Tech Notes

Why selling power to the grid is often the best choice

No change in lifestyle is needed when using this type of system.
Using solar panels to generate power is the easy part. Using the power, especially with energy storage is the hard part. Selling solar power to the utility company allows someone to have the benefit of a environmentally friendly power generation and financial gains in return.
The panels and inverter can all be installed on the outside of the home where they are out of the way and require no time or maintenance of any kind. They can simply be installed and forgotten.
It can be tempting to invest in solar thinking it is free power, but in reality it isn’t. Besides the upfront costs, there is depreciation and eventually the replacement costs. Utility companies exist for a reason. In terms of dollars to watts, they can’t be beat.

Heated debate for buy back cost of renewable energy going to the grid

The selling price of privately generated solar power varies dramatically by state and is constantly changing. It price can be as high as the retail cost of power to no option of selling power at all.
The operation of a utility company can be simplified into the following three fields; power generation, power transmission and power distribution. An example would be power generated at a hydroelectric dam, which in tern is sent over a large distance using high voltage transmission lines, finally reaching a residential area where low voltage distribution connects each home to the grid. Each of these stages have operating costs.
When someone generates power with solar panels with the intent of selling the power to the utility company they are contributing to power generation and the utility company is still handling the power transmission and power distribution.
The retail cost of power form the utility company is the cost to cover the three fields of power operation. If solar power can be sold to the grid for the retail cost, the utility company may be losing money because they are buying power at the cost of all three fields and only receiving the generation phase of production. This is where the debate gets heated. We encourage you to research for this yourself.

Unsaid truth about MPPT chargers

Solar panels have a specific operating point that is optimal for power generation. When operating outside of this optimal point, the amount of available power from the panels drops dramatically. This operating point varies based on sunlight and temperature. To stay at this optimal operating point, which is constantly changing, the load or device attached to the panels must also change. This is where MPPT chargers come in.
MPPT chargers have the ability to provide maximum power point tracking (MPPT). The MPPT charger is a DC-DC converter, which adjusts its load on the panels to stay at the panels maximum power point.
Before they became the norm, simple pulse width modulation (PWM chargers) were mostly used for charging batteries. This is also a type of DC-DC converter, but is mostly a fast on/off switch that is letting just enough power from the panels to have the output reach the desired charging voltage. No compensation for the solar optimal power point is done by a PWM charger.
From this brief explanation it would appear the MPPT is the way to go. Not necessarily. If the panel output voltage is properly matched to the battery bank voltage there is little difference in efficiency between a PWM and a MPPT charger. Another important note to make is in regards to the charger operation. Regardless of the type of charger, there is a constant current and a constant voltage mode. Unless the batteries are extremely low, most of the charge time is spent in constant voltage mode. A MPPT charger in constant voltage mode does not typically track the maximum power point of the panels. Why? Because it no longer needs the maximum power available, it only needs a portion of the panels power. At this point in the charge cycle there is no difference in efficiency between different types of chargers because the chargers no longer want all the power from the panels.
When MPPT chargers were new, there was a big difference in price between MPPT and PWM chargers. Due to the quantity of MPPT in the market, this is not the case. When in doubt, go with the MPPT charger.

Real efficiency of a solar system

Just as a chain is only as strong as its weakest link.
A solar power system can contain multiple components. Each component has its own efficiency, which is simply the ratio of power out to power in.
When one component in a system has a poor efficiency, the following components all suffer resulting in a low total system efficiency. This is called cascading efficiency, which mathematically is the product of all individual efficiencies.
The image below shows the major components in an off-grid solar system. For an on-grid system, the batteries and charger can be omitted.
Components such as inverters and battery chargers have a tendency to show high efficiency specs on the front of their marketing presentation. What they usually don’t show is how this spec is only relevant to a unique set of operating conditions. It is simply a best case situation, which does tell you what to expect on a perfectly sunny day, but is of little help when trying to understand how the system will perform in the real world. We have found, if you cannot find specs defining things such as the devices standby power and efficiency in non-ideal situations, it is probably not a good product to buy.

Real world example: Power from panels 95% (not example at max power, losses in lines etc), charger efficiency 92%, battery efficiency 87% (include charge and discharge), inverter efficiency including standby losses 90%. Cascading efficiency = 0.95 * 0.92 * 0.87 * 0.90 = 68%. an inefficient system!

Batteries are the future, they are great, aren’t they?

Batteries have become a hot topic in todays world.
Cost of batteries. Lifespan of batteries. Limitations of batteries. Recycling of batteries.
There no way to know if batteries are the future. It is safe to say the type of batteries we have today are not the future because they are inefficient. Energy storage devices that are easy to create, use and reuse are the future. Most renewable energy sources are intermittent by nature and require energy storage to allow us to use it when it is actually needed.