Budgets are being tested by cost increases across the country. It seems that there is little prospect of costs easing any time soon.

For those of us with a finite travel budget, staying at free or low-cost overnight camps is an attractive option, but only if we are well prepared. Gone are the days of minimal camping arrangements for many of us. We love our TVs, fridges, lights, fans and other desirables, but these require power, which is not cost free aboard RVs. Solar power facilities seem ideal, but they must be fit for purpose and this requires a degree of planning and some outlays. The old two-up game call of, “You need to speculate if you wish to accumulate” has some relevance here.

We are very lucky that there are so many wonderful places to stay in this country. Some have caravan parks, but many excellent campsites do not have mains power. Sadly, there is a significant shortage of power points on gum trees; you have to bring your own! Fortunately, this is now possible, albeit a little bit tricky.

Green power

GREEN POWER

Many new motorhomes, camper trailers and caravans come with a solar panel or two on the roof, used to charge the onboard house battery which powers LED lights, USB charge points, a water pump, and a TV, but not much else. Increasing the number of items needing to be powered and extending the time such items are used often requires additional batteries and the solar power to recharge them. Adding a set of folding solar panels may help, but a ‘cut and try’ approach can increase costs for less-than-ideal results.

A BETTER APPROACH

So, what is the alternative? You probably have a fair idea of all the major items that you wish to power and how much they will be used in any 24-hour period. Don’t worry too much about small items such as a mobile phone or two because you will need to factor in some  allowance for these later.

You also need to consider where and when you intend to travel because this can have a bearing on how much solar power you will need.

Fortunately, we came across an excellent website that really simplifies these aspects and which contains an online calculator that enables us to determine how much average power our solar panels may provide at any location in the world, in any month of the year! The website has several free tools including the Daily Solar Energy Calculator. The owner of FabHabs and developer of the Daily Solar Energy Calculator graciously allowed us to have free access to this remarkable feature, for the purpose of this article. Thank you, Thomas!

The calculator only deals with horizontally mounted panels (panels on RV rooftop), but if you have portable panels, the calculator will still be useful, and you will be able to harvest more solar power by tracking the sun - a nice bonus in poor solar weather conditions.

The calculator is really easy to use and can be accessed from the FabHabs website, www.fabhabs.com. First, input a location (this is followed by a results data section that you may like to examine later) but for the moment focus on the realistic average daily solar insolation by month figure expressed in kilowatt hours per square metre each day (1 kilowatt hour = 1000 watt hours). That figure can be multiplied by your choice of panel or array power to provide a result expressed in watt hours.

You may find it interesting to examine the minimum power figure for your chosen location. Have a play with the calculator to check out other locations, panels and/or months. Compare Hobart and Darwin in June!

Technicolor visitor

BATTERIES

While you are having a spell, we need to think about the socalled 12V batteries you will be using. You will note that battery capacity is normally expressed in ampere hours rather than watt hours or kilowatt hours that the calculator shows for your solar panels. We need to compare apples with apples – converting ampere hours to watt hours is simple – and all you need to do is to multiply ampere hours by the battery voltage.

For best service life, lead acid batteries, including AGM types, should not be routinely discharged below 12.20V which represents 50 per cent of their capacity. This means that a 100Ah AGM battery only has an effective capacity of 50Ah. Therefore, the capacity is approximately 50 x 12.20 = 610Wh or 0.610kW hours. AGM batteries should be charged with a three or four-stage charger that charges them to around 80 per cent capacity during the bulk charging phase, but may need a further four hours or more of charging during the absorption phase to reach full capacity and ensure long service life. This can be difficult at times, especially with solar charging systems.

For lithium (LiFePO4) batteries, the process is similar, however a 100Ah LiFePO4 battery has an actual capacity of almost 100Ah and a nominal voltage of 12.8V. Therefore, the capacity is approximately 100 x 12.8 = 1280Wh or 1.2kW hours. LiFePO4 batteries can be rapidly bulk charged at very high rates and do not require the time-consuming absorption phase that is necessary for lead acid types, including AGM. The net result is that LiFePO4 batteries can be rapidly recharged by powerful solar charging facilities.

The preceding figures are indicative rather than precise but nevertheless are good enough for our purposes.

Convict built bridge (1834) at Ross, Tas

PUTTING IT ALL TOGETHER

Now is the time for you to come up with the goods. You will need to find out and list how much current each of your significant items draws or how many watts of power they use in a 24-hour period. Multiply the current in amps by the battery voltage (12.8V) to calculate watts.

You now have the tools to use volts and amps to calculate watts over a 24-hour period and to express this as watt hours or kilowatt hours. The calculator will show how much average power solar panels can produce at a particular location, of your choice, in a month. Keep working through your list of items that you need to power. Handbook data on some items may show the current drawn by them or the power they consume in watts. It is also possible to use an ammeter to check currents and multiply the reading by 12.8V to calculate power in watts. You also need to know how many hours these items are used in each 24-hour day. When you are finished, calculate your daily kilowatt hour total.

240 VOLT OPERATION

Some appliances require a 240V power supply and if you wish to power them when you have no access to mains power you will need to fit an inverter. Any inverter powered from LiFePO4 batteries should be ‘lithium capable’, which avoids high startup current peaks that can instantaneously pull a LifePO4 battery below its low voltage cutoff point and cause the battery to shut down. This then requires the battery’s internal  management computer to be reset before the battery can resume normal operation. Your battery supplier can probably advise how to go about this.

A household power point provides access to 240V at 10A, which is capable of powering 2400W of appliances. With a 12V to 240V inverter, this means that a 2400W appliance would draw around 2400/12 = 200A from your battery. If you ran such an appliance for 10 minutes (1/6 hour), you would consume 200/6 = 33.3Ah or around 400Wh (0.4 kWh) from your battery, if it and its power cables would stand this. This 10 minute burst would consume around a third of a reasonable system’s battery power! Air conditioners, microwaves and coffee machines may be great in powered sites but pose significant problems for small solar-powered systems. Fortunately, gas appliances can provide some alternatives with slightly more effort.

Noddies in a Pisonia Tree, Lady Musgrave Island, Qld

SOLAR WEATHER

Now comes the big guess. What will be the actual (not average) solar weather during your time at the chosen location? You already have some experience with this because the solar  weather each night is really awful; however, you have managed to cover this by adding sufficient battery capacity. The same solution applies during poor daytime solar weather. In a practical sense though, there is a limit to how much battery capacity can be fitted aboard RVs and so the emphasis now changes to recharging your batteries from solar panels. Replacing lead acid batteries with similar ampere hourrated lithium iron phosphate (LiFePO4) batteries can almost double effective battery capacity and significantly reduce battery weight and recharging times.

SOLAR POWER

As with batteries, there is a limit to how much solar power most RVs can manage aboard. Roof area is limited and a pair of two panel portable arrays is probably as much as you would be prepared to carry aboard and deploy. These were the limiting factors for our small  RV. A suitably rated maximum power point tracker (MPPT) regulator effectively matches the output of the solar panels to the battery, whereas a pulse width modulated (PWM) type can comparatively reduce available charging current by 20 per cent or so at times.

When you are driving, a 40A DC/DC charger can usefully top up the charge level of your batteries. With two 100Ah LiFePO4 batteries, it would take around five hours of driving to recharge your almost fully discharged battery bank.

Part of the Australian Compact Array, Radio Telescope near Narrabri, NSW

OTHER OPTIONS

If your installation cannot keep up with spells of poor solar weather, you probably have two other options.

Go to a powered site and plug in. We carry a 30A LiFePO4 battery charger, which is capable of fully recharging our 200Ah LiFePO4 battery bank overnight and you can even run your 240V air-conditioner.

The other option is to buy a 240V generator and carry fuel. Some generators are fitted with a relatively low-powered battery charging facility. It is possible to improve on this by plugging a suitable 240V battery charger into the 240V output. A 240V AC, 40A lithium capable battery charger would require a sufficient generator rating to fully power it and to also run an air-conditioning unit at the same time. The generator capacity would likely be 3kVA or more. You should note that generators of this size may not be used at some campsites due to noise level restrictions, or alternatively you may be located in an isolated ‘naughty corner’.

A SOLAR SCENARIO

To get a feel for the overall process, let’s consider a simple installation capable of running a portable fridge and a few other modest items for a week at Mackay in Queensland during September, with good  solar weather for most of the week, but which would also cope with two cloudy days that only produce half solar power. We will examine this scenario with one installation that uses lithium (LiFePO4) battery power and compare it with another that employs AGM battery power. We will start our holiday with fully charged batteries but accept that our systems may only last for a week before the batteries need to be recharged with a 240V mains battery charger. In other words, the installations will be capable of running for a fine week with two half-power solar days.

Our power requirements will be the same in each case. The fridge will use around 1 to 2Ah, each hour, so let’s settle on 1.5Ah (i.e. 36Ah/day) for the fridge and add 1Ah (i.e. 24Ah/day) for everything else. This means that our total daily consumption is 60Ah/day.

Let us also assume that ‘Murphy’ has struck and our holiday starts out with the two half-power solar days followed by five full-power solar days.

Referring to the Daily Solar Energy Calculator, Mackay in September produces an average power of 5.213kWh/ sqm/day. A 100W solar panel would produce 521.3Wh in an average full-power solar day or 260.65Wh in an average half-power day. At a battery voltage of 12.8V, this works out at 260.65/12.8 = 20.36 (or roughly 20) ampere hours per day. Considering a 100Ah LiFePO4 battery, with our daily consumption of (-)60Ah and our (+)20Ah recharge, our remaining battery capacity is 100 – 40 or 60Ah. This means that we could manage a bit more than two days at half solar power. If we have good solar weather for the rest of the week, we can enjoy our holiday easily.

Loch Ard Gorge area, Great Ocean Road, Vic

If you have a 100Ah AGM battery, it only has a usable capacity of 50Ah. Given our daily consumption of (-)60Ah and a (+)20Ah recharge, we end up with 50 – 40 = 10Ah remaining capacity, at best. The matter is a bit more complex than this because the bulk charging phase for an AGM battery only restores around 80 per cent of battery  capacity and then enters the absorption phase, which can take an additional four hours or more to complete the full charging process. If we routinely go beyond the 50 per cent discharge figure or do not fully recharge the battery, we can shorten the AGM battery life. We are dealing with a very marginal situation in this case, and it would be lucky to survive for one half-power day, let alone two. It is also good to recall that the Daily Solar Energy Calculator model was based on the performance of horizontally mounted solar panels and the situation could be improved by using portable panels to track the sun. If our two poor solar days are separated by one or more good solar days, we may just make it. The safe way out of this is to either increase battery capacity and solar panel wattage or convert to a LifePO4 installation. A mains-powered site also remains an option.

The AGM model employed here could be significantly enhanced by adding another 100Ah battery and using an additional 100W solar panel.

Converting an existing AGM installation to lithium operation would require purchase of at least a 100Ah LifePO4 battery, a lithium capable DC/DC charger with solar charging capability and a 240V lithium battery charger. If your power needs increase over time, such a system could be easily expanded by adding more batteries and solar panels as required.

YOUR TURN NOW

You can use this process to check out your existing system capability or to aid development of your dream system. Give it a go!

OUR MOTORHOME

Before we had access to the Daily Solar Energy Calculator, we revised our onboard solar power by calculating how much solar panel power we could fit aboard and how much battery power we could carry (see The Wanderer June 2022 edition). We were able to fit two 130W panels to the roof and carry four x 120W portable panels aboard, giving a total of 740W. We were able to cram two x 100Ah LiFePO4 batteries aboard, providing 200Ah of battery storage. During a trip to Mackay in Queensland during September, we discovered that we used around 1kWh of power each day, as measured by our MPPT and were able to fully recharge our batteries before midday in good solar weather. We were lucky that our shotgun design approach matched our needs at the time.

Maria Island, Tas

REFINING OUR UNDERSTANDING

Now we were interested in using the Daily Solar Energy Calculator to see the real picture. We ran the calculator and entered our location, Mackay, and pressed ‘calculate’. This provided us with a realistic average daily solar insolation figure for September  of 5.213kWh per sqm per day. We then multiplied this by our 740W of panels, which revealed that we were able to generate an average of 3857.65Wh or 3.85765kWh of power per day under those conditions. Using our portable panels to track the sun, we could easily generate 4kWh per day in those same conditions, which would be three or four times the amount of power we use each day. Even in atrocious weather, we are still able to generate some power.

In addition, our 200Ah of LiFePO4 batteries would store 200 x 12.8V = 2560Wh or 2.56kWh of power, which means that we could power almost everything from the batteries alone, for more than two days with absolutely no ‘sun’ at all.

Re-running the calculator for June in Hobart reveals that we would, on average, be only able to generate 776.26W each day which is less than our daily power usage figure. This is where your portable panels can help you with tracking the sun and hopefully increasing charge current.

Contemplating ...

SAVE OR SPEND?

Our aim in this exercise was to build the best system for our purposes, which means that we wanted to be able to solar power all our stuff even in extended poor solar weather conditions. We are still limited by how much battery power and solar power we can carry, but the likelihood of us running out of power is now very low.

Looking back in time to when we only had 260W of panels on the roof of our motorhome and 160W of portable panels to charge a usable 100Ah of AGM batteries, we never actually ran out of power. We came close a few times but survived nevertheless and always had the option of going into a caravan park to recharge the batteries, but this never happened. However, we had reached the stage where we were getting close to the time when we needed to spend around $1800 to replace our aging, high grade AGM batteries.

Going to lithium technology required us to purchase LiFePO4 compatible components, including a 30A battery charger, a 20A DC/DC charger, a new MPPT and a single 100Ah LifePO4 battery to roughly mirror the performance that we already had. The $1800 that we had to spend to keep the old system alive came quite close to funding the basic lithium changeover. We could have left it at that and had a small but useful improvement. Part of the benefit was that the LiFePO4 battery could be more rapidly recharged than its predecessor and have a longer service life. 

Carved timber figures, North Tas

We decided that “we may as well be hung for a sheep as a lamb” and bought another 100Ah LiFePO4 battery and four new lightweight 120Ah portable panels which replaced our original heavy 160W portable array. We made up four fibreglass shells to house the new panels, to protect them and to make them easy to deploy. This gave us an extra 320W of panels. We also purchased and installed a Coulomb counting meter to act as our battery fuel gauge so that we can accurately measure the charge status of our now 200Ah LiFePO4 battery bank.

All of this roughly doubled the overall price of our total LiFePO4 upgrade, but all being well, this investment should allow us to comfortably free camp for around 10 years and the consequent savings should go some way towards covering increases in fuel costs, essential supplies and vehicle expenses, allowing us to see more wonderful parts of this country.

Your existing installation and power requirements may be able to be upgraded for less expense and comfortably meet your needs. The calculator and the processes described  should enable you to determine this and make any useful adjustments.

Our system has some limitations. When we need to wash clothes, we simply visit a laundromat. If the weather is excessively hot or cold and we need to run our air-conditioner, we go to a powered site.

Well, enough about setting up our solar power system. The central purpose is to hit the road again, to escape and see many more interesting places and meet more fellow travellers.

Entrance to Macquarie Harbour, Tas, (about six times the size of Sydney Harbour)

LIMITATIONS

This article is intended to convey some information about how we went about using the calculator to help with assessing our system performance and making some changes. It is not a comprehensive engineering appraisal or guide. You should seek independent advice about your system and any changes to it from a reliable automotive electrical professional to avoid any risks to insurance or warranties.

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Written: Thu 01 Sept 2022
Printed: September, 2022
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