If you are preparing to use backup power to get you through the next emergency power outage, you’ll might wonder if your generator can handle all the appliances you want to run. To complicate things further, solar power stations are becoming more capable, taking the place of many traditional portable power generators.
In this article we will fully explain what running watts and starting watts mean. We will show you how to calculate total power requirements no matter what appliances you use.
Having an appropriately sized generator can help you save fuel, and prevents safety hazards. It’s also important in making sure your backup electricity stays dependable.
Table of Contents
What Are Watts
Watts are units we use to measure electrical power. For anyone who prefers a simple visual, think of energy as something that electrons deliver to an appliance.
Voltage, measured in volts, represents the amount of energy electrons deliver to an appliance. Current, measured in amps, represents the number of electrons that travel through the wire to deliver the voltage. Power, measured in watts, is expressed as the number of electrons (current) times the amount of energy electrons brings (voltage).
As long as you know the voltage and current, you can figure out the wattage. If you know the voltage and the wattage of an appliance you can figure out the current. Knowing any of the two values can help you figure out the third.
Power (watts) = Voltage (volts) x Current (amps)
Current (amps) = Power (watts) ÷ Voltage (volts)
Voltage (volts) = Power (watts) ÷ Current (amps)
Appliances come with energy labels that indicate the current and voltage, so the wattage can be easily calculated by multiplying the voltage and current. A hair dryer is going to require more power to run than a fan. If both of those appliances are plugged into the same 120 volt power source for the same amount of time, you instinctively know the hair dryer is going to use more electricity. Let’s see if the math confirms that.
When a 2000 watt hair dryer is connected to a 120 volt power source, to figure out the current you would divide the wattage by the voltage.
2000 ÷ 120 = 16.7
What you get is 16.7 amps of current for the hair dryer.
When a100 watt fan is connected to a 120 volt power source, to figure out the current you would divide the wattage by the voltage:
100 ÷ 120 = 0.83,
0.83 amps of current for the light bulb.
So yes the light bulb draws much less current than the hair dryer even though they are hooked up to the same 120 volt source. It makes sense because we know hair dryers take more power to run.
Running Watts vs Starting Watts
All electrical appliances need to consume a basic amount of power to keep it from shutting down. Electricity is supplied by a constant stream of electrons flowing from the power source to the appliance. When you flip the off switch, this flow of electrons gets cut off so the household appliance also turns off. This basic amount of continuous energy needed by the appliance is the running watts.
Most appliances only require a constant stream of electric energy. But appliances that have motors need to kick into high gear mode during start up and at various intervals. The burst of energy consumption is referred to as starting watts, also known as surge watts and peak watts. Whenever this happens, your generator has to be able to accommodate this extra power draw for a few seconds.
After initial startup, these appliances throttle their energy usage back down and consume electricity at the rate of running watts, until they need to throttle back up again. In some appliances like refrigerators and air conditioners, you can hear them go into turbo mode as they work harder to get the temperature back down.
Think of surge watts as energy consumed in overdrive mode and running watts as energy consumed in maintenance mode.
What Types of Appliances Require Starting Watts
There are three general categories of appliances and only one of them requires surge watts.
- Resistive load. These appliances use resistance against current to create heat. An incandescent light bulb does this by heating up the filament until it glows. Space heaters also fall into this category. Energy usage is pretty much constant for this type of load so they only use running watts.
- Capacitive load. Typically battery chargers and cell phones are capacitive loads. Their main function is to store and dissipate electrical energy. The energy draw is also constant with this type of load.
- Inductive load. Inductive loads use an inner coil to create a magnetic field in order to propel physical movement. Appliances that contain an electric motor or compressor are typically inductive loads. These loads require starting wattage to get the motor turning in the beginning.
Here’s a list of some common appliances with their corresponding running wattages and additional wattages. Additional watts can be 2 to 3 times the running watts, with some appliances going up to 5X as much. For accuracy, you should always get the exact wattages based on your specific appliance model.
| Appliance | Running Watts | Additional Watts |
| Gas Furnace | 800 | 2350 |
| Sump Pump (1/2 hp) | 1050 | 2200 |
| Well Water Pump (1/2 hp) | 900 | 2000 |
| Washing Machine | 800 | 1600 |
| Electric Clothes Dryer | 5400 | 6750 |
| Window Air Conditioner (12,000 BTU) | 1500 | 2000 |
| Window Air Conditioner (5000 BTU) | 500 | 1500 |
| Refrigerator | 700 | 2200 |
How to Size Your Load
Once you have a list of all the appliances that you want to run on your backup power source. You need to get each appliance’s corresponding running wattage and additional wattage. As we already know, not every appliance will require additional wattage so put a 0 for those appliances. The steps to size your consumption are below or use our load calculator.
Add up all the running watts of appliances you plan to use, what you get is the total running watts needed from your power supply on a continuous basis. This number represents your minimum energy requirements, your generator and solar power station should accommodate at least your total running watts.
For all the appliances that require surge watts, list all of their peak watts and find the maximum wattage in the list. This number is your maximum additional watts. Now add the maximum additional watts to the total running watts you calculated previously. What you get is the estimated total peak watts. Your generator’s peak watt rating should be higher than the total peak watts you just calculated.
Step 1: Add up all the running watts to get the total running watts.
Step 2: Get a list of additional watts from appliances that cause power surge and find the largest, that’s your maximum additional watts.
Step 3: Add the total running watts and maximum additional watts to get your estimated surge wattage.
For example, let’s say you plan to run a refrigerator, a sump pump, an electric cooktop, and 10 LED lights at the same time. Adding up all the total running watts gets you 3035 watts. Out of those appliances, the refrigerator needs 2200 additional watts from time to time, and the sump pump needs 2150 watts. The largest additional watts is 2200.
Adding the maximum additional watts to the total running watts gets you the total surge watts of 5235 watts.
What this tells you is that, with all of those appliances hooked up to the power source at the same time. The power source has to be able to supply 3035 running watts continuously and be able to handle up to 5235 peak watts every once in a while.
| Appliance | Running Watts | Additional Watts |
| Refrigerator | 700 | 2200 |
| Sump pump (1/2 hp) | 1050 | 2150 |
| Electric cooktop | 1200 | 0 |
| 10 LED light bulbs | 85 | 0 |
| Total Running Watts (lower bound) | 3035 | |
| Maximum Additional Watts | 2200 | |
| Estimated Power Load | 5235 |
How to Size a Generator as Backup Power
Now that we have the total running watts and the estimated surge watts, these numbers will be useful in choosing a generator. Traditional generators create electric current by turning a coil inside a magnetic field. To do the work of turning the coil, generators burn fuel. The amount of power that a generator can produce at any given time is indicated by its wattage rating.
Generator specs come with running watts and peak watts. Make sure your load’s wattage values are within the generator’s. A safe rule of thumb is to multiply your estimated surge wattage by 1.25 and get a generator with peak watts ratings higher than that number. It gives the generator some safety buffer.
In addition to wattage requirements, you may also have to look at the generator’s amperage limits on the outlets themselves. Some outlets might state a maximum amperage of current that it can handle. It’s good to make sure the generator supplies enough current to run specific appliances. During turbo mode, appliances increase wattage by drawing more current.
For example: If your refrigerator has a running wattage of 700 watts and additional watts of 2200 watts, making its peak wattage 2900 watts. You are considering a generator that has 3200 running watts and 4000 peak watts. When you multiply 2900w surge watts by 1.25, gets you 3625 watts. Since 3625 is lower than the generator’s 4000 peak wattage and the refrigerator’s 2200 running watts is under the generator’s 3200 running watts, you might assume this generator would be suitable.
Upon further inspection, the generator’s 120 V socket has a maximum current of 13 amps. Now you need to figure out the refrigerator’s current draw to see if that stays within the generator’s limit. The refrigerator’s running wattage is 700 w. To get the current, you would divide 700 watts by 120 V, to get 5.83 amps. During surge, you would divide 2900 watts by 120 V, to get 24 amps. 24 amps is much higher than the generator’s 13 amp limit so this generator wouldn’t work. Make sure you check the wattage, voltage, and current of generators to find a good fit. Overworking a generator can lead to damage and fire.
An important safety tip is that you should never run a portable generator indoors, not even in a garage with the doors open. The exhaust puts out dangerous levels of carbon monoxide. Always run the generator outside, in a well ventilated area. To run indoor appliances, some people route extension cords inside. Ideally, you want to hook up the generator to a power inlet box and then install the power inlet box on your main electrical panel.
How to Size a Solar Power Station as Backup Power
Solar power stations differ from generators in that electric power is stored in the battery rather than generated on the spot. Except for routine maintenance, generators can keep going as long as there’s fuel. Batteries get depleted and it takes time to charge them back up again. Solar charging depends on the amount of sun and the equipment. It’s a time consuming process compared to the way generators “refuel”. When you size a solar power station, you have to consider the battery type and its size to determine if it can accommodate your needs.
Solar batteries also have running watts and surge watts to indicate what it can handle at any given time. In addition, it has a watt hour (Wh) rating to indicate how long the battery will last. 1000 watt hours means you can use 1000 watts for 1 hour or 500 watts for 2 hours. It’s a function of wattage and time.
For example: Let’s say you have a 2000 watt hour power station with a 2200 w inverter. You want to run a 1200 watt electric cooktop to boil some water. Divide 2000 by 1200 gets you 1.67 hours which is 100 minutes of use. Since the electric cooktop has no surge watts, it will only use 1200 watts at maximum which is within the 2200 watt limit of the unit.
Pass Through Charging
Most solar power stations allow for pass through charging which means you can charge it as you use your appliance. But the electric power created by solar panels can’t be directly used by appliances.
The electric current collected from the sun first goes to a charge controller to be evened out to prevent damaging the battery. Then the electric current goes to the battery to be stored. Finally, the current goes from the battery to an inverter to get converted from direct current into alternating current. All homes and appliances run on alternating current. Solar power stations have the charge controller, battery, and inverter built in.
Even with pass-through charging, power stations still have to convert the sun’s energy into something your home appliance can use.
Battery Type
Bear in mind when you make your purchase that the battery type has some impact on the capacity of the solar power station. The two most common battery varieties are lead acid and lithium. They have different characteristics.
Lead acid batteries are cheaper but have shorter life spans than lithium batteries. Lead acid batteries can only be discharged to 50% of their capacity where lithium batteries can be discharged up to 95% of capacity before having to fill them up again. For example: if you charged a 2000 watt hour lead acid battery, you can only use about 1000 watt hours before you need to charge it back up again. A 2000 watt hour lithium battery would need to be charged back up after you use 1600 watt hours.
Among those differences, lithium batteries are also more efficient where 95% of solar energy gets stored by the battery whereas lead acid batteries can capture 85%. It requires less lithium batteries to perform the same amount of work as lead acid so the power stations that use lithium batteries will weigh less. Lithium batteries also take up to half the time to charge up than lead acid. If cost isn’t a big issue, definitely go with the lithium variety.
What Happens When the Generator Overloads?
A circuit gets overloaded when an appliance draws more current than the circuit can safely supply. Since the voltage is already determined by the power source, a high wattage appliance will try to get power by drawing more current. If the generator can not accommodate the amount of current flowing through it, it will begin to create resistance in the form of heat. As the high current keeps flowing, a number of things can occur. Heat will continue to build up until the generator burns out, or worse, starts a fire.
Sometimes, when a generator is overloaded, it drops in voltage. This can cause permanent damage to the generator and cause the other devices running on the generator to compensate by overdrawing current, which leads to overheating. A generator that’s overloaded can start to produce intermittent power which would damage whatever appliance connected to the generator.
Signs of an overloaded generator are overheating, soot in the exhaust, and unusual sounds. Most modern generators have circuit breakers installed to detect overloading and will shut off automatically. But in the event your generator doesn’t have a circuit breaker, keep an eye out for signs of overloading and immediately turn off your generator and wait for it to cool down. Restart with a smaller load to make sure the generator isn’t damaged.
