Designing Renewable: Good Practices
Think Energy Efficiency
Large electrical appliances such as hot water heaters, electric baseboard or portable space heaters, electric clothes dryers, electric furnaces, electric ranges and ovens, even incandescent light bulbs can force the cost of a renewable energy system beyond what it would be practical to spend on a system. If there is an energy-efficient alternative, find it and employ it in your home power system. Most heat-producing appliances such as stoves, furnaces, hot water heaters and clothes dryers can be readily purchased at most home appliance stores.
Wherever Possible, Keep Loads at 120 Volts AC
240 volt appliances typically are heat-producing and therefore better operated from LP (Liquid Propane) or Natural Gas. One exception to this rule is the use of a residential water pump where long wire runs and deep wells require the use of electric pump motors larger than ½ horsepower. Typically, home submersible pumps larger than ½ horsepower will be manufactured only in 240 volts AC. A 240 volt load will require either a 120 volt to 240 volt transformer installed in the system or stacking and interfacing two 120 volt AC inverters to meet the needs of the 240 volt load.
Allow Extra Power For Inefficiencies, Losses and Cold Batteries
No system is 100% efficient. Plan accordingly. At Sunelco, we typically use an overall inefficiency factor of 25% when designing a 120 volt AC system and it has worked well for us. That is, once you have calculated your total daily power consumption, multiply that number by 1.25 to find out how much power needs to be generated to meet your electrical needs. If this seems like a high inefficiency factor, consider that electricity reaching your home via the electrical grid has an inefficiency of 60%.
Use Real Data, Not “Best Guess” Assumptions
If the solar site analyst says you get 5.5 hours of full sun equivalent in the month of May, believe it. You may know that you have more daylight than that but daylight hours are not the same as full sun hour equivalent or Kilowatts per meter squared per day. The same can be said for Available Wind at a site or Gallons per Minute of water in a micro-hydroelectric system. Be as accurate as possible when calculating your power consumption for the same reason. Miscalculations here can be disappointing and expensive.
Allow For Later Expansion of Your System
Life styles change and so do energy requirements. It is important when designing your system to allow for later expansion. Oversizing of charge regulators, wiring and inverters or allowing for duplicates later on can save you money in the long run when you realize you need more power.
Keep it Simple
Employ as few components to do the job as possible. Many manufacturers now produce multi-function components such as inverters with monitors, generator start mechanisms, back-up battery chargers etc built in or offered as options. Wherever possible and if your budget allows, plan ahead and use these components
Keep Batteries Warm
Batteries perform best at 70 to 80 degrees Fahrenheit. You will get the maximum amp hour capacity out of your batteries if they are operated at room temperature or slightly higher. Since batteries have a natural self-discharge rate, the rate of discharge can be reduced by storing unused, charged batteries in a cool environment at about 40 degrees Fahrenheit.
Size Battery Banks for Several Days Operation
Once you have calculated your daily power consumption, size your battery bank to give you several days of operation and pretend that there will be no charging source available during that time. Here at Sunelco, we would typically use 10 days of autonomy as our design figure. This would roughly calculate to 10% discharge per day with the plan being that the battery bank would be fully recharged by the renewable energy system the following day. In extremely sunny or windy locations we can reduce the size of the battery bank by as much as 20% to 30% because there is less fluctuation in the day to day re-charge cycle. However, if the 10 day rule is followed and the batteries are discharged 10% daily we can expect longer life out of the battery bank than if we are discharging the battery 20% or more.
Avoid Long, Low Voltage, DC Wire Runs
The lower the voltage of the circuit and the more amperage we need to carry over a circuit the larger the wire needs to be. For this reason, we need to keep the battery charging source as close as possible to the battery bank. For more information on this aspect go to Basics of Electricity and Wire Loss Chart
Electrical Load Planning
Use this form to evaluate your electrical needs:
Sizing Batteries and Solar Arrays
Use this worksheet to plan the size of your battery bank and Solar (PV) array:
Find out about the many financial incentives available for renewable energy