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A backup generator is a fantastic investment since it keeps you prepared in case the power in your home goes out. This type of generator will immediately start sending power to your electrical system during an outage. That means you’ll mostly be able to use your electrical system as normal until the power is restored. However, it all depends on whether or not the generator you install is sufficiently large to meet your home’s needs.
How an Electrical Load Calculation Measures Your Power Needs
Accurately determining what size generator you need requires calculating your home’s total power requirements. This is done by performing an electrical load calculation. A load calculation involves adding up how many watts of electricity everything in the home uses. However, it’s not quite as simple as that since the generator won’t ever need to power everything at the same time.
To perform the calculation, electricians first use a specific formula to estimate the total load or current draw of all of the lights and outlets. The size of a house influences how many total circuits it has. That’s why the first part of the formula is based on the total square footage of the home. When calculating square footage, you also need to include the garage and any outbuildings that have power. The circuits that power outdoor lighting and outlets also need to be factored in.
The 120-volt circuits in a house are either 15 or 20 amps, which means their wattage is easy to determine. The wattage of the major appliances in a house tends to vary much more. That’s why the electrician will always check the information label on each appliance to determine how many watts it uses. Especially important is factoring in the wattage of the heating and air conditioning system. In terms of heating and AC, the electrician just looks to see which of the two has the higher wattage draw. Whichever wattage is higher is the one that they’ll add in to their calculation.
You obviously won’t ever have everything in your home on at the same time. That’s why the formula doesn’t just add up the total wattage of everything in the house. Instead, it only factors in everything up to 8,000 watts at 100%. The remaining load over 8,000 watts is only factored in at 40%. Let’s say that your total load or power needed to run everything at one time is 20,000 watts. In this case, the formula says that your typical maximum load at any given time would be 12,800 watts. That means you would need at least a 13-kilowatt (13,000-watt) generator. However, you would be better off installing a 14-kW generator just to be safe.
Starting Loads vs. Running Loads
All major appliances have two different loads: the starting load and the running load. The running load is obviously how much power an appliance draws when running. The starting load is how much power it takes the appliance to start up. The motors in appliances like a refrigerator, clothes dryer and an HVAC system all require large amounts of electricity to power up. The starting load for these motors is almost always at least three times as high as the running load. For some appliances, it can be even six times higher. The fact that the starting load is so much higher is why electricians take it into account and not the running load. If you just factored in the running load, the generator likely wouldn’t be powerful enough to turn on your HVAC system.
An Overview of Load Management and How It Works
Backup generators use a process known as load management or load shedding to ensure that they shouldn’t ever get overloaded. Preventing the generator from being overloaded is essential since an overload will cause it to shut down. Load management just means that the generator will always prioritize certain appliances and electrical circuits. The highest priority is always the HVAC system since it almost always has a higher current draw than any other appliance in a home. That’s why the biggest factor in determining what size generator you need is the size of your air conditioner or heat pump. The one exception is if you have an electric furnace since these units use far more electricity than air conditioners.
Let’s say you have a 4-ton air conditioner or heat pump. The running load of a newer unit this size is usually somewhere around 4,000 and 5,000 watts (4 to 5 kW). That means the starting load is usually around 12 to 15 kW. As such, you would usually need at least a 17-kW generator in order for a 4-ton AC or heat pump to turn on. If you have a 5-ton unit, you generally want to have at least a 20-kW generator. If the generator wasn’t this size, the combination of the HVAC, lights and outlets would likely overload it.
The way load management works is that the generator and your electrical system only send power to certain circuits at various times. Most of your 120-volt circuits will always receive power so that you can keep your lights on. If your refrigerator or electric water heater is currently running when your AC system needs to start, the generator may temporarily “shed” these other loads. That means it will stop supplying power to the other appliances to ensure it can sufficiently meet the starting load of the AC system’s motors. Most electric water heaters use between 3.5 and 5 kW. That means even a 17-kW generator likely wouldn’t be sufficient to turn a 4-ton AC or heat pump on while the water heater was running. This is where load management matters since it allows you to get away with a slightly smaller generator than you’d otherwise need.
Load management also typically comes into play any time your power goes out and the generator starts sending power to your home. If your air conditioning was running when the power went out, the AC system and your lights will receive power first. It only takes a few seconds for the AC to power up and start drawing its running load instead of the starting load. As soon as this happens, power will then start flowing to other things like your refrigerator and freezer, electric water heater, etc. Essentially, load management means you won’t need as large of a generator to meet the starting loads of your appliances and your other main electrical needs.
When trying to figure out what size backup generator your home needs, you mostly just need to determine your specific priorities. That means what things you absolutely need to power and run at the same time during a blackout. For instance, do you want to be able to use your electric stove and oven while also running your washing machine, dryer or dishwasher? If so, you’ll need a much larger generator so that you can make sure it can still turn on your AC and heating when needed.
For more than 20 years, Pipe Works Services has been offering expert home services to customers in Chatham and the surrounding areas. As one of the area’s leading home service companies, you can count on us for your electrical, HVAC, plumbing and home insulation needs. If you want to install a backup generator, contact us, and we’ll help ensure you get the size your home needs.
