Why Your Home Solar System Might Cut Out, Even in Bright Sun Shine

Given the rush for everyone who can afford it to install solar power systems on their homes, due to terrible load shedding, and it should be remembered that many cannot afford it, it might be useful to discuss how those systems supply and store energy.

The thing to remember in a home solar system is that the battery bank is the actual source of power, when the council power is not available. Your solar panels just charge those batteries.

One way to envision this is think of consuming water from a dam. Where the water stored in the dam is analogous to the energy stored in your batteries. In a domestic solar system, water can flow into that dam through two different streams. One stream comes from the council, and another comes from the solar panels. Your inverter automatically manages which stream feeds the dam. When solar power is available, then the flow into the dam from the council is switched off and water only flows in via the solar panel “stream”. The flow from solar panels can be intermittent, and vary significantly in strength, depending on the weather, so the amount of “water” (energy) flowing in each day from the solar panels will vary.

When there is no solar power available, then the council flow into the dam is switched back on again. In this way the dam is kept topped up from either of those two sources, depending on the availability of solar energy.

Now down stream from the dam, you as the home user, consumes water from one of two separate sources, the one “stream” bypasses the dam, coming directly from the council, the other is from the dam itself. You can set the inverter when to switch between the two.

Now, in order to protect that dam (battery) from damage, there are rules programmed into the inverter that when the dam (battery) reaches a certain level of emptiness, then the source of supply to your home, is switched from the dam, back to the council supply. This is regardless of water flowing into the dam at the other end.

But if at that time, there is no supply from the council, you will be left without any power supply, until either the council power comes back on, or the inflow to the batteries tops them up to a much higher level, which satisfies the rules set on the inverter (as per the battery supplier’s recommendations).

Many people do not realise, or understand this problem.

The best approach here is simply being quite conservative when estimating how much energy you intend to consume every day (solar systems cycle daily), and then making sure you have sufficient battery storage available. You need to take into account two issues when sizing your battery (once you have a good estimate of how much energy you will drawing from them daily).

• Batteries should only be discharged down to a certain level to not become damaged, so only a portion of the total battery storage figure quoted should be used,
• Your system will have an overall efficiency rating which effectively means a proportion of the stored energy available will be lost in the form of heat.

Both these factors reduce the actual useful capacity of your battery, and that needs to be factored into your calculations.

So in general, one should keep in mind that any circumstances which result in the dam (battery) being drained more than expected, can lead to this problem of unexpected power cut off, even in bright sun shine.

One of the pervasive problems with home solar systems is users are often reluctant to be completely honest with themselves about how much energy they plan to use each day, they tend to underestimate that, or be reluctant to even make any estimates at all. In many cases they expect the solar system supplier/installer to do that for them.

But the fact is we all have very different energy consumption behaviours. We have all got used to the plug in the wall supplying an infinite amount of energy, when ever we need it. That difference in consumption behaviour, between different people only becomes relevant much later, when we pay the bill, and there usually are no actual limitations imposed upon us at the time of consumption.

It can come as a bit of shock to some people, that once they install a solar system on their home, that at certain times, they are now being supplied by a very finite source of energy, dictated by the size, and condition (batteries wear out), of their battery bank. Once they drain that stored energy, then their inverter will cut off to protect the battery, and thus leave them with no power if the council power happens to be off at the time (unless they have a generator backup or something).