First, you will need to calculate how much energy you will need... Because your system uses batteries instead of the utility to backup your power, you cannot take the excess energy generated during the summer and "bank" it for winter. You will only be able to bank a few days of energy with batteries.
So, if 500 watts per hour is your average maximum and it occurs during the winter you will have:
30 days x 24 hours x .5kW = 360 kWhrs per month.
360 kWhrs/month sounds like a reasonable amount of energy for here in the US assuming that you don't use a lot of electric heat or cooling. For example, I can get to a bit above 200 kWhrs per month with 2 adults, 2 young children, gas cooking/hot water, city water--basically just for lights, mostly laptop computer, refrigerator, freezer a couple small fans and cloths washer.
For the US, we have a map that gives us information on how much energy we can get from solar panels based on time of year and location.
US Solar Radiation Map Hopefully I can find one for Malta...
Looking around, found a short Malta university
article on solar in Malta. There are a couple of names you can contact that may help you. The article says that flat roofs are common so you may need to build something to hold the panels at a ~36 degree angle (your latitude) or at +15 more degrees (~50 degrees) if you want to maximize your winter power (summer maximum would be at -15 degrees). You can also accept less power per panel and mount them flat if that works better for you. The US information allows you to account for the exact mounting arrangement.
Also, before proceeding too far, you may also want to look at solar hot water too... In my case, hot water probably accounts for a good portion of my gas costs (excluding winter heating). It may be a better match for you if your utility does not support "net metering"...
Here is a page with
general European links (plus one to Malta).
Here looks like a good
Solar Irradiation Map for Europe...
So, for:
Location: 35°51'48" North, 14°30'52" East, Elevation: 3 m a.s.l,
Nearest city: Valletta, Malta (0 km away)
Optimal inclination angle is: 32 degrees
Month
Irradiation at inclination: (Wh/m2/day) 40 deg.
Jan 4269
Feb 5205
Mar 5564
Apr 5786
May 5875
Jun 6048
Jul 6101
Aug 6597
Sep 6156
Oct 5475
Nov 4484
Dec 3866
Year 5452
There is a link that you can follow on this page that calculates the monthly estimated energy usage by your PV solar system. I believe, that you can use 0.95 (95% efficient) for a grid tied system or down to 0.75-0.80 for a battery interfaced system (or even less). I used 75% and 45 degrees facing south.
You can decide where your optimum point for spending money will be. At ~40 degrees, you will get almost 60% more power at summer peak than at the winter minimums...
Of course, this calculator does not account for exact cell types, or perhaps, even temperature... But it is a good start for you.
It looks like a 3kW peak system will generate just around ~300 kWhrs per Month for you
during winter months. A one year average just over 360 kWhr and for the summer months, around 390 kWhrs.
Now for some links to various sources of hardware... Some will be manufacturer's sites which may not have prices, others, just a link to a solar site that has OK retail pricing. You may be able to save 25%-50% if you get a good wholesale price or can buy some used panels.
Here is one US manufacturer
Xantrex. Happens to be the one I have installed (Grid Tie). A 3 kW peak GT inverter is something like $2,500 list (this will be your cheapest cost to install--anything else will be more because you will need charge controllers, inverters, some sort of grid interface, and of course batteries)... But you are looking for a battery based system--Xantrex has those too.
At this point, I am not even going to recommend something too you from this, or any other line... You will have to decide if you want to be 100% independent from the Grid, use the Grid as a manual backup, or something else.
Just for the sake of discussion, lets choose this unit.
SW3048E a 3 kW, 48 VDC 230 VAC at 50 Hz unit with CE mark. Also has a generator input (perhaps, you can plug your utility power into this port for "backup".
This place seems to be pretty cheap at $2,600 (list is around $3,500???). You will need a maximum of 88 amps at 44-66 VDC solar panels to power this unit. That is a minimum of ~3,900 watts of solar panels.
This same site also offers a bunch of solar panels... They seem to run, roughly $4.40 to $4.60 USD per Watt. Call it $4.50 per watt, and 3.9 kW will cost you about $18,000. Remember, even the best panels degrade at 0.7% per year and after 30 years will only be running at 80% of what they did new... So eventually you would need enough room for 25% more panels to keep the same amount of power (or ~$22,000 worth of panels at todays prices). By the way, while electronics are going down in price, PV panels and copper are going up in price because of the increased costs in energy and more people buying solar panels.
Next, batteries. Say you want 12 kWhrs per day of energy with 3 days of storage plus you only want to cycle down by 50% over those three days... And your inverter/power system is 80% efficient.
You would need 12kWhr x 3 x (1/50%) x 1/80%) =307.5 kWhrs of storage. Big fix needed here… It should have been =90 kWhrs of batteries… Fix everything below.
This is the equivalent in size of ~90 average, full size car batteries. Assume that this will give you an average life of 7 years with good batteries and nothing going wrong (you may easily get between 5 years and 10 or more years of useful life). Long life batteries should be in the area of 12-15 years.
Lets go to the same solar site and look at
their batteries.
Trojan batteries are a good mid-range quality maker. And lets look at there biggest
420 AH battery. 420 AH at 6 Volts = 2.52 kWhrs or 20 kWhrs per 8 battery string. 5 strings would give you 100 kWhrs of storage. That would be 40 batteries at $238 x 40 = $9,520 (list). If you only want 1 day storage, you probably can drop to 1/3 (2/5's is the closest here) of the number of batteries and only pay $3,808. You won't have the 3 day storage, and the batteries may not last quite as long because of deeper cycling, but 16 batteries are going to be much easier to maintain than 40 batteries.
We can also run the same calculation on a different brand of batteries that should have twice the life as the Trojans. Surrette, pick one of their
larger batteries. Lets assume a 100 hour rate which is 9.25 kWhr capacity (8 volts). A string of six gives us = 55.5 kWhrs at $899 x 6 = $5,394. 2 strings would give us 111 kWhrs at $10,788.
It looks like the better batteries are worth it... They both cost about the same price in kWhrs (Trojan was 20 hr rate, Surrette was at 100 hr rate; at a 20 hour rate the Surrette price would be ~1/3 more or the 3 day Trojans 1/4 less--so this is not full apples to apples comparison) because of the longer life for the more expensive battery.
And, those Surrette batteries are heavy 193 kg (424 lbs) each, wet. Or 2.5 short tons of batteries.
At this point, your system would cost (list):
$2,600 3kW inverter/charger
$18-$22,000 for panels (20% loss for life, 20% loss for inverter)
$3,800-$9,500 for batteries. (20% loss for batteries/inverter)
Or somewhere in the neighborhood of $30,000 +/- for the major parts… Excluding copper wiring, fuses, bus bars, PV panel mounts, battery building, labor, permits, shipping costs and any local import duties.
END of fix for stupid math error…
Not accounted for. Wiring, mounts, losses due to shading (especially during winter), and labor.
As you can see, a huge part of the price is for panels and batteries… If you can reduce your power usage from 800 to 500, or even to less than 300 watts per hour, you can save a huge amount of money on the panels and batteries.
I really liked the idea of battery backup solar power for my area, but it just did not make any sense. If you can convince your utility (maybe with the help of your local university) to give you some type of Grid Tie option, your costs will be cut in half. Also, you will have almost zero maintenance for the next 30+ years…
I have to go now... But let me know if this is what you are looking for.
Sincerely,
Bill
PS Fixed a bad math error above and adjusted the costs below to account for the new values. -BB