I’m no expert on Solar, if you want to run a microwave or espresso machine while watching satelite TV and drying your hair, then go see a Solar Company for serious advice, but visit the bank manager first. As our power requirements are low I’m going to keep my install very basic.
There are 3 things to consider :
1. How much power do you require each day
2. How much power can your battery store
3. How many sunshine hours do you expect each day.
The confusing thing with working out the power requirements is some devices show the amperage and some show Watts. Batteries are rated in AH (Amp Hour) capacity, and solar panels in Watts. So let’s work out everything in Watts.
We all learnt about power in High School, but if you had no life interest in electronics it probably got flushed out of the grey matter along with log, sin, cos from Math class. Power = volts x amps (current). It doesn’t matter whether its 12v, 120v, 220v, 110kV. Power is power, so for a 45W laptop it needs 0.37 amps at 120V. But your 12V invertor will need draw 3.75 Amps from your battery to generate this (maybe a bit more due to conversion losses). But at the end of the day its still 45W, so convert everything to watts.
Amp Hours is the current capacity of the battery discharged over a 10 or 20 hour period. So for a 105 AH battery (the size that came with our RV), that’s 5.25 amps per hour for 20 hours. Higher discharge rates will shorten the time. To keep it simple let’s just say the wattage of the battery is 12 volts x 105 amps = 1260 watts. Or more correctly 63 Watts per hour over 20 hours.
Then consider the devices that will use power over a day.
45W laptop for 3 hours = 135W
2x 10.5W USB chargers for 2 hours = 42W,
20W lighting for 2 hours = 40w,
There are other items to consider too,
the water pump(90W)
kitchen extractor fan (40W)
say both run for 10 minutes per day. That’s 15W for water and 6W for the kitchen fan.
If you have a Dometic propane fridge it will also consume 12v power for the regulation control, and in fact has a 12v heater strip between the fridge and freezer compartments. If you want to maximise your 12v capacity consider disconnecting the red wire to the Light/Heater connection on wiring panel behind the external inspection hatch – will save you about 12W x 24 hours = a massive 288W per day. Still, after removing that link you should still allow 80W per day (guess) for the regulator and fan.
All up the total load is 318W
So for our 105AH battery, in perfect condition, should run about 3 -4 days without top up. To keep it fully charged we need to replace the 318W everyday. Choosing the right size panel depends on how long to expect to get full sunshine, no clouds, no shade If you can get 6 hours good sunshine then a 60 watt panel would do. I’m planning on 3 hours per day, so a 120W panel is required.
The Solar panel will output 18V, so the manufacturers current rating stated is based on 18V. The solar charge controller will manage the charge rate to suit your battery type. There is no point in buying a 300W array of Solar if your batteries cant store the power while the sun shines unless you have a full on charger / invertor package like a Meanwell 1500 – that can divert surplus solar power direct to the invertor while still charging the battery. So we decided on a foldable 140w panel with built in controller. Solarandwindtechnology from amazon.ca CAD$370. So in perfect conditions 3 hours charging should replenish a days use. Sometimes there will be cloud or shade that will reduce the output, so having a slightly larger panel is a safe option. It’s fairly large, I wouldn’t recommend going any bigger for a foldable panel.
We didn’t go for a fixed mount on the roof. If the RV is parked in the shade – or partial shade – the output is reduced dramatically. However the advantage of a roof mounted panel is it charges during the day where ever you are parked, Walmart for example, so you can get longer daylight charging, so a smaller cheaper panel can be used. It all depends on the way you travel – moving every day or dry camping in one spot for 5 days. Also the tilt bracket means we can optimise the angle to the sun – but it does need to be moved occasionally to follow the sun during the day. The charge controller did not have a digital readout, nor did our RV battery monitor, so I got a simple 12V plug in digital display to see the voltage more accurately. 13.5 is the ideal charge voltage for AGM batteries.
This is as basic as you can get. At least I can see the voltage increase as the battery charges. For larger installs you would want to see current flow from the panels, battery charge current and load current. Its no good having that on the controller, as it will be mounted near the battery – you need an remote display or a Bluetooth app to your cell phone / tablet. The Meanwell 1500 looks like a good place to start – but check out other reviews.
To make it easy to connect I cut off the clips that came with the panel and used some Anderson connectors to a lead terminated to the distribution panel under seat.
For the 120v inverter we got a 300w invertor from Canadian Tire, CAD$70. Not that we should ever need to run that much – that’s 33 amps at 12V – our battery would be dead in a couple of hours !!! .
So all up our Solar kit was under CAD$500. The Honda generator, although it could run the microwave and charge the batteries, was $2500, so for what we need we hope the solar panel will do the job.
This little converter was well worth getting, converts 12v to other DC voltages for direct connection to a laptop or other DC device with out the need to use an inverter
Don’t run your battery till stuff stops working. Standard batteries should only be discharged to 12v. AGM batteries can be discharged to 11.8v. After that you will damage the battery.