# Supply and consumption in Kilowatts

Registered Users Posts: 20

Can you solve this equation for me please?

Given that I am in Cambodia where there is abundant sunshine.

1)      What is the maximum consumption of the following:

a)      10 x 3 Watt LED lights for 8 hrs

b)      1 x domestic fridge for 24hrs

c)       1 x computer for 8hrs

d)      1 x internet router for 24hrs

Therefore:

How many solar panels of how much wattage?

What size (amp size) controller?

What voltage?

Plus what size inverter (wattage if pure-sign wave)?

How many batteries of what Ah rating?

Tagged:

You really need to measure the power usage of each device--Or at least have some sort of estimate. There can be wide variations in power usage for refrigerators, computers, etc.:

1)      What is the maximum consumption of the following:

a)      10 x 3 Watt LED lights for 8 hrs

• 10 x 3 Watts * 8 hours = 240 Watt*Hours = 0.24 kWH per day

b)      1 x domestic fridge for 24hrs

• 1 x 1.0 kWH per day = 1.0 kWH per day = 1,000 WH per day (typical very good US full size fridge at 365 kWH per year rating)
• 1 x 250 WH per day = 250 WH per day = 0.25 kWH per day (chest freezer converted to refrigerator--Best you can do)
• Example: 1 x 120 Watt average load * 0.50 duty cycle (motor on 50% of time) * 24 hours per day = 1,440 Watt*Hours per day (estimated based on measured power and duty cycle)

c)       1 x computer for 8hrs

• Computers--You can run from ~20-30 Watts for typical laptop to ~250 Watts for desk top + monitor
• 30 Watts * 8 hours per day = 240 WH per day (laptop)
• 250 Watts * 8 hours per day = 2,000 WH per day (desktop)

d)      1 x internet router for 24hrs

• For me, I need both a Cable Modem and a router--Probably each uses 10-25 Watts:
• (15 + 15 ) watts (est.) * 24 hours per day = 720 Watt*Hours per 24 hours (if not turned off when computer is)

Therefore:

Add everything up... My suggestion is to keep the energy usage to ~3.3 kWH per day as a "reasonably" cost effective system that should be large enough to live a "near normal" electric lifestyle (conservation is your friend here).

1 kWH per day refrigerator

0.24 kWH per day Lighting

0.24 kWH per day computer

0.72 kWH per day modem+router (you can probably do better):

==============================================
2.2 kWH per day above... Leaves ~1.1 kWH per day for charging cell phone, water pumping, washing machine, etc.

Next, size the battery bank--I will assume 3.3 kWH per day--But you can use your own numbers (more or less energy). Pick 24 volts (experience). 1-3 days storage, pick 2 days. 50% maximum discharge:
• 3,300 Watt*hours per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank
I suggest that you keep the battery bank to 800 AH or less, if possible. If your battery bank is larger, then go to the next higher voltage.

Next, sizing the solar array. Two calculations. One is based on 5% to 13% charging current, the second based on hours of sun per day. First charging current:
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,218 Watt array minimum (part time/seasonal use)
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,437 Watt array nominal (full time living off grid)
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,168 Watt array typical "cost effective maximum"
Continued on next page
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Next, sizing based on hours per day of sun and load. Estimated hours of sun per day:

### Phnom PenhAverage Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 78° angle from horizontal:
(For best year-round performance)
 Jan Feb Mar Apr May Jun 6.16 6.00 5.57 5.21 4.96 4.58 Jul Aug Sep Oct Nov Dec 4.36 4.01 4.22 4.60 5.40 5.86
If you do not want to use a generator very much, the minimum sun per day is 4.01 hours per day for August:
• 3,300 WH per day * 1/0.52 end to end AC system eff * 1/4.01 hours of sun per day = 1,583 Watt array minimum (based on loads and sun)
I suggest that batteries are expensive and have a "short life" and solar panels have never been cheaper and have a long life--So, if possible, you should add as much solar panel as your budget would allow... But somewhere in the 1,583 to 2,437 Watt range minimum would be a good start for full time off grid living. Of course, your actual hours of sun will vary with weather conditions--You should plan on your "base loads" using ~66% to 75% of the predicted output--And on clear/sunny days you can run more loads (pumping, computer, etc.).

How large of AC inverter--That really depends on both your loads and your battery bank size. For a typical Refrigerator, a 1,200 to 1,500 Watt (good quality) AC inverter minimum.

And based on the battery bank, a "workable" AC inverter would be (based on flooded cell lead acid battery bank):
• 647 AH * 24 volts * 0.85 AC inverter eff * 1/20 hour discharge rate = 660 Watt average load (5 hours per night, 2 nights, 50% maximum discharge)
• 647 AH * 24 volts * 0.85 AC inverter eff * 1/8 hour discharge rate = 1,650 Watt inverter maximum continuous discharge
• 647 AH * 24 volts * 0.85 AC inverter eff * 1/5 hour discharge rate = 2,640 Watt inverter max continuous discharge (minutes to an hour or so)
• 647 AH * 24 volts * 0.85 AC inverter eff * 1/2.5 hour discharge rate = 5,280 Watt inverter max surge (seconds to minute--Remember that most good quality AC inverters will support 2x rated output for surge load)
So--realistically, you would need (guessing) a minimum of 1,500 Watt AC inverter--And not much more than 2,000 Watt based on a 647 AH @ 24 volt flooded cell lead acid battery bank.

Of course, my numbers are based on an estimated usage of 3.3 kWH per day system. Your actual loads may be more or less than my suggested starting point.

Getting a Kill-a-Watt type meter and/or AC+DC Current Clamp DMM can help you get better measurements:

http://www.reuk.co.uk/Kill-a-Watt.htm (230 VAC 50 Hz version)
http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P (Sears "good enough" \$60 clamp meter)

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20
Thanks so much Bill, you certainly seem to know your stuff!
The reason why I posed the question is in reference to providing power in Cambodia.
I wanted to make a model which showed the typical usage for a small household.
Here in Cambodia the base rate for power is 2 dollars a KW
I want to take the cost of the equipment needed, (including replacement of batteries and upgrading of controllers every 2 years) during the 18 years lifespan of the panels and come up with a cost per kilowatt.
My target figure is 1.5 dollars
Can this be achieved?
Prices here are as follows:
Panels are 1 dollar a watt
100 amp hour Narada deep cycle battery is 175 dollar each
1500 pure sign wave inverter about 300 dollars
Bulb plus holder about 2 dollars each
Plus wiring, switch gear, breakers and fuses estimated allowance is 100 dollars
I know I am asking for an algorithm to be written but I am interested to know the economics
You can figure out the numbers pretty easily... Something like (18 year life, assume 5 year batteries, 5 years controller + AC inverter and that people only use, on average 65% of the systems possible power output over 1 year);

[\$2,437 panels + 4x 18 batteries * \$175 each * 4x \$200 charge controllers + 4x \$300 AC inverters + \$100 misc] / [3.3 WH per day * 0.65 ave usage * 365 days per year * 18 years] = \$1.21 per kWH over life of system

Batteries--I would like to see you use better/different/larger batteries (at least for 3.3 kWh per day system). Charge controllers and AC inverters (at least good ones) should last 10+ years, with some early life failures in 5+ years.

In the end, batteries are the weak part of any off grid power system... It is very easy to kill batteries with improper operation/maintenance (you can kill batteries in weeks or months pretty easily).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Via PM from Juliano... Was \$0.20 per kWH for power, not \$2.00 per kWH (currency conversion mistake)...

With the equation, you can see where your expenses are and what can be changed to (hopefully) reduce the costs... But, in general, you will be paying much more for off grid power than for utility power.

In the US, Grid Tied solar (panel+GT inverter connected directly to the grid) is as cheap as, and even less than utility power (can be less than \$0.10 per kWH). Notice, that there is no Battery, no backup/off grid power, and no extra stuff (like DC charge controllers, battery chargers, etc.).

However, GT Solar really only makes sense when subsidized by utility customers. In the US, we are presently on a path where subsidies for home based GT solar is slowly going away (for example, they just doubled my minimum metering charge--Other companies are dramatically reducing payouts for GT Solar power and or raising minimum billing fees).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20
This is distressing, presumably the fossil fuel burners have got something to do with blocking the path.
However I am not deterred from providing power to rural communities in Cambodia. These people are completely off the grid and in much need of good quality lighting for a start.
I am using Tweet as a mediam for networking and I have a facebook page called oursolarpower it will have in brackets (Siem Reap)
Perhaps you would be so kind as to follow me on tweet@Solar4Cambodia and or 'like' my page and become a FB friend.
Social media will help me to expose my crusade, thanks.
• Solar Expert Posts: 1,220 ✭✭✭✭
More bad news. While i dont doubt that people build systems on the cheap, a 3kWh/d system, that is to be reliable, and capable of replacing the grid long term, will not use a 300 dollar inverter, or 100Ah batterys. It just wont. Bang for buck batterys are at least 200-400Ah. For inverters \$1000+. They will also have at least \$250 worth of BOS parts. Around about \$10K should cover it. As far as your quest goes, you can try to scale it down a bit to something like a midnite kid + a pair of GC2s. Also dont forget these systems are not plug and play. A reasonable amount of expertise to both install and operate.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Registered Users Posts: 20