What do you need?

How much solar are you harvesting?


No matter what type of battery you're using, you need enough solar to make up for the amount of power you're taking out of your battery.

Our advice is to install as much solar as you can fit and afford on your roof, but if the budget doesn't permit a splurge on solar panels, or if you just want to know if you're breaking even, this page may help.


You can pick a RV traveller quite often in shops - they pick up an appliance that they're interested in, and the first thing they do is turn it over.  Why?  To check its power draw.

Power consumption is important in an RV and you probably already know what you use.  But how much power to do make, and is it enough?

The question of how much energy you can harvest from your solar can get very complicated, factoring in the time of year, how much solar energy each part of Australia gets over the course of a year, the longitude and latitude of where you are.  It's enough to make your head spin.

So we've come up with a method that we think is a bit simpler.  It's not going to be accurate down to the last amp, but the weather doesn't have a habit of being too consistent either.  First, let me explain some terminology we use for this formula (I've thrown in some example figures too), and then we'll look at how all that information can be put into a simple formula to calculate your solar harvest.


AdvW
Advertised capacity of solar panel
This is the number of watts your panel is rated at.  If you look at the label on the back of your panel, this is the Maximum Power Rated figure and it's expressed in watts.
100
LHLikely harvestWith your solar panels mounted flat on the roof, don't expect to see more than 70% of the advertised capacity each day.0.7
VYour system voltageMost RVs run 12 volt DC connections, but quite a few run 24 volt.12
PSHPeak sun hoursPeak sun hours are all the hours from sunrise to sunset, concentrated as if the sun were directly overhead that whole time.  Obviously the sun moves, so the effective peak sun hours you will get in summer amounts to about 5 hours, and the effective peak sun hours you will get in winter is about 2.5 hours.  The further south you travel, the lower those peak sun hours become.
5


The formula looks like this:

For example, a 100 watt solar panel, flat mounted on the roof will give you almost 30 amps per summer day:


That same 100W panel in winter:


Things that will affect your solar harvest

  • If your panels are tilted toward the sun, you will get a better harvest.  If you track the sun, you will almost double your harvest.  This is where portable panels have an advantage, but only if you take the time to move them periodically throughout the day.
  • If your panels are dirty or there's a shadow from a power line or a tree over them, you'll severely limit your harvest, or if the situation is bad (the shadow of a power line across the entire panel for example), you will lose the output from that panel entirely.
  • Severe heat will adversely affect the harvest from your panels.
  • You know those days where there's a lot of cloud about, but somehow also a lot of glare outside - enough to make your eyes hurt?  Those are actually very good solar days.

How much solar do you need?

If you're using lead acid batteries

If you're using lead acid batteries, you need roughly one watt of solar for every amp hour of battery, just to keep your battery alive.  You then need to factor in the amount of power you're using and add this to your solar requirement.

As an example, let's say you have 240Ah of lead acid battery, which means you can use a maximum of 120Ah.  If you use 120Ah in a day, you'll have to replace 150Ah.  The reason for this is the inefficency involved in recharging lead acid batteries.

What this means in terms of your daily charging routine is that you need your lead acid battery to be above 14 volts by lunchtime.  This is so that the remainder of the day's solar is used to do the absorption stage of battery charging.  If you don't reach 14 volts by lunchtime, you're steadily going backwards.  In other words, you don't have enough solar and you are damaging your batteries.

If you're using a  lithium battery

There is no need to bring a lithium battery up to any particular charge by the end of the day as they do not lose voltage as their charge drops.  You simply need to know your power consumption and you need to be able to more or less replace it each day.


There is no such thing as too much solar.