Words and Images by: Collyn Rivers N8054

As electricity charges escalate, a few caravan and motorhome owners are using their RV solar systems to supply home base electrical power whilst parked between trips.

Bruce Bailey (W65365) uses the substantial output from his converted Hino bus solar system to assist power his home when he is not on the road.

We did similarly whilst living near Broome. Our Nissan Patrol’s 200 watt system ran a, permanently on, 60 litre Engel fridge/freeze to hold our (since deceased) Great Dane’s many kilos of dog meat between 50 km return trips into town. We used the Tvan’s 50 watt system for outside (LED) lighting.

Environmental architect and CMCA Member (N382) Dick Clarke’s solar powered A-class catamaran’s 500 watt (24 volt) output powers computers and printers, via a stand-alone Redarc 700 watt inverter, in his Sydney office. As power is needed only during the day a 55 Ah (24 volt) battery amply copes. At weekends the boat often goes cruising.

If an RV’s 230 volt inverter output is to be connected into a home’s fixed wiring, including the cabling to it, the design must be done by a licensed electrician, and installation and associated work likewise.

This is a complex area as it requires a specific type of inverter plus an  obligatory break-before-make switch to isolate the input from the grid supply. This may limit usage to lighting and similar circuits only, so that grid power is still available for heavy loads.

There are also complications such as our obligatory domestic requirement to have the MEN (Multiple Earth Neutral) link located within those premises. With Australian and New Zealand RVs however, that link is formed within the caravan park’s or other power source. This is done for safety reasons related to supply cable protection etc. Knowing how to handle this requires serious expertise.

MCA Member (N382) Dick Clarke’s solar powered boat in Myall Lake, NSW.       

12/120 Volts dc.

A simpler and safer approach is to use 12 volt or (preferably 24 volt) power directly from the solar charged battery.

Applications might include pumping rainwater to an elevated tank, local LED lighting (using two 12 volt LEDs in series at 24 volts if necessary) or, as Dick does, to assist power his office. This is easy to do and legal to do yourself - if at less than 120 volts dc.

Voltage drop is of concern, but eased by using 24 volt or more volts as for the same power, not only is current halved (from its 12 volt equivalent) but, as acceptable voltage drop is a function of percentage of that voltage, this enables cable at 24 volts to be only a quarter the size of that at 12 volts.

As an example, the acceptable voltage drop, for uses such as this, at 12 volts is 3% (and thus 0.34 volt). That for 24 volts is still  3% (thus 0.68 volt). That alone enables cable size to be halved, but as there is half the current for the same power at twice the voltage, cable size can be halved yet again, i.e. 25% of 12 volt size.

At 48 volts it need be only 12.5% the size as at 12 volts.

Another approach, if power is needed at a distance, is to use a dc-dc voltage converter to bring that dc voltage up to whatever you wish (as long  as under 120 volts dc). This enables 2.5 mm2 domestic 230 volt cable to be used over at least 50 metres. Then to charge batteries (at the usage end) via an MPPT solar regulator. Some accept 120 volts dc or more, yet can be programmed to charge 12, 24 or 48 volt batteries.

Yet another way, if you have solar modules in series, and an MPPT solar controller in the RV, is to take the feed directly from the highish voltage solar module output to that remote MPPT solar regulator. 

There’s any number of such alternatives that do not involve 230 V.

Time Shifting Solar

At our present grid-connected home in Church Point (Sydney), we are experimenting with ways of time-shifting the daytime excess (otherwise exported) solar input by about five hours so that it can be stored for use after dark, and to run our two super efficient fridges all night.

This is worth considering in states such as NSW where users (of post June 2011 systems) currently receive a mere 7.7 cents or so per kWh, and only for energy exported. Depending on time of day, that from the grid costs 14-50 cents a kWh. An option, which we currently use, is to opt for the flat rate (in our case) of about 24 cents per kWh.

There’s many ways of doing this, including charging batteries from surplus daily solar, and using battery power at night. This is also handy back-up for loads (such as computers and lighting) during power outages.

Whilst tempting to disconnect from grid, it currently makes more sense (money wise) to store about 14 hours energy, and to use the grid as a virtual battery during times of little solar input.

This may change if lithium batteries become affordable, their longevity proven rather than forecast, and electricity rises above $1.00 a unit. But right now this seems only worth doing if existing energy usage is reduced below 10 kWh or so a day, and gas used for cooking.

Doing this is readily possible. Our current home’s previous energy usage exceeded 24 kWh/day. With little effort, and despite adding a second 320 litre fridge, we reduced that to 6.0 kWh in summer and 7.0 kWh in winter. Further tweaking will enable us to reduce that by a further 1.0 kWh/day.

The present solar system, a nominal 2.4 kW and an actual 2.2 kW, produces an average 5.5 kWh/day in  mid winter and up to 17.5 kWh/day from April to October.

Several companies already offer virtually ‘turn the key’ packaged systems for this purpose that nevertheless must be connected by a licensed electrician.

As with RV solar, this can also be DIY done (preferably) at 24 volts to reduce energy loss, via an efficient battery charger, with the battery bank configured to run whatever required.

(Please note that copyright of this article is the property of Caravan & Motorhome Books.)

Collyn Rivers

Category: Technology
Written: Thu 01 Aug 2013
Printed: August, 2013
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