Tech Notes

Unfortunately there is no magic number. This has to be estimated with a little math. We can help with this, but we still recommend reading the steps below to have an understanding of what is involved.

The first step is to determine how much power your solar panels generate each day. This is based on numerous factors including; panel size, orientation, location, shading, temperature, time of year. The easiest and quickest way to estimate this is to use the online calculator from National Renewable Energy Laboratory (NREL). The link is located at the end of this page. You will be required to enter your zip code and system specifications. The result will give you the average power generated each month. Remember to divide this by the number of days in the month to get a daily estimate of power generation. If you are using power on site (battery bank), you will have to subtract what you are using on average each day to see how much available power you have left over.

Second you need to determine how much hot water or heat you can generate with your available solar power. A space heating calculator and hot water calculator are located at the end of this page. If calculating for space heating, you have have to convert units from BTUs to kWh. A conversion tool is also located at the end of this page.

The result is how much heat or hot water you can generate each day with the power you have available.

To determine you rate of return, compare the product cost against the cost savings each day for the heat or hot water generated with your solar panels.

For a new system you will work backwards. Start with how much hot water or heat you want each day and then determine the size of the solar array needed.

Select if you have a new or existing system and fill in the information below. If something is unknown, leave it blank. We will work with you to determine the best solution for your needs.

New System

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Energy Goals [New System]

• Utility Credit (cash back)
• Energy Independence
• Hot Water
• Space Heating

Other

Location (zip code)

Water Water Location

Hot Water Needed Each Day (gallons)

Incoming Water Temperature (F)

Outgoing Water Temperature (F)

Heat Needed (BTUs)

Room Size (square feet)

Your Name *

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Last

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Existing System

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Email Temperature (gallons)

Energy Goals [Existing System]

• Utility Credit (cash back)
• Energy Independence
• Hot Water
• Space Heating

Other

Size of Solar Array (kW)

Battery and Charger in System?NoYes

Battery bank Size (nominal Voltage)

Amount of Battery Power Used Each Day (aH)

Location (zip code)

Hot Water Needed Each Day (gallons)

Incoming Water Temperature (F)

Outgoing Water Temperature (F)

Heat Needed (BTUs)

Room Size (square feet)

Your Name *

First

Last

Email *

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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 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 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 a decrease in efficiency over time and eventually 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. Its price can be as high as the retail cost of power or there can be 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 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 from 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. For more information we encourage you to research this for 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 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 used for charging batteries. This is also a type of DC-DC converter, but it is mostly a fast on/off switch that is letting just enough power from the panels through 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 remember a chain is only as strong as its weakest link. A solar power system often contains 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 at the forefront 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 tells 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. When looking at efficiency it is best to see performance across all conditions, including standby conditions.

Example: Power from panels 95% (including losses in wiring), charger efficiency 92%, battery efficiency 87% (includes charge and discharge losses), inverter efficiency (including daily standby losses) 90%. Cascading efficiency = 0.95 * 0.92 * 0.87 * 0.90 = 68%. This is an inefficient system!

Batteries are the future?

Batteries have become a important resource in today’s world. There’s no denying the importance of batteries and the advances that have been made in recent years, especially with lithium batteries. There is an annual increase in energy density of lithium batteries every year since they entered the market. However, the cost for lithium batteries has remained high and often cannot surpass the simpler lead acid batteries in terms of watts per dollars over the expected lifespan of the battery.

The cost, size and weight of batteries remains high. For energy storage with solar power they are essential, but for many solutions they are not the best choice. That is why there is so much research and development into different types of thermal storage. Heat and hot water are essential requirements to life. They are both prime examples of where thermal storage surpasses 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 far future because they are inefficient and require materials that are limited on our planet. Energy storage devices need to be efficient to be effective. However, they also need to be made from resources that are abundant to us. They need to be easy to manufacturer, easy to use and recyclable at the end of their long life.