Sizing an Off-grid Solar Power System

There are many considerations when specifying the components for an off-grid solar power system, these include:

• Load
• Battery type
• Global location
• Local temperature
• Solar grid size
• Inverter type
• etc

I spent several years installing small scale (single and dual and 4 panel) systems for powering roadside driver information systems in Ireland.

This instructable will attempt to guide you through some of the obvious and not so obvious considerations. The last step will be the provision of a downloadable excel sheet to simplfy the process.

We are looking here at an off grid system, this means that there is no mains alternative.
There are mains/generator backed systems as well as assistive systems where the mains and solar power system work in tandem, the considerations and calculations differ for these systems.

Often, when designing a power system, people are tempted to start at the generation source, i.e. the solar panels. This is fine if you are going for a top down system and will only load to the available capacity.

However, if you have a load, house or shed in mind, then chances are you want to build in capacity for all of your appliances. So this is where we will start.

Every mains powered device will have a sticker somewhere that indicates its power requirement in watts, you need to identify all of the loads (appliances) and record their power requirements.

Now make a map of what devices are likely to be on at the same time (it would be a rare event that everything you own would be turned on at the same time)

This total is now your peak load, we will carry this forward to our next step.

Chances are that most (if not all) your appliances are mains powered AC loads. Your output from your solar power system will be DC.

In order to convert this you will need an inverter.

In the last step, we calculated the peak load. This will inform the continuous load rating of the inverter, there will also be a peak load rating on the inverter, this is the amount of power that can be supplied for a short duration, loads, particularly those with motors will have a large current inrush on startup and this needs to be accommodated.

There are 3 types of inverters to look for:

1. Square wave inverter: these are usually the cheapest type, they may be used to power some tools and universal motors but generally won't work with more complex electronics.
2. Modified sine wave inverter: these are an evolution of the square wave inverter, the wave is still squarish, but there are additional steps in the wave to emulate a sine wave, these will work with most equipment.
3. True sine wave inverters: these are the most expensive type but will most accurately emulate the power that comes from a generator of the power company, they will work with all types of equipement.

Batteries are at the heart of the solar power system, these are the storage for all of that lovely solar energy you have collected. Without batteries, you can still have a solar power system but you must use the power as it is collected and depending on the conditions, the output may be unstable.

There are many types of batteries, however, due to cost and practicality, normally only Sealed Lead Acid (SLA) and Gel batteries are considered.

Typically you will select between 12V & 24V cells with12V being more common.

Batteries will come with a capacity rating in amp hours (Ah), to work out how long one battery will run a load first find the wattage for the load, to keep it simple we will call this load 100W.

Our battery is 12V with 50Ah capacity.

The Wh capacity of our battery is 12V * 50Ah = 600Wh

So from 100% charge to 0% charge in the battery, our run time is: 600Wh capacity/100W load = 6 hours run time, easy huh? well, no..

In general, batteries will have a discharge curve, meaning that as the capacity of the battery declines, so too does the batteries ability to push that charge. The amount of battery used is called the depth of discharge (DOD) so a battery at 100% charge is @ 0% DOD, a completely empty battery is @ 100% DOD.

Typically batteries do not deal well with deep discharge (more than 75% DOD), some may never recover a charge and for others, the overall life cycle of the battery will be reduced.

There are guidelines for each battery type but a general rule would be to prevent discharge beyond 50% DOD.

So now our run time is gone from 6 to 3 hours... oh wait, there's more

This value is dependent on some other factors, if the load is AC then we are converting the current from DC to AC via the inverter discussed on the last step but there are losses associated with the conversion, these losses range from 5 -15%, there are also losses in the wiring so depending on the spec of wiring on both the DC and AC side there may be additional losses of 2 - 6%.

Taking the solar element out of the equation for a moment, if our 100W load from above was AC and we stuck with the 12V 50Ah battery with an inverter, at a minimum we would expect to see an additional 7W (this could be up to 21W in this case) load hitting the battery so:

600Wh capacity - 50% for DOD = 300Wh/(100W load + 7W losses) = 2.8 hours run time.

Ambient temperature can also have an effect, I have never been able to find a good calculation for how temperature relates in numbers to discharge rate but the rule would be colder batteries discharge faster and warm batteries slower. However colder batteries will have a longer life cycle (more charge cycles) than hot ones. For every 8.3 deg C increase in temperature, the life expectancy of a battery reduces by 50% (reference)

The solar controller is a vital component of the solar power system.

It is responsible for taking the fluctuating power from the solar panels and converting it into usable power both for direct use and for charging the battery bank.

There are many different types of solar regulators, the ones I would recommend sit as a central controller in the system. There are 3 sets of terminals:

1. Input from solar panels
2. Output/input to and from battery bank
3. Output to load

The main reason for using a solar controller is to protect the batteries. The controller does this is several ways.

Using voltage and current monitoring it changes the charging characteristics for the battery bank, adjusting for a deep charge, maintenance, and float charging. The controller will prevent over and under charge on the battery by stopping the charge or cutting load, this is important for maintaining the life of the battery bank.

The controller will also load balance during hours of sunlight, so if the battery is full or close to it and there is excess solar energy available then the controller will direct some of the charge to the load and keep the battery in reserve for times when the PV cells are not providing power.

Now that we know all of our loads we can work our way out to the solar array.

This a may be an array of cells in a panel or an array of panels.

The number of panels required in the array is dependant on some more variables.

These are:

• The loads and losses as previously calculated
• The efficiency of the panels
• The geographic location of the panels with respect to solar irradiance (the amount of sunlight that will fall on a given spot)
• The inclination and rotation of the solar panels
• Any shading, dirt or snow causing losses in the array itself

No power generation system is 100% efficient, in fact with solar panels the efficiency is quite low only about 22% of all the energy they collect is converted to usable power.

The angle and orientation of the solar panels vary by location, this page will help you set the optimum angle for fixed solar panels for your area. Tracking systems may be used, but they use some of the power and may actually require additional panels just to power the system.

Ambient temperature will also affect how efficient the solar panel is at converting solar to electrical energy, if you are feeling like a bit of reading, this paper explains the relationship in a few short equations.

The last factor we have to consider is autonomy, this is how long you want the system to operate for without input from the solar cells, this is required as you will not get guaranteed sunshine for the full average time and at the average irradiance. Also if you had a snow storm or similar, your panels may not produce for several days, this is when your battery bank will pick up the slack.

I have included a simple excel calculator to help you size up your system.

I have included some sample numbers simply fill in the areas with green shading and let the orange ones do the calculation. There are round ups on the number of batteries and solar panels to stop the system being undersized. Remember, when filling in the losses, if you are unsure go bigger than you might think, this will prevent your system from being under specified.

I hope this is of some help.