How much solar should you have ?

This is a question I often see asked here, and it is a difficult question to answer, there are a number of key factors affecting the answer. First how much power do you plan to use, then also how much power can you store in your battery.

In the following I will try to provide a high level guide, I will do my best to avoid making it technical.

You want solar to provide power to charge your batteries, thus allowing you to use electricity for the things you need basically overnight. Thus you need to have enough battery capacity to be able to store enough energy to keep you going overnight. Indeed most people would like to have enough to keep them going for two or three nights (allows for bad weather).

So the first place to start is to understand how much energy you are going to use in a typical 24 hour period, you would measure this in amp hours (one amp hour is taking one amp of current from your battery for one hour). I will not go into how to calculate your usage here, but for the purpose of the example below lets assume an estimated usage of 40ah per day.

Now the amount of battery you need depends on when your power usage is, for this exercise lets assume it is all between sunset and sunrise, so we need to be able to provide all 40ah from the battery.

How big does the battery need to be?

That depends on the type of battery, lets assume it is a simple lead acid, where you do not want to discharge it below 50% charged, and you typically can not charge it past about 92 or 93% in the length of a solar day, thus a 100ah battery will typically yield about 42ah a day.

Thus with a 40ah per day usage, and a lead acid battery you would just make it through one night with 100ah, you would be better off with at least 200ah to get you close to two days. For the purpose of the example, lets assume we go with 200ah of lead acid battery, we aim to discharge the battery no lower than 50% and we want to build our system to get the battery back above 90% in a good sunny day. To refill the battery we will need to supply at least 80Ah.  

How Much solar do I need?

I would assume five productive hours of sunlight per day, thus we need to divide the power we want to generate by 5, and that tells us how much we need to put in our battery each hour.  

Now batteries do not charge in a linear fashion, thus we can put more in early in the charge dropping down to just a trickle when they get closer to full, so in reality we need to provide more to be able to get close to filling the battery. So I would recommend building a system capable of providing a quarter of our total power requirement each hour, remember it is always better to have too much power.

So in our example we need to supply at least 80ah, I would plan to put 20ah into the battery each hour, that means we need 20amps of current from going into the battery.

In a perfect world that would mean we needed 240 watts of solar (20 * 12 = 240, being Amps * Volts = Watts).

Unfortunately solar sitting flat on a caravan roof is not perfect, so I would assume the solar is only 70% efficient, and we would need to increased how much solar we need to compensate for this.

So now our 240W of solar panel needs to increase to about 340W (240 *100/70 = 342).

Confused?

It is not really that bad, just start out by working out what you need/want to run, how much power they take, and multiply it by the number of hours you want to run it for. Example Laptop charger 2.5 hours at 6amps = 15amp hours), then add them all together.

Then work through the story above, I am not able to tell you the percentage of useable power from the different common batteries being AGM and Gel, (I do not use them, I only use Lithium). But I am sure someone else will provide that information.

Good luck with working it out, and please remember it is always better to have more solar than you need rather than less.