November 9, 2008
Application Note
Recommended Design Practices of Basic Off-Grid Solar Photovoltaic (PV) Systems
Jakub Mazur – B.S. EE, Michigan State University
Abstract:
This document will outline design practices that should be taken into consideration
while designing a basic “off-grid” system, particularly for deployment into rural areas.
This includes the following topics: determining load power consumption, solar
insulation, solar panel array size, battery array size and voltage. This document will use
examples of calculations used by Michigan State University’s Team 2 senior design
project for deployment near Arusha, TZ.
Keywords: Solar System Setup, Photovoltaic power
Application Note: Basic Off-Grid PV System Design
November 2008
Table of Contents
Introduction
3
Block Diagram
4
Section 1: Determining Photovoltaic Array Size
5

Load Power Consumption
5

Determining Solar Insolation Levels
6
Section 2: Sizing Battery Array
8
Section 3: Wire Sizing and Connections
10
Section 4: DC-AC Inverter
12
Section 5: Charge Controller
12
Conclusion
14
References
15
Appendix 1: Solar Panel Selection
16
Appendix 2: Battery Selection
17
Appendix 3: Inverter Selection
18
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Application Note: Basic Off-Grid PV System Design
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Introduction:
There are many types of solar systems but most can be categorized into a variation of
the following: A “grid-tie” system where there are no batteries and the power grid
provides back-up power. A hybrid “grid-tie” system where the power grid provides backup for the solar panels and batteries act as a backup for the grid. In cases where there is
no access to grid power an “off-grid” system is used, in which the battery bank stores
and provides all the energy for the system without a backup. Since this is generally the
case in under-developed areas this will be the system discussed here. There are also
systems with generators as backups, they are comparable to “grid-tie” systems and will
also be omitted from discussion here.
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Block Diagram of Team 2 PV System
Figure 1
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Application Note: Basic Off-Grid PV System Design
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Section 1 - Determining Size of Photovoltaic Panel Array:
There are several steps involved in sizing the PV array, determining load power
consumption, accounting for losses and dividing by solar insolation levels for
deployment region.
Determining Load Power Consumption:
The first step is to determine how much power the total system load will draw. Power is
measured in Watts:
P = V x I (Joule’s Law)
However, the power rating is more useful when looked at in terms of time, this is indeed
how electric companies charge consumers. For example a 200Watt light bulb running for
24 hours uses 4.8 KWh.
200Watts x 24hrs = 4800 Watt-Hours or 4.8 KWh
A list of all devices connected to the system should be made with their appropriate
power draw available from specifications sheets or better yet, actual measurements.
Power Measurements (11/11/2008)
Component
Lenovo S10 (Idle)
Lenovo S10 (Full Processor + Hard Drive)
Lenovo S10 (30% Duty Cycle)
Satellite Router (Idle)
Satellite Router (Busty)
Est. Typ. Satellite (30% Duty Cycle)
17” LCD Screen x 4
Total
Power (Watts)
91
116.76
98.73
53.8
72.5
59.41
(20*4) = 80
238.14 Watts
Figure 2
* Headsets, Keyboards & Mice are currently not included in calculations because the
team is not in possession of them and their power consumption should be minimal.
Since these devices are designed to plug into AC power, a DC-AC power inverter is
needed. The power inverter ideally operates at 90% efficiency. Therefore the maximum
inverter draw from batteries is:
238 Watts / 0.90 = 264.60 Watts
This system power draw is then multiplied by the amount of hours per day that it will
operate.
264.60 Watts * 8hrs/day = 2116.80 Watt Hrs/day
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Application Note: Basic Off-Grid PV System Design
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To compensate for system losses during battery charge/discharge cycles the total system
power consumption is multiplied by a 20% compensation factor (Sunwize).
2115.52 Watt Hrs/day * 1.2 = 2540.16 Watt Hrs/day
Determining Solar Insolation Levels:
In order to determine a good approximation of how much power the PV panels will
output, solar insolation levels need to be considered. Solar insolation is the amount of
incoming solar radiation incident on a surface, for PV applications the surface of interest
is the earth’s surface. The values of solar insolation are commonly expressed in
kWh/m2/day, which is the amount of solar energy that strikes a square meter of the
earth's surface in a single day. This is commonly referred to as a “Sun-Hour-Day”. The
amount of insolation received at the surface of the Earth is controlled by the angle of the
sun, the state of the atmosphere, altitude, and geographic location.
World Insolation Map, Figure 3
This map divides the world into six solar performance regions based on winter peak sun
hours.
It is important to keep this in mind when designing the system because as seen
below in Figure 4, during the winter you have a much smaller ‘Solar Window’. Worst
case scenarios should be calculated as it is better to have extra energy in the summer
than not enough in the winter. Therefore the “Sun-Hour-Day” values for December
(since December days are shortest) are generally used.
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Application Note: Basic Off-Grid PV System Design
November 2008
Figure 4
Solar Insolation Levels for Arusha, the prototype deployment area are seen below in
Figure 5.
Figure 5 (Hankins)
The compensated total power consumption per day value calculated above is then
divided by the solar insolation values for given deployment region to determine
minimum PV panel array power output requirement.
2540.16 Watt Hrs/day / 5.5 = 461.84 Watts
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Application Note: Basic Off-Grid PV System Design
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Section 2 - Sizing Battery Array
Nearly all large rechargeable batteries in common use are Lead-Acid type, although
there are three variations, flooded, gelled electrolyte (“Gell Cells”) and absorbed glass
matt (“AGM”). Flooded is the oldest and cheapest technology used but can be
dangerous, in case of a malfunction acid can spill. Gell Cells contain acid that has been
"gelled" by the addition of Silica Gel, turning the acid into a solid mass, therefore even if
the battery where cracked open, no acid would spill. Gell batteries need to be charged at
a slower rate (capacity / 20) but this is not a concern in the PV setup as the panels will
not be outputting nearly this much current. AGM batteries are the newest technology
and have all the advantages of Gell Cells without the charging limitations.
All deep cycle batteries are rated in amp-hours. An amp-hour (Amps x Hours) is one amp
for one hour, or 10 amps for 1/10 of an hour and so forth. The accepted AH rating time
period for batteries used in solar electric and backup power systems is the "20 hour
rate". This means that it is discharged down to 10.5 volts over a 20 hour period while the
total amp-hours it supplies is measured (Windsun).
The compensated total power consumption per day value is used again to calculate
minimum battery array size.
2540.16 Watt Hrs/12 Volts = 211.68 AmpHrs/day
Number of days of autonomy to support: 1 (8hrs)
211.68 * 1 = 211.68 AmpHrs
“Battery life [deep cycle] is directly related to how deep the battery is cycled each time. If
a battery is discharged to 50% every day, it will last about twice as long as if it is cycled
to 80% DOD [depth of discharge]. If cycled only 10% DOD, it will last about 5 times as
long as one cycled to 50%. Obviously, there are some practical limitations on this - you
don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD.
The most practical number to use is 50% DOD on a regular basis (Windsun).”
Depth of discharge for battery: 0.5
211.68 / 0.5 = 423.6 AmpHrs
This means that after 8 hrs of use without sun the battery will be discharged to 50%
8 Hrs of autonomy and battery depth of discharge at 0.80 (Half the life-span of 0.50):
264.60 Amp Hrs
6 Hrs of autonomy and battery depth of discharge at 0.50:
264.6 Watts * 6 Hrs = 1587.6 Watt Hrs / day * 1.2 = 1905.12 Watt Hrs / day
1905.12 Watt Hrs/12 Volts = 158.76 AmpHrs
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142.8 AmpHrs / 0.5 = 317.52 Amp Hrs
4 Hrs of autonomy and depth of discharge at 0.50:
238 Watts * 4 Hrs = 1058.4 Watt Hrs / day * 1.2 = 1270.08 Watt Hrs / day
1270.08 Watt Hrs / 12 Volts = 105.84 Amp Hrs
105.84 Amp Hrs / 0.5 = 211.68 Amp Hrs
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Section 3 - Wire Sizing and Connections:
Another important consideration for the system is the electrical wiring. All wiring needs
to safely accommodate the amount of current draw of the system with an acceptable
amount of losses. In a DC system losses quickly become an issue. This is especially a
concern PV systems as they can only handle a small voltage drop as there must be
enough potential to charge the battery array, and of course it is good practice to keep
energy loss sourced from the sun to a minimum. Generally a 3% drop between PV array
and batteries is acceptable. Also, “any type of connection bigger than AWG 10 should
have a proper compression connector, with appropriate joint compound and
preparation. This does require special tools and dies. Otherwise you are running the risk
of burning up your connections if you get any kind of heavy current flowing.
(SolarForum)”
Losses associated with transmission of DC power:
CM = (22.2 x A x D)/VD
CM = Circular Mills In Copper
A = current in amps
D = one-way cable distance in feet
VD = Voltage Drop
22.2 = Constant for Copper
For wiring from the PV panels to charge controller the maximum PV short circuit current
specification (from PV data sheet) is used.
Maximum Solar Power Output:
24 Volt Systems:
Configuration
6 x PW080
3 x ST-165
4 x KY125
Max Current Out (Amps)
3 x (5.14A-ISC) = 15.42
20.63
20.83
Figure 6
12 Volt Systems:
Configuration
6 x 80 Watt
3 x PW165
Max Current Out (Amps)
6*(5.14A-ISC) = 30.84
41.25
Figure 7
Using the loss equation above the following result was obtained for the selected system:
Distance: 50ft
Voltage Drop: 0.72
Current: 15.42 Amps
Circular Mills: 23772.5
AWG: 6
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Application Note: Basic Off-Grid PV System Design
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Inverter to Battery Wiring
For current level estimates from the battery to inverter maximum power draw levels are
used although this distance is generally short and maximum available wire gauge is
recommended. This is also due to the fact that the system will encounter surge currents
as various components are ‘turned on’. Since the system used as an example here is not
continuously running and is to be turned off every night and back on in the morning this
was a serious issue that needed to be tested. (Refer to figure X).
Maximum Power Draw:
Component
Lenovo S10 (Full Processor + Hard Drive)
Satellite Router (Busty)
17” LCD Screen x 4
Total
Power (Watts)
116.76
72.5
(20*4) = 80
270 Watts
Figure 8
Assuming the inverter that will be sourced in deployment area is operating at 90%
efficiency:
270 Watts = 300 Watts x 90%
Maximum current draw in 12 Volt system = 300 Watts / 12 Volts = 25 Amps
Maximum current draw in 24 Volt system = 300 Watts / 24 Volts = 12.5 Amps
Power-Up DC Current Draw
35
30
25
Current (Amps)
20
15
10
5
0
1
1570 3139 4708 6277 7846 9415 10984 12553 14122 15691 17260 18829 20398 21967 23536 25105 26674 28243 29812 31381
-5
Samples
Figure 9
Figure 9 shows DC current draw as measured during power-up of Lenovo S10
Workstation (custom configuration) and L193p Monitor. Although the system is only
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Application Note: Basic Off-Grid PV System Design
November 2008
drawing 5 amps while running the surge current spikes are clearly visible. This is indeed
one of the reasons why proper electrical connections are crucial.
Section 4 - DC-AC Inverter:
Since the computer and monitors are designed to plug into AC power and accessory
plugs for phone charging are a project specification an inverter is necessary. There are
two types of inverters, pure sine wave and modified sine. Most devices will work from
modified sine, this is what common uninterrupted power supplies provide and what was
selected for this system. It is important to make sure that the inverter is rated to provide
enough power for everything running off of it.
Section 5 - Charge Controller:
The charge controller chosen for this system is the Outback Power FlexMax 60. This
decision was based on versatility, efficiency, robustness, and availability in deployment
area. The Outback can accept a wide range of voltage inputs as well as various battery
arrays, this was important for this specific system as ultimately whatever solar panels are
in stock at the time of deployment in the region will be used.
Note, the efficiency curves (Figure X and X) are for the Flexmax80, they are identical to
the Flexmax60 other than the fact that the FX60 does not accept 85 and 100V.
Figure 10, (Outback)
The highlighted area on the graph represents the highest efficiency while charging a 12V
battery array. The charge controller is operating at about 95.5% efficiency with an input
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Application Note: Basic Off-Grid PV System Design
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Voltage between 17-34V. Typically a 12V PV panel's Voltage at Peak Power is around 17
Volts.
Figure 11 (Outback)
The highlighted area in this graph represents the optimum efficiency if the system where
charging a 24V battery array. The charge controller is operating at about 98% efficiency
with an input Voltage around 34V. Two 12V panels in series will typically have 34 Volt
equivalent Voltage at peak power.
In an ideal setup the FlexMax 60 would operate at 98.1% efficiency with an input of 68V
while charging a battery array at 48V. This would be the case with the optimum PV panel
chosen in section 1, the Kaneka G-EA060 as the VPM is 67Volts.
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Conclusion
Designing an off grid photovoltaic system involves many steps and although the math is
simple all calculations should be double checked. If the calculations for one component
are off chances are the whole system will not work, every stage relies on the previous
one. Designing the system for worst case scenarios is good practice, it is better to have
extra energy than not enough. All safety precautions should be followed especially on
electrical connections that have a possibility of carrying a lot of current. Breaker boxes
before and after battery connections for easy power disconnect should be implemented.
These breakers should be rated for DC voltages.
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References:
Hankins, Mark, and Francis Njeru. Solar Electric Systems for Africa : A Guide for Planning
and Installing Solar Electric Systems in Rural Africa. Ed. Timothy Simalenga.
Beverly: Commonwealth Secretariat, 1995. 7-8.
http://www.knowledgehound.com/topics/solar.htm
World Insolation Map: http://www.sunwize.com/info_center/insolmap.htm
Sun path chart: http://www.oksolar.com/images/solar_path_large.jpg
Outback Power Flexmax User Manual:
http://www.outbackpower.com/pdfs/manuals/flexmax.pdf
Wiring Safety Concerns: http://www.usbr.gov/power/data/fist/fist3_3/vol3-3.pdf
Also, http://www.solarpowerforum.net/forumVB/showthread.php?t=1890&page=3
** Source: Sunwize PV installation guide as well as other installer personal experiences
<http://www.sunwize.com/catalog/images/design_sunwize_guide.pdf>
http://www.windsun.com/Batteries/Battery_FAQ.htm
<http://www.windsun.com/Batteries/Battery_FAQ.htm>
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Appendix 1: Solar Panel Selection
Optimum Panel available in North America:
Kaneka G-EA060
Power: 60 Watts
IPM: 0.90 Amps
VPM: 67.00 Volts
Price: $227.00 USD (Retail)
Value ($/Watt): $3.783
Locally Available in prototype deployment area Arusha, TZ:
24 Volt Setups:
PV Panel
Quantity Total Wattage
Price
Value ($/Watt)
Photowatt PW080
6
480
2706
5.64
Suntech ST-165 (24V) 3 (//)
495
3150
6.36
Kyocera KY125
4
500
2768
5.54
Suntech ST-080
6
480
3180
6.63
12 Volt Setups:
PV Panel
Quantity
Total Wattage
Price
Value ($/Watt)
Photowatt PW080
5
400
2255
5.64
Photowatt PW165
3
495
2778
5.61
Suntech ST-060
7
420
2870
6.83
A 24 Volt setup is recommended, one reason being that current levels are reduced. This
allows for easier current measurement and smaller wire gauge which results in more
manageable wiring and lower costs.
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Appendix 2 – Battery Selection
Batteries locally available in deployment area
Description
Voltage
AH at c/20 Type
Retail Price
Value ($/AH)
Surrette 27-HT-90 12
90
Flooded
$190.00
2.11
Surrette T12-250
12
200
Flooded
$500.00
2.5
Deka 8G4D
12
210
Gel
$575.00
2.74
The Deka 8G4D batteries is chosen because it is the only non-liquid acid battery
available. Two are suggested as they will allow for a 24V setup and provide enough
capacity to run the system for approximately 8 hours with a discharge depth of 50%.
With this discharge depth the Deka batteries are rated for 1000 cycles (at 77*F).
Batteries self-discharge faster at higher temperatures. Lifespan will also reduce at higher
temperatures, the Deka datasheet does not include this information and their engineers
claim that temperature will not affect the lifespan. Most other manufacturers state a
50% loss in life for every 15 degrees F over a 77 degree cell temperature. Temperature
and depth of discharge data will be collected to determine the actual temperature
effects.
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Appendix 3 – Inverter Selection
Inverter used in first 12V lab prototype:
Xantrex Xpower, 1750 Watts, Peak efficiency 90%, Retail Price: $210.00
10-15VDC Input, 115VAC 60 Hz (+/- 4Hz) Out
Inverter suggested for 24V setup:
Samlex PSE-24125A, 1250 Watts, Peak efficiency 90%, Retail price: $250.00
20-33 VDC Input, 120V (+5% / -10%) 60 Hz (+/- 5%) Out
Inverters available locally in Arusha, TZ:
24 Volt:
Xantrex DR1524E, 1500VA, Peak efficiency 94%, Retail price in Arusha: $1050
24VDC Input, 230VAC 50Hz Out
12 Volt:
Xantrex CR1012E, 1000 Watt, Peak Efficiency 86%, Retail price in Arusha: $554
10.5 - 14.5VDC Input, 230VAC 50Hz Out
Charge Controller Prices:
Outback Flexmax60 sourced in MI for: $563.44
Available in Arusha, TZ for: $744
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