do this with a very small battery, I would like to produce "live" more or less what my AC, fridge and pc use with panels and store maybe a 1hour supply of it. The idea is to recharge the battery faster than it drains during peak solar times. Does this sound like a good plan? How little money could I do it with?
is there a way to automatically switch back to grid power once the battery is drained?

The reason this appeals to me is that the times when i run the highest loads are in the summer during the daytime, when I have the ac running full blast for 6 hours straight. There's always plenty of light, so I don't see the need to store the energy, I just want to use it there and then.

BB.

Re: is this even possible?

Even if you can get a 5-10 kW grid tied system, that would be a better setup (cost much less, no batteries, smaller solar array for same kWH per day than an off grid system with battery bank).

In the US, people can save money vs a utility power bill if they have fairly expensive electricity (i.e., somebody that pays $0.6 to $0.15 per kWH will usually not save money with GT power. In areas where we pay $0.30 to $050 per kWH--especially for Time of Use billing with high summer afternoon rates, GT solar can really save a lot of money).

And looking at Off Grid power, it runs (very roughly) 10x the cost of utility power (i.e., roughly $1.00 to $2.00+ per kWH when you take all the costs of Off Grid solar into account--Batteries are expensive and need to be replaced every ~5-8 years or so). It is usually almost impossible to "save money" with off grid solar power if you already have access to "reasonably" price utility power.

An off grid system is less efficient and the costs/place to keep the battery bank, etc. make Off Grid systems kind of counter to what you need. The only drawbacks for GT systems are you need utility approval and GT systems do not work if the utility power has failed (storm, brownouts/blackouts, etc.).

My first suggest would be conservation. Have you looked into getting the most efficient air conditioning system you can (some of the Mini-Split A/C systems are very efficient too, and you could limit cooling to just a couple of rooms).

Other conservation would be to install double pane windows with "Low-E glass" (low emissivity glass to reflect heat back outside, and keep it inside during the winter), insulation in walls/ceiling, etc.). And replace all lighting with LED (or CFL) bulbs (you use less power, so save money. Plus, since you use less power for lighting, you waste less heat that your A/C system needs to move outside, so save money again). Other energy efficient appliances in conditioned space (very efficient refrigerator, laptop computer vs desktop computer, new LED type TV's, etc.) can also save on initial electrical costs and waste heat too.

In some cases, people can save 1/2 of their bill (or even more) if they have never attempted "extreme" conservation before. And, this is usually a better "investment" of your money vs GT or Off Grid solar power system.

-Bill

BB.

Re: is this even possible?

I should add, if you have an "all electric" apartment, you can heat hot water and your apartment during the winter using Heat Pump systems.

Heat Pump heating/cooling systems (basically an A/C system that runs backwards during the winter, and available in Mini-Split systems too) are ~2x or more efficient than normal resistance type electric heaters.

And for hot water, you can take the heat from an AC system to heat water (de-superheater--basically circulate hot refrigerant through your hot water tank). Or you can now find heat pumps made to connect to a water heater, or even electric heat pump based water heaters (with backup resistance heaters for very cold weather, high water usage when guests come by).

And the neat thing about heat pump water heaters is that, besides using 1/2 the electricity, they output cold/dry air as their "waste" product--So they help keep the home cool (if your water heater is installed inside the unit). Electric heat pump water heaters are getting to the point where they cost less than natural gas/fuel oil water heaters to run too (in many cases).

-Bill

Cariboocoot

Re: is this even possible?

Welcome to the forum.

Technically it is possible to produce electricity faster than you use it. In the off-grid world we do it all the time as it's the only way to keep things running while charging the batteries.

However, you will find that A/C and refrigerators consume a lot of electricity. Surprisingly, so do PC's if they are desktop. Typical A/C window unit when running: 500 Watts. Refrigerator: 150 Watts. PC can be the same. The latter will draw all the time, whereas the first two will cycle. As such you'd need over 900 Watts of panel (depending on exact loads) to handle all three at once. Then when all three are not running at once much of that panel is "wasted" because there's no place for its power potential to go: a bad investment.

Look at it like this (simplified math leaving out losses for demonstration purposes):
The 500 Watt A/C cycling 1/2 the time over 6 hours = 1500 Watt hours.
The refrigerator using 150 Watts cycling 1/3 of the time = 300 Watt hours.
150 Watts of PC on all the time = 900 Watt hours.
Total consumption in 6 hours = 2700 Watt hours.
1000 Watt array in that time frame produces 6000 Watt hours.
Wasted potential: 3300 Watt hours.

This is why OG systems are planned around kW hours used daily to calculate battery capacity and then the array sized to recharge that battery capacity in the sunlight hours available. If we apply this logic to the 6 hours use we get:

2700 Watt hours total / 24 Volt system = 112.5 Amp hours used, needing a minimum 225 Amp hour battery @ 24 Volts.

Figuring the array based on the Icarus formula:
2700 Watt hours with 52% over-all efficiency and 6 hours of equivalent good sun = 865 Watt array.

Checking the recharge ability of that array we find it would handle 277 Amp hours @ 24 Volts (865 Watts * 77% efficiency / 24 Volts = 27.7 Amps peak rate).

So it would work. But, where are you going to put an 865 Watt array?

NorthGuy

Re: is this even possible?

Whether or not you can save money on your electricity bill depends on two factors:

- cost per kWh that you pay to your power company.

- cost of the system, including materials, installation etc. If you install by yourself, it'll be cheaper, but it is not very easy to do.

It is impossible to tell if it's a good idea because prices are different in different countries.

In most places, using grid electricity is much cheaper, so you're unlikely to save any money with solar. However, if you anticipate huge increases in electricity rates, solar may make sense.

Photowhit

Re: is this even possible?

Well your in a condominium of some sort...

Other options are to plant shade trees to shade your residence (I bet your 3 floors up...) and also being sure you AC compressor unit is in the shade, if it's in the sun it works much harder.

Starting from scratch 6 years ago, to make a place I could AC from solar, I...

Built a small place (less area to air condition)
Put in lots of insulation, 6" in the walls
Built it in the shade
I even put a 3 1/2" of foam insulation over one of the windows
Purchased an energy efficient AC

I understand you have a fixed place already, but could you limit the amount of area your air conditioning? add insulation? plant shade trees, insulate some windows? Purchase a more energy efficient AC?

phaidinho

Re: is this even possible?

ok, a note about conservation, I already have double glazed windows, good insulation, energy efficient appliances, LEDs etc. I also have a solar hot water boiler which covers 95% of my needs year round. My power consumption is already low, not many savings to make there.
this is my one ac that I run in the day
http://www.daikin.com/global_ac/products/residential/new_wall/energy.html and I have a very efficient fridge as well. I will make shade for the outdoor component, thanks for that advice.

also, there's no way i can put my solar on the grid, I own the penthouse and the roof but every household in my building would have to sign a document approving it and this is just not possible, too many crazy/greedy people.

So, I need a 865 watt array, that's 4 220w panels which is no problem at all (I have room for maybe 100). The cost, on the other hand, seems high. I'm looking to save on a power bill that is only over 40euros a month 3-4 months of the year, so spending, say, 5000 now to save 500 a year doesn't make much sense.
I see what you mean, cariboocoot, about wasted power, the main reason I don't care about it is that once the sun goes down all I have running is a fridge and maybe a TV so I don't really use a lot of electricity until the sun comes out again and sends temps to 40C, at which point I'm generating it anyway. I'm just trying to bring the cost of the system down by minimizing battery size.
What kind of price am I looking at for 800watts of panels and a battery that lasts one hour?

Thanks for all the quick and informed answers guys.

BB.

Re: is this even possible?

OK, sounds like you have a good location for solar. And starting with your planned solar array... We can suggest a size of system using the standard rules of thumbs and adjust from there.

Assume 865 watt solar array, 4 hours of sun (9+ months a year) minimum and ~6 hours of sun per day for sunny summer day (southern Greece has much more sun than northern Greece).... Such a system would produce roughly (rough guesses):

• 865 * 0.52 system efficiency * 4 hours of sun minimum = 1,799 kWH per day typical minimum for sunny day of AC power
• 865 * 0.52 system efficiency * 6 hours of sun minimum = 2,699 kWH per day typical for sunny summer day

Using our rules of thumb for battery charging (~5% to 13% rate of charge), with 10% to 13% a good "balance", particularly since you want to maximize your day time production:

• 865 Watt array * 0.77 panel+controller eff * 1/0.10 rate of charge * 1/24 volt battery bank = 277 AH @ 24 volt battery bank nominal
• 865 Watt array * 0.77 panel+controller eff * 1/0.13 rate of charge * 1/24 volt battery bank = 213 AH @ 24 volt battery bank suggested minimum battery bank

In general, I would suggest that such a system would power a typical refrigerator + some lights... Or possibly your AC during the day if you assume running on low (guess 300 watts) at 10 hours per day would be ~3 kWH per day (I could not find typical power usage numbers for your listed AC system).

Is this what you were looking for? Did I make good guesses about your sun and power needs? Do you have some sort of Kill-a-Watt type meter to measure the energy used by your major appliances per day?

My guess is you would need 2-3x larger system (minimum) to take the home off grid. And you would still need some sort of backup power source for cloudy days. A small generator (such as a Honda eu2000i class 1.6 kW) would be enough for a small system. But you would need to haul/store fuel, or convert the genset to natural gas/propane (if available).

Or, keep your utility connection for poor weather--Which can be an issue. In many places, the utilities are seeing that solar power is going to cut into their profits. So, they are doing things that make solar less competitive. For example, they are reducing the kWH pricing but increasing monthly connection fees (makes conservation and various solar power systems less competitive if utility power is still connected)--High monthly bill ($25-$50 per month just for utility connection, even if no power is used at all.

-Bill