advice?

System · June 2011

I am interested in purchasing a solar system for a 300 sq ft building to run a deep freezer, small window unit ac and lights 24/7. Any insight on how much a solar and back up system will cost? and what components will i need?

Cariboocoot · June 2011

Re: advice?

Welcome to the forum.

The first thing you need is a Kill-A-Watt meter. If it is at all possible, get some real-world numbers on the power consumption of the things you want to run. You really do need to know what the maximum Watts will be and how many Watt hours per day you'll use. The whole system gets designed around those figures, so having at least some type of "likely target" is necessary.

Two of the things you mention, a freezer and an AC unit, are notorious power hogs. They have big start-up surges because they use AC induction motors running compressors. The freezer may draw less than 200 Watts running, but when it starts it will "blip" over 1000 - too fast for the K-A-W meter to read but fast enough to shut down the inverter. The AC unit will have that problem and consume pretty high on the Watt hours daily, depending on what the temperature is like. That's the third trouble with this type of load; they come on when they need to, and you don't have much control over it (unlike with a light). Hot weather, open the door more = higher power consumption than isn't easily predictable.

So we'll ask the holistic question: what is this for? Off-grid cabin with no utility power available? You may as well be warned in advance; solar electric is expensive.

BB. · June 2011

Re: advice?

It is really difficult to guess what your power needs will be... But as a start:

1 kWH per day freezer
1 kWH per day lights (24x7)
3 kWH per day for small A/C in efficient cabin
=5 kWH per day

Assume >4 hours per day for sun (typically 9 months of the year for reasonably sunny locations), assume 0.52 system efficiency, 2 days of battery storage and 50% maximum discharge:

5,000 WH per day * 1/0.52 system derating * 1/4 hours of sun = 2,403 watts of solar panels

Battery wise, assuming 85% efficient inverter and 24 volt battery bank:

5,000 Watt*Hours * 1/0.85 inverter eff * 2 days of storage * 1/0.50 max battery discharge * 1/24 volt battery = 980 AH @ 24 volts

You can price out the hardware at Northern Arizona Wind & Sun (our host) if you wish.

But depending on what you end up with (and doing the labor yourself), you probably would be in the $15-$20k range minimum.

In the end, solar PV power is expensive. Extreme conservation (appliances, insulation, building design) and measuring your loads before you purchase hardware would be a better start.

I have no idea if my power estimates are even close to what you will really use.

Freezers, A/C, 24x7 loads all add up quickly.

-Bill

ggunn · June 2011

Re: advice?

josh wrote: »

I am interested in purchasing a solar system for a 300 sq ft building to run a deep freezer, small window unit ac and lights 24/7. Any insight on how much a solar and back up system will cost? and what components will i need?

Square footage is nearly irrelevant, power consumption is what counts. This is a backup system for a building that already has grid power? A year's worth of bills is a great place to start if the building is metered independently.

Another important consideration is how long will your system need to supply power when there is a grid outage, worst case. Part of the "worst case" scenario is to assume there is little or no sunlight available during that time.

Another is how often do you anticipate this happening? The less frequent it is, the farther you can tolerate discharging your batteries.

Once you slog through all this (power consumption, days of autonomy, and tolerable depth of battery discharge), you'll determine the size of the battery bank you will need. Then you will size your PV so that it can provide the power the batteries need to recharge them in a reasonable amount of time. THEN you can estimate how much it is going to cost.

We can throw numbers at you, but until you run the analysis on your particular situation, it's nothing but a WAG. Grid tied PV is pretty simple stuff, but when you bring batteries into the picture, things rapidly get complicated and expensive.

arkieoscar · June 2011

Re: advice?

Since the OP said 24/7, I'm going to assume that it's off-grid.
I just put together a system that's close to what he's wanting. It consists of 20, 200 watt panels, 2, XW150/60 charge controllers, 1 XW6048 inverter/charger and 775 amp hour, 48 volt battery mounted on a south facing shingle roof. The owner is going to use a Generac ecodrive 8.5 kw generator from a local dealer that I will set up. Cost will be around $25K, without the generator.

Spiffster · June 2011

Re: advice?

Im very much a noob too but I do know that there are Chest freezers designed just for such applications: http://www.amazon.com/Sundanzer-Solar-Powered-Chest-Freezer-30in-L/dp/B002EP1ALC

This one only requires a "175 Watt solar panel, 250 Ah battery and 15 Amp charge controller". Keeping your food cold is probably at the top of the priority list IMO.

mike95490 · June 2011

Re: advice?

Recently, the Energy Star appliances, have surpased the "speciality solar" chillers, and for a product that is sold by the hundreds of thousands, compared to the "hundreds" sold in the specialty market, with the expensive repairs and parts, my money goes to the energy star appliances.

And just buying a "kit" is only appropiate with enough hours of sunshine, and with restricted content shuffling.

Each application should be looked at carefuly, and reviews of products, read.

System2 · June 2011

Re: advice?

thank you for the replies, thank you bill. when i go back to my cabin im sure i will need more info. i am a beginner i just dont want to waste a bunch of money from having no experience. i have not purchased any appliances yet. i will post more info an details soon thank you.

techntrek · June 2011

Re: advice?

Mini-split air conditioners (also come as heat pumps) are discussed often around here and will most likely be more efficient than a window A/C. Some here run them off-grid.

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