Calculating a Solar Power System

A Quick Guide:

Please use the following quick guide when calculating your solar power system– a more detailed version of the guide (complete with explanations and examples) can be found below.

(1)  Estimate energy usage (per day)

(1a) Calculate the total power consumption (Watts), and divide the total power consumption by Volts to give Amps

(1b) Estimate the total hours of use per day/night (how long will it be on?)

(1c) Calculate the total Watt/hours and total Amp/hours


(2)  Select Solar Panels

(2a) Calculate the daily average sunshine hours for your location

(2b) Double the Watt/hours (estimated in 1a above)

(2c) Divide Watt/hours by the average daily sunshine hours

(2d) Choose your solar panels


(3)  Select batteries

(3a) Double the Amp/hours (you estimated in 1c above)

(3b) Choose the redundancy level of your storage potential

(3c) Choose battery (or number of batteries)

Solar Kit

Expanded Guide:

To work out your solar power system, you need to know what your energy use will be. This is a combination of how many lights you are running and how long they will be running for. In order to do this, you will need to estimate the total hours of use per day (i.e. if you are running LED lights, how long will you need the lights to run each day/night?).

Think about how critical it is for you to have your system operating for your estimated time all year round. In other words, does it matter in winter, when the daylight hours (and thus solar charging potential) are shorter, if your system operates for only half your estimated time? If it is for garden lights for example, it may not be an issue if it doesn’t operate for a day or two in the winter. Alternatively, if the system is for lighting in a shed that is used every night/morning, it must have capacity to operate for the desired time in the dead of winter when charging potential is at its lowest.

What we are considering here is the redundancy of the system – more redundancy gives you more reliability (less chance of running out during winter), but will cost more (in extra/bigger solar panels and batteries). While having a system with less redundancy will be cheaper to set up, it will have less reliability (more potential to run out of power in the winter when there are fewer sunshine hours). In a solar power system, the two main areas where we can build in redundancy are the charging potential (where we consider the potential sunshine hours of your location and the number/size of solar panels) and the storage potential (where we consider the size/number of batteries in your system).

In the following calculation for a solar power system, we will build in a reasonable amount of redundancy; so that your system will be able to provide the energy you need, for the daily number of hours you need, through the middle of winter (to handle 2-3 days in a row with 0 sunshine/charging hours). If it is critical for your system to operate as specified (providing the required energy for the required time per day – such as for a work-shed that is used daily/nightly), then you may want to consider adding in more redundancy. If it is not critical for your system to operate as specified, such as a system for garden lights, then you may consider reducing redundancy (to lower the overall cost of the system). As you go through the calculation below, we will include notes to indicate where/how you can add or reduce the redundancy of your system from the ‘reasonable’ level we have set.


1) Estimate energy usage (per day)

a) Calculate the total power consumption (Watts), and divide the total power consumption by Volts to give Amps

E.g., if you are running 1.5m of strip light that draws 14.5W per meter, you have a total power consumption of 21.75W. If the strip is 12V, your Amps are calculated as 21.75W divided by 12V, which equals 1.8125 Amps.

Note: Overestimating energy usage, and/or rounding up your calculation of Watts/Amps, will increase your system’s redundancy.


b) Estimate the total hours of use per day/night (how long will it be on?)

It is best to overestimate how long it will be on – and to take into account the fact that we generally have lighting on for longer over winter (when the daylight hours are shorter). E.g., We want the light on from dusk to 10pm, which in winter is from about 5pm for 5 hours.

Note: The more you overestimate your daily use, the more redundancy you build into your solar power system.

c) Calculate the total Watt/hours and total Amp/hours

For convenience, we will use Watt/hours to calculate the number of solar panels, and Amp/hours to work out the battery size. To calculate Watt/hours, take the Watts you calculated in (a) above, and multiply that value by the total number of hours per day/night from (b) above. To calculate Amp/hours, take the Amps you calculated in (a) above and multiply that value by the total number of hours per day/night from (b) above. E.g., Watt/hours = 21.75W x 5 hours = 108.75 Watt/hours. Amp/hours = 1.8125A x 5 hours = 9.06 Amp/hours.


2) Select Solar Panels

a) Calculate the daily average sunshine hours for your location

If you are located in New Zealand, the following website has graphs of the average sunshine hours per month for various locations:

Choose a location that is closest to you, and divide the monthly sunshine hours by the number of days in the month to work out the average daily sunshine hours for a given month. To build the most robust solar system that will provide the energy you need through the short days of winter, we suggest that you base your calculation of average sunshine hours on the month of June (the month with the lowest average sunshine hours in New Zealand). Also, you need to keep in mind where you are going to put the solar panels, and any potential shading (from trees or buildings) they may get through the year (as the sun changes angle in the sky). If your solar panels will be located where they will get some shading, you need to take into account how much shading they will get on average, and lower your daily average sunshine hours accordingly. E.g., Hamilton in June has an average of 112.80 hours of sunshine. We divide that number by the number of days in June (30), to get a value of 3.76 for our average daily sunshine hours.

Note: You can add or reduce redundancy here by reducing or increasing this value, i.e., rounding the value down, to 3, will increase redundancy, while rounding the value up, to 4, will decrease the redundancy.


b) Double the Watt/hours (estimated in 1a above)

Your solar power system will have enough ‘extra’ storage (batteries) to provide your required energy use for a few days without charging (more on this in the ‘battery’ section). In the middle of winter there will be some days with less sunshine hours than average, and so there will be times when you start draining the ‘extra’ storage. When this happens, you need to have enough charging potential to cover your usual days use, and to replace the ‘extra’ energy you had stored. By doubling the Watt/hours of estimated use, and selecting solar panels based on this doubled value, we are creating a solar power system that can produce enough energy on one average day of sunshine hours to cover two days of your estimated usage. E.g., We estimated our daily use to be 108.75 Watt/hours, and so will double that to be 217.5 Watt/hours, and use this doubled value in our calculation for solar panels.

Note: You can decrease or increase the redundancy of the charging potential in your system by decreasing or increasing the Watt/hour value you use in your calculation.


c) Divide Watt/hours by the average daily sunshine hours

Here we are working out how many watts of energy we will need to generate per hour. E.g., The Watt/hour value we are working with is 217.5 (from 2b above), and the average daily sunshine hour value we are working with is 3.76 (from 2a above), giving us a value of 57.85W of energy we need to generate per hour with our solar panels.


d) Choose your solar panels

Depending on the particular solar panel(s) and solar controller you choose, there will be some charging inefficiencies, and in general your system will have some losses. As a way of taking these things into account we can calculate our solar system based on the solar panels operating at two thirds of their rating – so for a solar panel rated 30W, we will assume (for our calculation) that it produces 20W of energy per hour (in ideal lighting). We then calculate how many panels we need to produce the energy per hour (calculated in 2c above). E.g., We need our system to generate 57.85W of energy per hour, and we are going to use solar panels rated to 30W (20W for our calculation). 3 x 20W = 60W, which is just above our value of 57.85W. So, by these calculations, 3 x 30W solar panels should provide enough charging potential for our system.


3) Select batteries

a) Double the Amp/hours (estimated in 1c above)

You should never run your batteries down until they are completely depleted, as this will damage them and shorten their life. As a general rule, you should plan to not run your batteries down below 50%, and so to calculate your battery size, we begin by doubling the Amp/hours you have estimated will be used in one day. E.g., We estimated (in 1c above) that we would use 9.06 Amp/hours for one day, so doubling that value gives us 18.12 Amp/hours to use in our calculation.


b) Choose the redundancy level of your storage potential

We have already calculated what battery capacity we need to provide us the energy for one day, but if in winter we get a couple (or more) days of less than average sunshine hours, we risk running out. To add redundancy, and make our system more robust, we will add more days’ worth of storage potential. We will base our calculations on a system that has enough storage potential for three days of our estimated use. E.g., In 3a (above) we calculated the storage capacity (Amp/hours) we need for one day’s use, so we take this value and multiply it by three to give us the storage capacity we need for three days use. 18.12 Amp/hours x 3 days = 54.36 Amp/hours.

Note: If your system is quite critical, such as a milking shed that must have lights every day during the middle of winter, you could increase redundancy by calculating to have storage potential for four or five days’ use. While if your system is not critical, such as garden lighting, then you could calculate for a system that only has storage potential for one day’s use.


c) Choose battery (or number of batteries)

In 3b (above) we calculated the storage capacity we need for our solar power system, now we just need to get 1 (or more) battery(s) that have a greater storage capacity than the Amp/hour value we worked out in 3b (above). E.g., We calculated that our system needs 54.36 Amp/hours of storage capacity. At LEDstuff, we sell 7Ah & 18Ah batteries. 3 x 18Ah batteries will give us 54Ah of storage capacity. As our system is not critical, we would be okay with this small 0.36Ah shortage.

Note: If your system is critical you may decide that the cost of another battery (such as a 7A/h) is less than the cost of potentially running a bit short in the middle of winter (and doing so will build in more redundancy to your solar system).