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Solar Electric Fundamentals
( Remember this a reference only, prices are used to illustrate the process and will vary)
Basic Principles
Solar panels at present can convert about 10% of this sunlight to electricity. Thus a one meter square solar panel will produce about 100 watts of electricity. We will use $600 for a reference and ease of calculation..
During the day the sun moves across the sky and the sunlight must travel through different amounts of the atmosphere and in doing so loses some of its energy. If the panel does not face directly at the sun it will also produce less energy. If the panel is not on the equator both of these effects are exaggerated. The amount of sunlight also varies with the season and amount of clouds.
On a typical summer day a one meter square solar panel might be exposed to sunlight for 8 hours at 60% of its maximum capability and will thus capture about 500 watt hours of energy (100 watts x 60% x 8 hours = 480 watt-hours).
Of course, a solar panel only produces energy while the sun is shining. If electricity is desired at other times some sort of storage is needed. Most solar electric systems use batteries to store the energy from the solar panels. The size of batteries needed for a particular system depends primarily on the number of days the system must operate without sun
The daily energy capacity of the solar panel must be at least equal to the daily energy usage requirement plus the amount needed to fully recharge the battery after it is discharged during several sunless days. A typical number of sunless days to design for is three and a typical recovery period is five days.
Solar Electric Design Rules of Thumb
Energy Requirements
The total daily energy usage requirement is the sum of the daily energy used by all appliances (fans, lights, electronics, etc). Each individual appliance energy is found by multiplying the appliance power rating by the number of hours it is used.
| Ea = Pa x Ha |
where
Ea = Daily Energy used by an Appliance (watt-hours)
Pa = Power Rating of Appliance (watts)
Ha = Daily Hours of use of Appliance |
Add the Daily Energy used by all Appliances to get Et (Total Daily Energy Used)
Battery Size
All of the solar energy will be generated during the day but most of the energy will be used at other times. The battery is usually sized to store several days of energy so that energy is available during days when the sun does not shine.
| Eb = Ds x Et |
where
Eb = Battery Energy needed (watt-hours)
Ds = Days without Sun
Et = Total Daily Energy Used (Watt-hours) |
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| AHb = Eb / Vb |
where
AHb = Amp Hour Rating of Battery (amp-hour)
Eb = Energy Rating of Battery (watt-Hour)
Vb = Battery voltage (volts) |
Solar Electric Panel Size
The solar panel must be able to supply the daily energy required plus recharge the batteries after several sunless days.
| Ep = Et + Er |
where
Ep = Total Daily Energy from Panel (watt-hours)
Et = Total Daily Energy Used (watt-hours)
Er = Energy Required to Recharge after Sunless Days
(typically 5 days so Er = Eb / 5) |
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| Pp = Ep / (Hs x Eff) |
where
Pp = Panel Rated Power (watts)
Hs = Hours of sunshine per day
Eff = Panel Effectiveness (typically 0.6) |
Usage Requirements Example
We have a particular usage requirement, what size of solar panel and battery storage do we need?
| Energy Usage |
power
(watts) |
usage
(hours/day) |
energy
(watt-hours) |
Lighting4 - 60 watt lights
2 - 10 watt compact fluorescents |
240
20 |
4
6 |
960
120 |
Entertainment
TV
Radio |
60
10 |
4
8 |
240
80 |
Stove
oven
burner |
1500
500 |
1
1 |
1500
500 |
| Refrigerator |
250 |
2 |
500 |
| Water Pump |
250 |
1 |
250 |
| Furnace (fan) |
250 |
4 |
1000 |
| Total Energy Use per day (watts-hours) |
5150 |
| Number of days without sun |
3 |
| Battery capacity required (watt-hours) |
15450 |
|
|
| System voltage (volts) |
12 |
| Amp-hour capacity (15450 / 12) |
1288 |
| Each battery amp-hour capacity (typical) |
225 |
| Number of batteries (1288 / 225) |
6 |
| Battery cost (6 x $250) |
$1200 |
|
|
| Solar Panel Size (to fully charge the batteries in 5 days) |
|
| Daily Energy Requirement (watt-hours) |
5150 |
| Recovery Requirement (watt-hours) (15450/5) |
3090 |
|
|
| Total Energy per day (watt-hours) |
8240 |
|
|
| Hours of sunlight |
8 |
| Panel effectiveness |
60% |
|
|
| Panel Size (8240 / (8 x 0.6)) |
1700 |
| Panel size (10% conversion efficiency, 1000watts/sq.m) |
17 sq meter |
| Panel Cost ($6 per watt) |
$10,200 |
( Remember this a reference only, prices are used to illustrate the process and will vary)
Small Solar Energy System Example
We have a small solar panel, what sort of appliances can we use?
| Panel Size (watts) |
200 |
| Panel Size (2 meter x 1 meter)(6'x3') |
|
| Panel Cost |
$1200 |
| Energy Per Day (8 hours @ 60% effectiveness)(watt-hours) |
960 |
| Battery size (3 day capacity)(watt-hours) |
2880 |
| Number of batteries (12 volt, 225 amp-hour = 2700 watt-hour) |
1 |
| Battery Cost |
$250 |
This will allow the following use per day:
|
power
watts |
usage
hours |
energy
(watt-hours) |
Lighting
4 10 watt compact fluorescents |
40 |
4 |
160 |
Entertainment
TV
Radio |
60
10 |
1
6 |
60
60 |
| Small Refrigerator |
100 |
2 |
200 |
| Water Pump |
250 |
1 |
250 |
| Total Energy Usage (watt-hours) |
730 |
| Daily Energy available for recharging battery (960 - 730) (watt-hours) |
230 |
| Number of days to recharge battery after 3 days without sun |
12 |
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