To start we need to define some terms:
• Volts (V) – is electrical “pressure”
• Amp (A) – or Ampere is the electrical current.
• Kilo (k) – is one thousand (1,000)
• Watt (W) – real power (0.0013 x watts = hp)
• Horse Power (hp) – power (hp x 745.7 = watts)
• Volt-Amps – is apparent power (volts x amps), or the 100% efficiency
• Power Factor – is the efficiency rating of an electrical systems (PF=VA/W) Higher the power factor the more efficient the electrical system, PF is always between 0.0 to 1.0 and PF x 100 gives the efficiency in percents.
• British Thermal Unit (BTU) – BTU = Watts x 3.413
• Engine hp to generator watts rule of thumb is 2hp engine for every kilowatt.
• Motor start:
Residential – Dual phase or capacitor start is 2-3 time running amps (common in home A/C and fan motors).
Commercial – 3 phase induction motors require up to 7 times the starting amps versus the running requirements.
• Air Conditioning – Tons = 12,000 BTU/hour or 3.517 kilowatts(kW)
In choosing a generator the type of system and fuel are two of the main decisions to be made, conditional on run time and load and environmental circumstances. There are three ratings, Standby or Emergency rated, Prime rated generating units and Continuous run rated units.
Standby Generator Sizing
Standby generators are used to provide power if the grid goes down. Typically they utilize spark ignition engines that run on natural gas or propane. Smaller, portable units may run on gasoline. Larger units my use diesel but stationary units usually run on propane or natural gas and by their nature don’t require refueling by the owner. Natural gas is a relatively low cost, efficient and non-polluting fuel. Propane costs more and requires on-site fuel storage but fuel costs are reasonable. Standby generators are required to provide short duration emergency power, and not to run in conjunction with a public utility electrical power source as these two systems are not synchronized. There is usually no overload capability built into these units. Standby units need to be run regularly in a test mode to make sure they are operating and in dependable condition. Modern, electronically controlled units will run these tests automatically at programmed intervals, like weekly, bimonthly or monthly. These test routines usually run quietly at reduce rpm and are seldom noticed. Another consideration in earthquake zones is damage to the natural gas lines. Propane or a diesel alternative may need to be considered.
Prime Power Diesel Generators
Prime rating is the maximum load rating accessible of a variable load, and they can be run for long hours on a yearly basis. Run time is up to 750 hours per year at lower load levels.
Continuous Rating Generator
Continuous rating is 100% load for unlimited hours per year. Units rated ‘Continuous’ are usually operated in off grid situations such as mining, agriculture and military operations, and of course ‘off grid’ residential living.
Other considerations are site elevation and ambient temperature. These conditions must be factored in to accommodate de-rating. Engines lose power and efficiency at high elevations or operating under elevated temperatures. However, turbocharged engines can compensate for higher elevations to a degree. At 3,000 foot elevation and up to 100 degrees Fahrenheit, they operate at full rating. Higher and hotter and they take a performance hit. Estimated loss is 3% per 1000 ft altitude over 3,000 ft and 1% loss for every 10 degrees Fahrenheit in temperature. Prime rated may go to 5,000 ft with the same temperature ratings.
Clean Diesel Generators for Lease, Rent or Purchase
Stay tuned for next weeks blog. Part 2 of Standby VS Prime VS Continuous Rated Diesel Generator Power.