Solar Power System Design – Part I

I’m designing my offgrid solar power system at the moment. The plan is to get my head around what I need, locate a supplier for those components in Auckland, consult them, negotiate a price, take the car on the ferry to town, bring the equipment back to the Island and install the system.

The plan ends with me sitting on the deck in the sun, enjoying a cold beer and feeling proud of myself.

solar_powerA naive schematic for a basic system. The solar panel converts sunlight into electrical energy, the charge controller manages it and charges the battery, which stores the energy, and the inverter transforms direct current into alternating current.

A sophisticated system is a bit more complicated. Solar panels are connected in an array (PV array), they need to be installed appropriately and oriented for maximum sun exposure over the year, the charge controller will contain a battery monitor (a shunt needs to be installed), a deep cycle battery bank, circuit breakers, fuses, splines, switch boards will be used, every component should be earthed and both DC and AC loads can be connected.

Understanding units and what you are dealing with, reading manuals, appropriate consultation, a detailed wiring diagram and double checking every aspect of calculation and installation is a must. As usual, the internet is full of ‘Do It Yourself’ systems and plenty will be dodgy, especially when up-scaled. This, following articles and downloadable material to which I link to, shall NOT be treated as advice, instruction or a form of guideline for any sort of power system. I have downloaded two documents and believe they contain all I need to know.

A sound approach to system design should include:

1. Quantify Energy Usage – How Much Must The System Supply?
2. Quantify Energy Storage – How Much Energy Must Be Stored?
3. Quantify Energy Input – How Much Energy Must The PV Array Supply?
4. Which Components Fulfil Those Needs?

Ideally, a system designer works out 1-4 in detail and takes into the following into account:

  • efficiency of all components
  • different energy usage during winter and summer
  • days of autonomy (how long the system can work even when it is extremely cloudy?)
  • battery maximum depth of discharge, discharge rate and temperature derating
  • for the solar panels: solar irradiation varies, so does daily energy production. Dirt, temperature and manufacturing tolerance of modules are important. Further, an oversize factor needs to be accounted for when a generator will not be used.
  • the type of charge controller depends on PV array and how many connected in series
  • inverter sizing should include surge capability

I’ll skip all of this for both the sake of clarity and since for me it is more important to know what I can afford than what I need. My needs will change, so it is important to assure the system can be up-scaled in the future.

Before writing about the PV power system I want to purchase, I’ll provide you with worthwhile information.

  • If you require more than 1 kWh per day, go for a 24 V battery bank.
  • Purchase an MPPT charge controller, one that can handle about 50% more panels than you want.
  • Energy efficient appliances can cost significantly more than their pendants. It may be cheaper in the long run when investing more into PV power system than buying energy efficient appliances.
  • If you are planning to install components inside, you should know how much noise individual components make.
  • Maintenance free batteries (AGM or Gel) do not require any maintenance!  They  don’t require equalization either. Instead of spending lots of money on a generator to equalize batteries in winter months, spend more money on PV array and batteries. Oversize PV array, use surplus energy to keep batteries fully charged even during winter and use the surplus energy in summer months to do other things. Pump water up a hill, perform electrolysis or just turn it all on.
  • Work out Ah per Dollar for batteries.
  • Work out W per Dollar for panels, work out how many you can link up in series to controller.
  • Don’t lie to yourself when working out your daily energy requirements. Take into account that other people in the household might be notoriously uninterested in details about power consumption. Educate them!
  • Peak inverter efficiency is under full load. Don’t purchase a 2000 W inverter when you require a permanent load of 200 W (for a freezer for instance) and sometimes more power. You’d be better off buying more than one inverter in terms of efficiency.
  • Where you can, use DC loads to bypass inverter losses. Most household appliances take AC from the power socket and transform it into DC.
  • Don’t forget, circuit breakers, aluminium feet, switchboards, wiring, etc. cost money and need to be considered in budget.
  • If you leave the entire system design to a trades-person or expert, chances of disappointment or inadequate solutions are greater than working together with an expert. Basically, you need a sound understanding of your system and alternatives solutions.
  • Finally, find a good supplier, negotiate a price. Their input and assistance will be useful and you can expect a better level of support from one than from multiple ones.

The focus of Solar Power System Design – Part II will be the components I like to purchase and an explanation of my rationale.

4 thoughts on “Solar Power System Design – Part I”

  1. You really should educate Rani about not leaving the light on all the time 😉

    Interesting read so far, looking forward to see what you plan to install/build on your site.

    1. Well, lights are usually not the problem. The electric kettle, vacuum, hair dryer, washing machine, etc. Some like to turn these on in the morning before going to work. Yeah, I’m excited myself and hope to get some free time to deal with the project. I’ll do a bit more writing soon. Cheers Lars, Ben

Leave a Reply

Your email address will not be published. Required fields are marked *