A while ago we wrote about power calculations and we are going to revise this in this post.
If you want to look back at that post, you can find it here: POWER – calculation — RenewSolar
We take a long look at power needed within the home if we intend for the system to be off grid or have a zero grid import. There are key points to work out, which include the actual loads and when they occur, the peak load, and also the seasonal changes. The power needed and available will vary, but we are pretty accurate with our math as we have many many years experience.
Load:
The load on a system is the usage, this is split into two parts, the normal load and the peak load. To translate this to plain English, this means how much power you use per hour and what is the most power that you use at any one time.
PV ( array power yield):
Solar power and what is claimed to be the power do vary, Most panels are rated as STC ( Standard Test Conditions) but more so you will see NOCT ( Normal Operating Test Conditions). NOCT are more like the real world when it comes to solar panels and how they perform. But we use a general rule that we take off 35% from the STC rating for roof mounted panels.
As the seasons change the power from the sun will rise and fall. You can read more about this in this post about angles There are 3 Articles in relation to solar panels and setting them up correctly.
As you can see from the image above, you have a voltage and current curve, the current relates to the power of the sun, so in winter you may see 200w per meter squared and in summer 1000w, you may see higher and lower numbers depending where you are and what season you are in. This is important to work out how much power you will produce over the year at given times.
The law:
The law I am referring to is Ohms law, it is the preservation of energy and how this effects solar production.
Voltage is fairly uniform and easy when it comes to solar, however current relies on the sun.
When applying the law, voltage will translate to current. the best way to explain this is like this:
100v and 1a is 100×1 This gives you watts. so the answer is 100W
50v and 2a is 50×2 which again is 100w.
as we rely on the sun to give current, in winter when we struggle to make amps a low voltage system will suffer more with power.
The power conversion which I have explained a little more in this article called power factoring may be helpful. But here is a quick example:
200v 2a solar, going to charge a 24v battery.
200/24 = 8.33
Therefore we then multiply 2a by 8.33 giving us 16.33a
As you can see the current goes from 2amps to 16 amps as the voltage is converted to current.
Sun Hours:
Sun hours is the next important part of the math. This accounts for the time we make the most power from solar.
Again the angles will dictate the peak times of the array and the season will dictate the number of hours, and the array angle also matters.
We use 275 days of the year to do the annual calculation as for the most part we have 275 sunny days and 90 days that are grey.
6 hours is a good average but sun hours can reach 8 hours or be as low as 2 in winter. find out more about Sun hours.
What you get to what you need:
Understanding what power you can get from solar and when plays a big part in how the system will work for you.
In the UK it is fairly safe to say that solar is feast and famine. You have to consider where the value point is for you, it maybe that you ignore solar and work on energy trading where you buy power at cheap rate and use it all day, therefore avoiding the additional costs. Most of our batteries information page will tell you the values and returns.
We often also look at token solar. This is where solar just adds to the power rather than being a main source. typically the solar only meets the normal day time load. This is polls apart from systems there the solar needs to charge the battery and provide the load, which will depend on the usage of power.
Battery charge and use:
When you have a solar battery most will just average their power use and that is the battery that they want. it can work, but this is not how it works in reality.
As a buffer, the battery provides power for when the sun hides behind clouds and the solar cannot provide the load. This means that the battery is in use all of the time, rather than when the sun has set until the next dawn.
Most homes will have a good sized solar battery storage of around 15kWh, The battery must be charged, so if we use this power from the battery, we know that we need to make 15kWh to charge it. But when you have solar the day time load can be taken off the daily total, therefore leaving some power reserve in the battery, but we have to consider the buffer use and poor solar production, as in winter months.
The math to address the solar array size is going to be 15/4 which is 3.75kw per hour.
In winter we can then work out that we could make 7.5kWh of power from the same array. This is however compounded as we need to cover the day time loads, which vanish into solar during summer.
For example 600w base load is 4.8kWh over 8 sun hours and 14.4kWh over a 24 hour period
Of course a home will not have a base load without peaks and drops, For example in my home the power over night is around 111w, in the day is around 570w and peaks are around 3.44kWh.
All these times are pretty normal for most homes where you can group the loads into these time zones. The solar production times can also can be calculated over the zones and this will effect the power needs that you will have at the time together with the charge and discharge of the battery through the year.
Going back to the math, we know that for 4 hours of sun in the later part of the year, we will need to make 3.75kwh to charge the battery, we also will have a base load of 570w which has to be accounted for This moving the solar power need to 4.32kWh
You may see now that we recommend a 4.6kw solar array, with 5.6 being better as we have to take off ratings for roof mounted systems.
All of this information is how we calculate the power needs and system requirements when installing solar as part of our project management service. and why we do ask a little more detail for the power use and within periods of time, so that we can best match you to a system that works for you.
The inverter:
Your inverter size is a matter of balance. the rating should meet most of the needs that you have in terms of the peak power. if you have 3 hours in the evening where the loads are 3kw then a 3.6 G98 will work for you.
lets say you have a power shower and that is 8kw. You will use solar for the 3.6kw and then grid will supply 4.4kw
If you are not on the grid then you have to look at the peak load that could be possible and what you can accommodate, as you can limit what high use items are on at anyone time.
if you want to test yourself for an inverter, we can send out a programed box which can be set at a inverter level and will alarm when you exceed this load.
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