RV Camping
with
Solar Power

Operating your RV's electrical system or charging batteries with solar power is simple to do. But, you need to know some basic information and you need to have a plan. I hope this information and examples will help you to plan your system for an RV, cabin or home. I created this information at the same time I was designing and documenting my solar installation into a pop-up camper used for family camping. I started the quest for solar power for the RV during the first year of RV camping. I found during "dry" camping that the battery was OK for a two day weekend, but a recharge would be needed if I was going to camp for a longer period. I was also planning a four day "dry" trip that would require the battery to be recharged, somehow.

The picture above was taken during an early spring star party camping trip where the planning (from this page) was worthwhile. The telescope is equiped with a solar filter, the camper has a 75 watt solar panel and 225 Ahr battery capacity "under the hood". It was a great four day trip, until the weather changed, saturday afternoon. The temperature went from a high of 81°F in the afternoon, to wind and rain by early evening, turning to SNOW around 3am Sunday. Without some planning, the normal RV battery wouldn't have lasted for the full trip.

Do I NEED solar? No, I can charge the RV battery with a small gasoline generator or the vehicle alternator. So why solar?

THE PLAN:

Establish the Load.
 Determine battery requirements.
 Determine the amount of sunlight available for your location.
 Purchase a solar panel.
 Determine specifications for charge controller.
 Purchase charge controller.
 Determine system losses.
 Wiring it together.
Determine solar panel mounting requirements.
 Evaluate the data -- another look.

Click on a RED ball to jump the the section desired or scroll down to see the entire report. I have created this report as a single web page to make it easier for you to print a copy.

Establish the Load:

General specifications:

The Loads and Usage -- Determine all the loads and the "worse case" duty cycle of each.
See the example data at the end.

Batteries:

Batteries are a very important part of the solar system as they provide electrical power at night or on rainy days. A battery is a small chemical plant that converts electricity into stored chemical potential and back into electricity on demand. Just as the EPA watches over industry, you too need to watch your little chemical plant for leaks and emissions. The battery can leak acid, emit explosive hydrogen gas and produce enough electrical energy to cause a fire if not safely contained. A very good FAQ page can be found at the Northern Arizona Wind & Sun web site.

My RV came with a location to mount a series 24 or 27, 12 volt "deep" cycle marine battery. I had a 5 year old 105 Ah series 27 battery I used for the first two years of camping. I learned a lot more about batteries during the first year. Here is what I knew and learned.

BATTERY TYPES

FLOODED FLOODED SEALED
(Valve regulated)		 SEALED
(Valve regulated)
Maintenance Free
(Don't add water)		 Requires Maintenance Absorbed Glass Mat
AGM		 Gelled
electrolyte
Lead Calcium Lead Antimony Lead Calcium Lead Calcium
50% DOD 80% DOD 50% DOD 50% DOD
100 - 300 cycles 300 - 700 cycles 100 - 300 cycles 100 - 300 cycles

A sampling of battery specifications will give you an idea of differences and give you a start in your search for your "ideal" battery. Capacity, price and weight all need to be taken into account for your RV's needs. This is my derived data from selected batteries off the Trojan Battery Company and Interstate Batteries (manufactured by Johnson Controls) web sites and used as an example of battery capacity.

BATTERY CAPACITY

Battery type Voltage Ah at 20hr
rate		 Amps at 20hr rate Reserve Capacity
Minutes at 25 Amps		 Weight in
pounds		 DOD
in %		 kWh at 12 Volts for 20hr rate to max DOD kWh at 12 Volts for 25A rate to max DOD Peukert's Coefficient
Golf Cart
T-105
U2200		 6 225 11.3 447 61 80 2.2 1.8 1.24
Golf Cart
T-125
U2300		 6 235 11.8 488 66 80 2.3 2.0 1.19
Golf Cart
T-145
U2400		 6 244 12.2 530 71 80 2.3 2.1 1.19
Floor Machine
U2500HC		 6 275 13.8 600 78 80 2.6 2.4 1.16
Stationary
L16		 6 360 18.0 805 113 80 3.5 3.2 1.22
Gell Series 27
SG-90		 12 86 4.3 169 61 50 0.5 0.4 1.11
Marine
27TM		 12 105 5.3 160 52 50 0.6 0.4 1.30
Marine
27TMH		 12 115 5.8 200 60 50 0.7 0.5 1.23

Peukert's Equation (information from the E-Meter Owners Manual) describes the effect of different discharge rates on battery capacity. As the discharge rate increases the available battery capacity decreases. The coefficient is derived by making two discharge tests, one at a high rate and one at a low rate, that bracket your normal range of operation. The coefficient data above was calculated by using the 20 hour rate and the 25 or 75 Amp discharge values. For the higher capacity batteries the difference between the two rate may not be enough to provide an accurate number, but are useful for comparison.

Peukert's Coefficient = ( LOG10(T2) - LOG10(T1) ) / ( ( LOG10(I1) - LOG10(I2) )

DAYS OF BATTERY LIFE*
without solar

EVENT 27TM T-105 T-145 L-16
Field Day(a) 0.4 1.2 1.3 2.0
Field Day(b) 0.6 2.2 2.3 3.4
Star Party (s/f) 1.2 4.1 4.5 6.6
Star Party 1.7 5.9 6.4 9.5
Camping 2.0 6.9 7.5 11.0

* Calculated at the allowed DOD AHrs x 12.5 Volts.

Widgets to help increase the life of your battery:

 

SUNLIGHT (Determine the amount of sunlight available for your location):

Before the size of the solar panel array is determined, the amount of sunlight for your location needs to be established. Then the number and wattage of the panels can be determined. I live near Wilmington, Delaware. The solar radiation for Wilmington will be used as my starting point for calculating how much power I can get from a single panel. I can then determine the radiation for other sites I may wish to visit in my RV.

The web pages of the Center for Renewable Energy and Sustainable Technology, CREST, or the National Renewable Energy Laboratory, NREL, are good places find out how much sun you get at different US locations and at different times of the year. The data all originates at NREL, it is presented in different forms at the two sites. A good place to start is an NREL map like the one shown above. The maps can show all the solar information for the entire U.S. (and possessions) for the entire year or just a single month.

The amount of SUN available at different times of the year to power the solar panel varies in different parts of the country as can be seen in the map. The needs of a cabin and a RV are different as the RV can be moved, batteries charged by the engine or tow vehicle or even a generator. For a cabin used year round, its best to determine a "worst case" sun generation capability by using the smallest minimum for the year (most likely in December) to calculate your available solar energy. Unless you will be using your RV for long trips at a fixed location, pick a National Solar Radiation Data Base (NSRDB) station site from the NREL data near your camp and use the average sun output data to plan the size of the solar array.

City: WILMINGTON * State: DE * WBAN No: 13781 *
Lat(N): 39.67 Long(W): 75.60 * Elev(m): 24 * Pres(mb): 1015 *
Stn Type:Secondary 

Flat-Plate Collector Facing South at Fixed Tilt=0
 

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Year

Average

2.0

2.9

3.9

4.9

5.6

6.2

6.1

5.4

4.4

3.3

2.2

1.7

4.1

Minimum

1.7

2.4

3.4

4.2

5.1

5.5

5.3

5.0

3.9

2.8

1.8

1.5

3.8

Maximum

2.3

3.5

4.4

5.7

6.6

6.9

6.8

6.0

4.9

3.8

2.6

2.0

4.2

The table above is an example of the CREST data for Wilmington, DE for a solar panel flat on the roof of an RV. Plotting the (fixed tilt = 0°) data it is easier to see the differences between the average, maximum and minimum daily sunshine. In June a flat fixed mounting position is best and it produces an average of 6.2 kWh/m^2/day. That's in thousand (kilo) watt hours per square meter per day. Solar panels are rated by the manufactures in watts output per 1000 W/m^2 (1kW/m^2). A 75 W panel in Wilmington, DE and mounted flat on an RV's roof in June will produce 465 Wh of electric power per "average" day. The 465 Wh/day is derived by multiplying the 6.2 kWh/m^2/day times the 75 W/kW/m^2. (Look at the 6.2 kWh/m^2/day as 6.2 h-kW/m^2/day and the kW/m^2 cancel leaving Watt-hours/day.) Now that you know the "real" math involved in determining the output of the solar panel, it's easier to look at the NREL data as just HOURS and the PV panel rated in WATTs or 75W * 6.2hr = 465Wh.

I used the average sun output for my calculations keeping in mind the way the results are presented. The minimum and maximum value from the NREL data is NOT the lowest value from the 30 year data for a particular month. It IS the smallest or largest AVERAGE value for that particular month. Note the chart of the daily sunshine for June, 1961 and you see a large variation in energy with an average for the month of 6.1 Kwh/m^s/day. This chart also helps show the reason for having enough battery capacity to power your system during the dark days.

 

Mounting orientation:

The orientation of the solar panel toward the sun makes a difference in the total power produced. A stationary system (at home) might use a tracking mount to produce the maximum energy from the sun. It is unlikely an RV would use a tracking system although it could be done. The simplest way is to flat mount the panel on the roof or mount it with one axis being adjustable. Using the NREL data it is possible to create a new table showing the single axis adjustment for each camping month and the corresponding average energy output. I only included the months that I might expect to be camping as my pop-up would not be comfortable in weather much below freezing at night with the limit being freezing water lines and wife.

Note, "Lat" stands for your current latitude and in the case of Wilmington, DE it is ~40°.

Wilmington, DE

Month Angle kWh/m^2
Apr Lat-15° 5.3
May Lat-15° 5.7
Jun Flat 6.2
Jul Flat 6.1
Aug Lat-15° 5.8
Sep Lat+0° 5.1
Oct Lat+15° 4.6
Nov Lat+15° 3.6

This graph shows the advantage of adjusting the tilt on the panel for each month. Although tracking mounts are not practical for an RV, making an adjustment to the tilt should be easy to do if the panel is mounted on an adjustable frame.

If you go to another part of the country, the solar radiation is different. The latitude for Charlotte is ~35° and you will note the difference in energy (kWh/m^2) between DE and NC. Not a big number, but it will make a difference in the energy produced. In November, the difference between Charlotte and Wilmington is 60 Wh's more energy in Charlotte, "on average". See the next section for a comparison between different locations and different camping events.

Charlotte, NC

Month Angle kWh/m^2
Oct Lat+15° 5.2
Nov Lat+15° 4.4

Calculation of the energy required (to run 100% solar) for different loads at different locations. Since my desire is to operate with a single panel I used a 75 watt panel as a starting point for my calculations. For star parties and family camping it would be nice to be able to recharge the RV's batteries fully from the sun. For amateur radio Field Day I don't expect to operate 100% solar.

As time has changed the data in the table to reflect the true load for spring and fall trips, it is clear I will not always be able to achive 100% solar for those trips. The weather, particulary the temperature has a large effect on the power consumed during a trip. Since all of my camping is only for a few days at a time and with a large battery capacity, I am not likely to add additional solar PV panels. Even with my spring and fall star party estimates at ~70% solar, most days I am able to fully recharge the battery.

Solar Wh's = PV Watts multiplied by sunlight from table or 75 W/kW/m^2 X [sunlight]kWh/m^2 (per day)

Event Nearest
NREL Site		 Month Solar
Wh's		 Load
Wh's		 % Solar
Star Party (spring) Baltimore, MD Apr 398 542 73
Family Camping Altantic City, NJ May 428 326 131
Family Camping Wilmington, DE Jun 465 326 143
Field Day(a) Wilmington, DE Jun 465 1,815 26
Field Day(b) Wilmington, DE Jun 465 1,046 44
Star Party Wilmington, DE Jun 465 379 123
Star Party (fall) Baltimore, MD Sep 383 542 71
Star Party (fall) Charlotte, NC Nov 330 542 60
Family Camping Wilmington, DE Nov 270 326 83

Purchase solar panel:

Now that the loads are known and the amount of solar radiation has been established, it's time to find a PV panel to meet the need.
I recommend an Alta Vista search to help you find current specials. Click "search" below to proceed.
The search will return an Alta Vista results page.

The cost of a PV panel, output and it's size limited my choice to a single Seimens Solar SP75 panel at 75 Watts from Northern Arizona Wind & Sun. Keep in mind that two smaller PV's rather then one large panel may be easier to store and transport.

Specifications for charge controller:

Why do you need a charge controller? The simple answer is to keep from destroying your batteries. This is not the case with small (low wattage) panels with the lower output voltage of 30 cells. With the full voltage of 17 - 21 Volts, the high power 36 cell panels will over charge a battery in short order without a controller. Some charge controllers can also be used as a load controller by shutting off the load on low voltage or used as a dusk to dawn light control. Since the maximum current required by the RV is greater then the 10 - 12 Amp range of the controller, load control was not considered an aspect of this design.

The specifications I wanted in my Charge Controller

Purchase charge controller:

I recommend an Alta Vista search to help you find current specials. Click "search" below to proceed.
The search will return an Alta Vista results page.

My choice was a Trace Engineering model C12 charge controller from Northern Arizona Wind & Sun .

System losses:

After generating and storing the energy, it's prudent not to waste it by using small gauge wire. Other then the choice of a charge controller, the wiring is about the only way you have to save energy. You may want to look at the size wire the manufacture of your RV used. I found #12 wire was used to wire the DC side of the 3-way refrigerator providing about 3.4 watt loss in the wire and a 0.3 volt drop over 18 feet of wire. It is unlikely you will power a DC refrigerator from a solar panel, but the RV has losses in all it's wiring. #8 wire would reduce the loss to 1.5 watt and 0.14 volts. The effect of wire size will also limit the charging current of the RV battery while driving even if the alternator is capable of 130 Amps. The voltage drop between the tow vehicle battery and the RV battery is the reason and a result of small wire size.

Solar Panel:

Anything that reduces the specified output power of the solar panel could be considered a loss. Some you can't do anything about, like clouds and the age of the panel. You can do something about tracking and alignment of the panel or dirt on the front glass of the panel. Age can cut the output by as much as 20% over the life of the panel and should be taken into account for your design. Most manufacturers will provide a warranty period and a minimum power output.

Wire & Connections:

The losses caused by wire size (or lack of size) is one area you can make an impact on the losses in the system. Ohms law defines the relationship between Voltage ( E ), Current ( I ), Resistance ( R ) and Power ( P ) by the equations, E = I x R and P = E x I. From those equations, you can derive the following equation used to calculate the loss.

P = I^2 x R

The table below shows the difference in resistance for different size wire and the resulting loss of power in Watts at 4.4 Amps, the maximum current from the solar panel. From the data in the table below, it should be obvious for the need to use the largest wire size possible from the solar panel to the controller, battery and then to the load. For example, if #14 wire is used for a single 75 Watt panel and a second panel is added, the maximum current is 8.8 Amps. The loss in the 10 feet of wire goes from 0.58 Watts to 2.32 Watts, a big difference.

Properties of Stranded Conductors
per MIL-W-81044

Wire Size
AWG		 Resistance
Ohms @ 20°C / 1,000Ft		 Lost Watts
@ 4.4A per
10Ft wire
#14 2.99 0.58
#12 1.58 0.31
#10 1.27 0.25
#8 0.70 0.14
#6 0.44 0.09

Charge Controller:

There are four losses associated with the controller. I don't intend to use the controller to detect low voltage or load control. So, the loss from the battery to the load isn't used in further calculations.

Controller
LOSS		 Trace
C12		 ProStar
12		 C12
Lost Watts
@ 13.8 V and 4.4A
Voltage drop
PV to Battery		 0.30V 0.70V 1.32
Voltage drop
Battery to Load		 0.15V 0.40V 0.66
Current required
While Charging Battery		 7ma 13ma 0.10
Current required
At Night		 3ma 10ma 0.04

The problem with modeling the losses the same way as the solar generation is it's not the same type of model. For example, the 6.2 hr time used to establish the energy from the solar panel is calculated as an on-off function, which it isn't. The 6.2 kWh/day number is the sum of the radiation from sunrise to sunset. The length of a June day is 14.8 hours. This is also true in the calculations below, as the [I^2 x R] loss in the #10 wire from the PV to battery doesn't have a constant current for the 6.2 hours and if half the current was supplied it would produces a fourth of the loss. The calculation still does help to determine "worst case" conditions.

Charging System
LOSS		 Watts Hr Watt-hours
#10 Wire
10' x 2
PV to Battery		 0.50 6.2 3.1
Controller
Voltage drop		 1.32 6.2 8.2
Power consumed
Charging		 0.10 14.8 1.5
Power consumed
At Night		 0.04 9.2 0.4
TOTAL     13.2

The 13.2 Watt-hours is a loss of 2.8% of the 465Watt-hours that could be produced by the solar panel or you can look at the loss as if the panel, wiring and charge controller only produced 72.9 Watts of usable power.

Battery:

Inverter:

Even if an inverter is not part of my design, I do want to include them in the loss information. The best inverter is only ~90% efficient at full load and draws current even with no load. Some units can "sleep" drawing less then a tenth of an amp. Short connections between the battery and inverter are VERY important with the high currents drawn by the inverter. Inductive loading in the wires is also a problem caused by the high currents and can be reduced by keeping the two cables together rather then seperating the wires to and from the inverter.

Wiring it together:

Don't forget the Fuses

Solar panel mounting requirements:

When I figure out how I am going to mount the solar panel to my pop-up roof, I'll add my results to the page. Most manufactures have limitations on how to add attachments to the roof and load limits that all need to be followed...
This picture shows the result of my first method of mounting the panel flat on the roof. You are looking at the back of the panel with a 1/4" x 2" aluminum bracket. The bracket has an aluminum spacer with one of four, 3" white plastic disks attached with three screws. The disk sits on the roof without scratching the paint and the screw eye attached to the bracket is tied to each corner of the roof with 1/8" nylon rope. A modification of this method will be used to allow for a tilt adjustment. With this method, I only mount the panel when I am camping, not while traveling.

EXAMPLE DATA:

The current load data is ONLY for 12 Volt devices, a separate load list would be needed if 115 VAC devices were used. Additional items to think about adding to the list are things like:

Family Camping

Device Number Watts hours Watt hours
LP gas detector 1 0.62 24.0 14.9
E Meter 1 0.32 24.0 7.7
Water pump 1 41.4 0.25 10.4
Heater Fan 1 48.3 2.0 96.6
Ceiling light, bulb #921 2 18.0 3.0 108.0
Ceiling light, bulb #921 2 18.0 1.0 36.0
Bunk light, bulb #921 2 18.0 1.0 36.0
Outside light, bulb #912 1 13.0 1.0 13.0
Trunk light, bulb #912 1 13.0 0.25 3.3
TOTAL       325.9

 

Star Party

Device Number Watts hours Watt hours
LP gas detector 1 0.62 24.0 14.9
E Meter 1 0.32 24.0 7.7
Water pump 1 41.4 0.25 10.4
Heater Fan 1 48.3 2.0 96.6
Red Ceiling light 2 4.0 6.0 48.0
Red Ceiling light 2 4.0 2.0 16.0
Red Bunk light 2 4.0 2.0 16.0
Telescope controller 1 27.6 6.0 165.6
Custom Red LED desk lamp 1 0.7 6.0 4.2
TOTAL       379.4

 

Field Day (a)

Device Number Watts hours Watt hours
LP gas detector 1 0.62 24.0 14.9
E Meter 1 0.32 24.0 7.7
Water pump 1 41.4 0.25 10.4
Heater Fan 1 48.3 4.0 193.2
Ceiling light, bulb #921 1 18.0 8.0 144.0
Bunk light, bulb #921 1 18.0 1.0 18.0
Outside light, bulb #912 1 13.0 8.0 144.0
Trunk light, bulb #912 1 13.0 0.25 3.3
HF transceiver -- Transmit 1 276.0 2.0 552.0
HF transceiver -- Receive 1 27.6 22.0 607.2
Desk lamp, bulb #93 1 15.0 8.0 120.0
TOTAL       1,814.7

The ICOM IC-706MKIIG transceiver specifications were used to model the transmit and receive current. As can be seen by the 2.0 A receive current (27.6 W), after a full day of operating the number of watt hours are a significant part of the total load. Other transceivers on the market have receive current in the order of 1.2 Amp (16.6 W or 364 Wh). A reduction in transmit power, QRP, will also help. See Elecraft for even better specifications.

Note: After the 1999 FD with the First State ARC at the WDEL - WSTW station grounds, I realized it had been a few years since I had "done" FD in Delaware. It gets hot in DE in late June! 95°F. I do remember years in the past when a heater was required at night, not this year! Also, since the children/wife didn't join me for the evening, I didn't need as many lights. We used a desk lamp and a single ceiling light. Re-evaluate as required, as I am sure I could have used just the desk lamp and reduced the load by another 144 Whrs.

Field Day (b)

Device Number Watts hours Watt hours
LP gas detector 1 0.62 24.0 14.9
E Meter 1 0.32 24.0 7.7
Water pump 1 41.4 0.1 10.4
Ceiling light, bulb #921 1 18.0 8.0 144.0
HF transceiver
50% T/R
20% SSB Duty Cycle		 1 31.2 24.0 748.8
Desk lamp, bulb #93 1 15.0 8.0 120.0
TOTAL       1,045.8

Evaluate the data -- another look: