How Many Solar Panels Do I need?
We often get asked “how many solar panels do I need?”. The person asking the question normally expects me to look at the size of their caravan/motorhome then immediately offer a valid technical opinion.
To offer any kind of meaningful answer a number of factors must be known:
- Average daily power consumption (normally measured in amp-hours).
- The size and voltage of the battery bank.
- The average number of sun-hours for the location and time of year.
A few years ago I developed a very comprehensive Excel spreadsheet to calculate all these and other variables. By spending about an hour talking to the occupants of a motorhome or caravan and making a few simple measurements, it is possible to produce a fairly accurate estimate of the amount of solar required to keep the battery system charged.
In developing the spreadsheet, one of my biggest challenges was to find data showing how many amp-hours should reasonably be expected from a given solar panel during the course of a day. After many hours of searching and a few emails, I finally found a table that showed the average sun-hours per day for xxx Australian locations for each month of the year.
It needs to be said that in this context, sun-hours is NOT the same as hours of sun. One sun-hour could arise from one hour of full sun OR two hours of sun at half intensity (or three hours of sun at one third intensity – you get the picture). This data is perfect for calculating the average energy production from a given solar panel or group of panels.
Here is the formula:
Firstly work out the number of amps you can expect from the solar panel when it is in full sun…
Amps = Total wattage of panels / (system voltage + 5) = amps
Example : 12v, 100watt panel…
100 / (12 + 5) = 5.88
So a 100 watt panel will produce about 5.88 amps when exposed to full sun.
Why volts plus five, I hear you ask? Well, you can not charge a 12v battery with 12 volts. So most panels have their peak power rated at about 17 volts for 12 volt systems (about 31 – 34 volts for 24 volt systems). This extra voltage must be taken into account when calculating the current value.
Now that you have the number of amps the panel will produce in full sun, you simply need to multiply this value by the number of sun hours for your chosen location and month.
A sample of the sun hours data is in the table below, the full table can be downloaded in CSV format by clicking here.
| Jan | Feb | Mar | Apr | May | |
| Adelaide | 7.31 | 6.58 | 5.39 | 4.06 | 3.00 |
| Albany | 6.67 | 5.75 | 4.75 | 3.56 | 2.86 |
| Alice-Springs | 7.41 | 6.79 | 6.60 | 5.74 | 4.59 |
| Box-Hill | 6.55 | 5.76 | 4.66 | 3.01 | 2.00 |
| Brisbane | 6.56 | 6.22 | 5.47 | 4.89 | 3.92 |
| Broome | 6.25 | 6.03 | 6.00 | 5.97 | 5.17 |
| Cairns | 5.86 | 5.67 | 5.53 | 5.25 | 4.47 |
| Canberra | 6.97 | 6.39 | 5.33 | 4.19 | 3.08 |
From this we can see that our 12 volt, 100 watt panel will produce about 35AH (amp hours) per day when in Broome in March. The same panel will produce just 11.7 AH when in Box Hill in May (where the $#% is Box Hill?)
Using the full version of the table we can estimate how much power a given sized solar panel or group of panels will produce per day at a given location at a particular time of the year. This is very useful information when deciding how much to spend on solar panels for your motorhome or caravan.
Tags: Caravan, Energy, Motorhome, Power
Has the information on this site been useful?
If it has, please consider using the donate button at the bottom of our home page. All donations are used to maintain and improve the website and we are very grateful for any support.
Ever wondered why are there Google Ads on the Hobohome Site?
It costs quite a bit of money each year to keep the Hobohome site running.
We have decided to allow advertisements on selected pages of the site to help offset the costs involved
in maintaining and hosting the Hobohome website.
You can read more about how this works
by clicking here.
Thanks and happy Motorhoming and Caravanning - Gavin & Tracey.
September 10th, 2009 at 9:03 am
This sun-hours table is really useful, and it’s important. Instead of just estimating the number of solar panels to be installed, we can take your approach, which is very professional, and calculate the exact number of solar panels we need wrt to the specifications of the solar panels stated.
November 24th, 2009 at 4:05 pm
You genuinely make it appear so uncomplicated with your presentation, but I find this topic to be really something which I think I would never understand. It seems too complicated and very broad for me. I am looking ahead for your next post; I will try to get the knack of it.
March 29th, 2010 at 3:41 am
First off wonderful website. I’m uncertain if it may be brought up, but when working with Firefox I can never get the entire weblog to load without refreshing several times. May possibly just be my modem. Enjoy!
March 30th, 2010 at 7:01 pm
Hi – thanks for the comment. FireFox works ok in my testing – however I have made a few changes to the site navigation and layout – hopefully that will sort your issues.
Hobo
March 10th, 2011 at 9:00 am
this site is simple enough for even me to understand.
on the basis that most caravaners would not expect their microwave,fridge,HWS or Air Con to work on solar power they would like their TV/radio and lighting to operate for say 6 hrs per day.
is it possible to average out the figures and suggest a minimum size that would supply demand in Box Hill
regards
peter
March 12th, 2011 at 10:27 am
Hi Peter,
As a rough guide, based on 200watts of consumption for 5 – 6 hours per day (50w of lights, 100w of TV and 50w of other), 250 watts of solar would probably be enough for most people. Keep in mind that close monitoring is always advisable to avoid deep discharges that quickly kill battery systems.
Gavin
September 3rd, 2011 at 11:43 am
Tims Solar Systems…
[…]Motorhome and Caravan Info Australia » Blog Archive » How Many Solar Panels Do I need?[…]…
June 5th, 2013 at 2:04 pm
Hi team I have difficulties trying to find the correct solar panel plus batteries and power converter for what I want to use. I have to water pumps that I would like to run the specifications are the following:
Pump 1:
1.1Kw. 1Hp. 50 Hz. 230-240 v. 4.5 amps. 2750r/min. 20Uf. 450v
Pump 2:
230/240 v. Kw out 0.55. 2.54 amps. 50 Hz
The unit that drives pump 1 is 240 v
The unit that drives pump 2 is 240 v 50 Hz
So far I haven’t be able to find or to make my system work with what I have and no one has been able to guide me. Can you help please respond.
Regards Cisco
June 8th, 2013 at 11:27 am
Depending on how long you want to run these pumps, it will require quite a large solar and battery system. I would recommend talking to some solar pump specialists. It may be more economical to replace the pumps with ones designed to run directly from solar panels.
March 21st, 2014 at 5:50 pm
Hello,
I have just bought solar cells totalling 200 watt if made in two DYI panels. I have 10/ 12volt 55 amphr batteries from a big UPS system. would it be best to run the solar at 12 volt or pair the batteries to 24 volt and can the system carry a 1500 watt inverter. Still debating on PWM or MPPT charge controller. Any thoughts would be helpful.
March 28th, 2014 at 6:59 am
Hi Mike,
firstly, I have seen lots of adverts for DIY solar panels but never talked to anyone who has done it – I’d be very interested in how it goes. Do you have the test equipment to confirm the output once the panel is completed?
To answer your question – higher voltages are better for larger inverters or where the cable runs are longer.
MPPT is better in colder climates where to solar voltage tends to be higher.
April 3rd, 2014 at 3:39 pm
Hi Hobo,
Yes I have all the test equipment as I am a 35 year computer shop owner. Was given 12 batteries from a backup system of 12 volt 55 AH. After sorting the cells I can build 1 x 100 watt panel and 1 x 40 watt. Because my system is going to be mainly lighting I thought I would split the battery banks in two. 8 for the big panel and 4 for the later. Cells tested at around 1.7 per and I used 1.5 as the base to get my totals. Whole system cost will be under $100. AU. ( less inverter at present ) I hope to post number updates on outputs and float ranges for anyone who has expectations on DYI and huge power gains. The first system will be basic 12 volts with 440 AH storage 100 watts to keep charged if it works
April 20th, 2014 at 6:41 am
Maths not my strong point so can you please tell me whether 300 watt solar with 4 x 75ah AGM deep cycle batteries (parallel wired) would power a 12volt Vitifrigo 130 litre (42 watt = 3.5ah) overnight? I also like to watch TV – any chance?
April 27th, 2014 at 9:10 pm
There are too many unknowns to give a definite answer, but my best guess would be “yes – sound ok to me”.
July 12th, 2015 at 4:28 pm
Hi Hobo,
You say that the output of a 100 Watt panel is 100/(12+5) = 5.88 amps. This is the out for a panel directly facing the sun. As the panel on a caravan roof is mounted flat there is a loss of output. In Broome this loss is around 10%. In Hobart the loss is 24%. There is another loss due to temperature. Solar panels produce there maximum rated output at 25 degrees C. For every 10 degree rise in temperature there is a loss of 5%. At 45 degrees which could easily be exceeded in Broome we have another 10% loss. Finally we have the regulator loss in transferring the power from the solar panel to the battery. Adding up these losses 10% for flat mounting + 10% for operating temperature + 20% for regulator and wiring loss comes to 40%. A rule of thumb would be to install double the wattage required. I would like to hear your comments on calculating size of panel wattage.
July 16th, 2015 at 2:26 pm
Hi ya,
You make some valid points – however …
Tilt Losses are dependent on location and time of the year. As we know, the ideal tilt angle is about angle of latitude. Losses are NOT linear. A 20degree deviation from ideal results in only a 5% loss of output.
Temperature losses – if you look at the temperature Coefficient of mono and poly panels, you will see that the reduction in wattage is due to the reduction in out voltage (not current). If the regulator is of the PWM type, this has almost no effect at all (during the main production parts of the day). If the regulator is MPPT then the reduction in potential charge is reduced according to the reduction in wattage.
I’m not sure where you are getting 20% losses in wiring and regulator – wiring losses should be less than 2% and I have not seen an MPPT regulator that has efficiency less than 90% (PWM regulators are almost 100% efficient).
The intention of the article was to provide a guide – it is based on the assumption of 5 sun hours per day (not the same as 5 hours of sunshine). If we were looking for absolute accuracy – this estimate of sun hours is the biggest source of inaccuracy.
As an illustration of the purpose of the article – I was contacted by a couple who were having a lot of issues with their battery/solar system (they had not had the caravan very long). They reported that the battery was “not holding a charge”. When I asked about the system components, capacity etc, they said that they had a “solar expert look at it, and he said that there was enough solar to run a house”. Further investigation revealed that they had 4 X 160w panels (so a total of 640w). They also had one 340ltr 240v fridge and two freezers running. If the “solar expert” had bothered to do some simple calculations it would have been very clear that they required many more panels to support the refrigeration load alone.
February 4th, 2017 at 12:34 pm
Hi Love your site, I might not be very bright but I can’t work out 100 divided by 12 +5 =5.88 I know 100 is watts size of panel 12 is volts(12volt system) +5 is to compensate for higher panel voltage, to me it =13.3 what am I missing. Jeff
February 4th, 2017 at 2:06 pm
12 + 5 = 17 – most 12v panels are rated at about 17 volts.
I=W/V
current = watts divided by volts
100 / 17 = 5.88
This the estimate is that a 100 watt panel will produce about 5.88 amps at about 17 volts