Schmidt Consulting

Portable Backup Generator



Tom Schmidt



Even though we live in a rural area the electrical utility is pretty reliable but we are occasional victims of multiday outages, typically due to winter ice storms. I consult so being without power for a week is unacceptable. We heat with cord wood so staying warm is not an issue. However we have a well so lose access to running water during an outage. I could not justify a whole house generator so opted for a small gasoline fueled portable unit. This paper discusses fuel choices and electrical wiring, plus none of my home improvement projects would be complete without some electronics so I designed a status panel to monitor the generator.



Table of Contents

Overview.. 2

Sizing. 2

Fuel Choices. 2

Generator Selection. 3

Electrical Considerations. 4

Mains Wiring. 4

Grounding and Bonding. 4

Battery Charging. 7

Generator Modifications. 8

Status Panel 8

Generator Wiring. 12

Generator Shed. 13

Fire Extinguisher. 14

Gas Can Modification. 14

Operation. 14

Power Outage. 14

Generator Shut Down. 15

Utility Power Return. 15

Long Term Generator Storage. 15

Finished Installation. 15




We live in a semi-rural area in New Hampshire. Overall electrical power has been very reliable. Most outages are due to severe ice storms in which case we, along with most of the of the state, can be without power for a week or more. We heat with cord wood so staying warm is not a serious problem. However we have a well so when we lose power we also lose running water.Luckily we have a nearby stream as a source of non-potable water for toilet flushing. But hauling water in five gallon jugs gets old fast. Our wood stove does double duty as a preheater for hot water so without electrical power have to operate stove at a low level to prevent boiling water in the heat exchanger. An outage of a day is an adventure, but it gets old fast if it persists much longer.

Our solution was to purchase a small electric start 7KW portable gasoline generator and install a generator interlock on our service entrance panel. The interlock is an important safety feature as it insures utility power and generator cannot be on simultaneously. I built a generator shed on the north side of the house to store the generator when not in use and protect it from rain and snow during an outage. A homebrew generator status panel is installed next to the circuit breaker panel to keep tabs on generator health.


Like any engineering problem sizing is a tradeoff, the larger the generator the less you have to manage electrical loads but it increases fuel consumption.Our electricity consumption is about 30 kWh a day, exclusive of domestic hot water (separately metered). If we assume the entire amount is consumed in 12 hours we have an average load of 2.5 kW per hour. Loading is highly variable so generator needs to be large enough to accommodate peak loads. During an outage we are not going to use high powered electrical devices reducing overall consumption. In researching generators 7kW seems to be a common size and even somewhat lower wattage rated generators tend to use the same size gas engine. The capacity of 10 AWG wire protected by a 2-pole 30A circuit breaker is 7.2kW so a larger generator needs the next larger breaker and wire, increasing cost. With a 7kW generator we should even be able to run the electric dryer as long as we were not using any other high power appliance. On balance 7 kW seems to be a good choice between convenience and minimizing fuel consumption.

Fuel Choices

This was a hard decision. Residential generators operate on: natural gas, diesel fuel, propane or gasoline. The other fuel issue that needs to be addressed is how much to keep on hand and how difficult will it be to replenish during an extended emergency. I discarded the first two, as we are not connected to a natural gas pipeline and have no fuel oil appliances, plus diesel generators are very expensive. Not an issue if you use it often but painful for something we may need for only a few days every other year or so. That left propane or gasoline.

Propaneís advantage is long shelf life, it does not degrade like Ethanol gasoline blend. Most propane generators are designed to use small 20 or 30 pound gas grill cylinders. Propane, called liquefied petroleum gas (LPG) has the very useful property of becoming a liquid at room temperature with modest pressure, several hundred pounds per square inch. Liquid being much denser then gas enables a lot of energy to be stored in a small volume. Propane delivery works because as gas is drawn off, pressure is reduced causing the remaining liquid to boil converting more liquid to gas. The problem occurs when demand is high, the tank is small and ambient temperature is low. Vaporizing a liquid takes energy, causing the temperature to drop. If it drops enough there is not enough gas being produced to operate the generator. Propane temperature dependency is not an issue in warm climates but here in frozen New Hampshire with winter temperatures below zero Fahrenheit it is a serious shortcoming.

We have a 200 pound Propane tank to supply our kitchen range. The large tank minimizes the effects of low temperature so it would be suitable to fuel the generator. Modifying our existing Propane system to accommodate a generator is a long term possibility but I decided not to spend the money at this time. If I decide to go that route down the road there are lots of dual fuel generators available or I can install a propane adapter kit on our gas generator.

So the winner by default is ordinary gasoline. Gasoline is a less than ideal fuel for backup generator use. Storage is an issue and modern 10% Ethanol blend degrades rapidly.Using gas stabilizer extends storage life but it still has limited life expectancy so we need to rotate stock to keep the gas fresh.†† The generator is rated at 2 hours of run time per gallon at 50% load. As a back of the envelop calculation operating the generator for a couple of hours in the morning and three at night consumption will be about 2.5 gallons per day.†† I purchased four 5 gallon gas cans that should provide enough fuel for a week. During past outages power in the center of town was unaffected so getting replacement gas and more importantly coffee was not a problem. The other common electrical outages risks in our area are hurricanes and tornados. We are at the northern limit of Atlantic hurricanes so that risk is small but New Hampshire has had occasional serious bouts of tornado activity. Traffic accidents occasionally take out a pole but as this is a localized problem with fairly rapid power restoration. In the case of a severe extended outage where gas is hard to get we will need to husband our supply and run the generator for fewer hours each day. To keep the gasoline supplies fresh, every 6 months or so need to use it in our cars and replace with fresh gas and add fuel stabilizer.

Generator Selection

Internet reviews of portable generators are over the map. With a gas generator the key to reliable operation is managing the gasoline supply so stale gas does not gum up the carburetor and to perform regular oil changes. Normally I shy away from Harbor Freight electrical equipment but the 7KW Predator generator received a positive review by Consumer Reports and online user reviews are generally favorable. Since over a decade the generator may only see a few hundred hours of operation I thought it was worth the risk. The generator has electric start, in addition to manual recoil pull cord. Electric start is a nice feature to have in the dead of winter as I get more decrepit in my old age but it requires diligence to insure reliability after months of inactivity.

Electrical Considerations

When using a backup generator it is critical the generator is not able to feed power back to the grid. This poses an electrocution hazard to power line workers during power restoration.Generator grounding can be confusing since the optimum configuration is different when the generator is used by itself (separately derived system) then when it is connected to a residence. Lastly electric start requires a small lead acid battery. Batteries are the Achilles heel of any electrical system. They need to be kept charged but undercharging or overcharging will result in premature failure. Nothing like losing power when it is -10F and finding the generator battery has gone bad.

Mains Wiring

Insuring the generator cannot back feed the grid is critical. There are several ways to accomplish this by using a transfer panel or generator interlock. A transfer panel splits off circuits that are to be powered by the generator and provides A/B switching to select utility or generator power. A generator interlock is a mechanical device that prevents the main breaker and generator breaker from being on at the same time. For new construction investigate the feasibility of using a generator ready breaker panel. I opted for a generator interlock from GenInterlock as we have an existing panel and did not want to replace it just for the sake of installing a generator. Using a generator interlock insures the generator cannot back feed the grid while providing a great deal of flexibility, limited only by the capacity of the generator to power the load.

Grounding and Bonding

This can be a confusing topic. In a nutshell the purpose of bonding is to make sure any conductive metallic surface has a low enough resistance back to the power source so in the event of a fault there is enough current to trip the over current device. Grounding on the other hand is important to bleed off static charge and to maintain the neutral conductor voltage near that of local Earth potential.

It takes very little current to produce a sensible shock and not much more to cause electrocution. These values are much smaller than the rating of a typical residential circuit breaker or fuse as shown in this chart:

Figure 1 Shock Values for the human body

So letís see how that affects using a portable generator.

Figure 2 Generator Bonding and Grounding

The figure above is a standalone generator, termed a separately derived system, meaning it is not connected to any other electrical system (particularly the neutral conductor as will be discussed later). In this case if the motor has a fault to the frame the Equipment Grounding Conductor (EGC) aka green wire will carry enough current to trip the circuit breaker eliminating the shock hazard. If the generator is equipped with a ground fault circuit interrupter (GFCI) when leakage current exceeds 5ma (5 one thousands of an ampere) it will also trip. Note for this function to protect users standing on the ground the generator needs to be earthed to provide a return path for the current. Letís assume there is a problem with the tool and EGC and our hapless victim touches the tool while standing on the ground. Now a path exists from the hot conductor, through the faulty tool, through the victim and through the Earth back the supply. The resistance of this path is pretty high and will not trip a 15 or 20 amp circuit breaker. This is where the GFCI comes into play. It compares the current in the outgoing hot conductor and the returning neutral. They should be identical, if not the GFCI trips protecting the person from electrocution. In the US this value is 5 ma +/-1 ma. The function of the EGC and GFCI is the same regardless of whether they are used in a portable generator or your residence powered by the utility company.

Note the neutral and EGC are both connected to the same point on the supply, why? Neutral is the current carrying conductor. The EGCís purpose is to provide a separate low resistance path back to the supply so in the event of a fault of the ungrounded conductor the EGC is able to carry enough current to trip the overcurrent device (aka circuit breaker). The EGC is connected to neutral at one location and one location only.

Now letís see what happens when you connect a portable generator to your homeís wiring. Remember the generatorís neutral is internally connected to the EGC.

Figure 3 Separately Derived System

Thanks to Mike Holt for the following pictures. Note the neutral and EGC are connected together at the Service Entrance and also within the generator. As mentioned there can only be a single connection between neutral and EGC for safety purposes. In this case we need to use a transfer switch that also switches neutral. That way at any given time there is only a single connection between the EGC and neutral.

Figure 4 None Separately Derived System

In a non-separately derived system the EGC to neutral bond is performed at the service entrance. They are kept separate at the generator. If the generator is always used for backup keeping the EGC and neutral separate is easy. The difficulty comes when using a portable generator. When connected to the residential electrical system neutral and EGC are kept separate. If the generator is disconnected and used by itself it becomes a separately derived system. In that case the EGC and neutral need to be bonded within the generator for it to be used safely.

Battery Charging

I wanted the convenience of electric start, didnít want to use the recoil starter in the dark at -10F. The generator has a small seal lead acid (SLA) battery for starting. Lead acid batteries are the Achilles heel of any electrical system.They have relatively short life expectancy of a few years and for a century old technology have pretty demanding charging requirements. For optimum life the battery needs to be kept fully charged to prevent sulfation with resultant loss of capacity but not overcharged resulting in excessive gassing causing loss of electrolyte. Over time all batteries self-discharges so for an application like this where the generator is only started every few months we need to use a battery maintainer to keep it fully charged but not over charged. Power Sonic has a nice SLA tutorial if you are interested in the gory details.

I chose the Deltran Battery Tender Junior. This is a smart 500 ma charger with a maintainer mode to keep the battery toped up without overcharging it.

The charger has a multicolor LED indicating operating mode.

ō  Flashing Red Ė unit powered but not connected to battery (or reverse polarity or battery voltage is extremely low

ō  Constant Red Ė battery is charging

ō  Flashing Green Ė charged to ≥ 80%

ō  Constant Green Ė fully charged, in battery maintenance mode

Generator Modifications

I wanted to make is easy to drain unneeded fuel at the end of an outage without having to disconnect the fuel line from the carburetor. Purchase a barbed T connector, shutoff valve and length of fuel line. When power is restored this modification allows any remaining gas to be drained back into the gas cans. Once the generator tank is empty the generator is run to remove the last bit of fuel from the carburetor.

Status Panel

None of my projects would be complete without some form of whizzbang electronics. The generator control panel is pretty simple. It has a digital voltage and current display for each 120V generator leg to make sure we do not exceed the current ratings. The meters use a toroidal coil to provide non-contact current sensing. A third meter displays: volts, hertz, run time and accumulated run time. The voltage reading is not all that useful as the other meters display the voltage of each leg. I use this meter to monitor AC frequency and runtime. The generator controls output frequency by running the engine at a constant speed. This is easier said than done as engine load constantly varies as the electrical load changes. So being able to monitor generator frequency is an important consideration.In addition the display has two run time counters, one is reset each time power is lost and a second shows cumulative run time. The volatile run time counter makes it easy to monitor how long the generator has been this cycle. The cumulative run-time counter is not resettable and tracks total run time. It is useful for managing generator maintenance such as oil changes.

On the DC side I wanted to monitor battery voltage to make sure the charger and battery were healthy. I designed a simple window comparator using a LM339 comparator and LM336 2.5 volt reference. The battery LED is green if voltage is between 12.7 and 14.5V. It turns red outside this range and if the alarm switch is set an audible alarm sounds. A small DC voltmeter intended for motorcycles displays actual battery voltage.

The top LED indicates utility power status. If the alarm switch is in the utility position an audible alarm sounds when utility power is active. This makes it easy to tell when utility power has been restored.

The bottom LED monitors the work light in the generator shed. When the door is opened an interlock switch automatically turns on the work light. Whenever the work light is on the door status indicator flashes providing a visual reminder the light is on. The generator battery is pretty small and if the light is left on for an extended period there will not be enough energy left in the battery to start the generator.

At the lower left is a USB power outlet. It is designed for motorcycle use so it has a plastic protective cover over the switch and USB A receptacle. I wanted one with a switch so there was no idle power drain of the battery. This allows us to charge USB devices without the need to run the generator during an outage. It draws power from the generator battery so is adequate for charging cell phones without too much risk of excessively depleting the generator battery.

The pictures below were taken while I was debugging the unit. The one on the left shows the unit when utility power is active. The one on the right shows what it looks like when the generator is running during an outage.

Figure 5 Control Panel - Utility Power

Figure 6 Control Panel - Generator Power


The panel is powered by two separate DC sources: 9 volts from a small SMPS attached to the electric water heater circuit and 12V from the generator battery. The 9 volt supply provides power to the logic circuits during normal operation. One of the disadvantages of using a generator interlock is determining when power returns because the main circuit breaker has to remain off. Sourcing 9 volts from the separately metered water heater circuit provided an easy way to sense utility status even when the main breaker is off. The audible alarm can be used to generate an alert when utility power returns.

The circuit draws about 30ma from the 9 volt supply. When utility power is available the DC voltmeter is the only component powered by the generator battery. The batter maintainer easily makes up for this small draw keeping the battery fully charged.

When power is lost the panel is shutdown. I did not want to drain the generator battery during an outage. A pushbutton on the front panel can be used to manually turn on the unit to observe battery and door status when the generator is not running during an outage.

When the generator is running the unit automatically powers up. An AC sensing optoisolator is used to sense generator output and through a 2N3906 transistor powers the logic from the 12 volt generator battery. This way the unit is only drawing battery power when the generator is running. The optoisolator is simply a neon pilot light assembly coupled to a 5516 CdS light dependent resistor.

The unit is built into an 8Ēx8Ē Hoffman box. DC components are located at the bottom of the front panel, the AC panel meters at the top. Each meterís AC leg is protected by a small fuse. The status panel is located to the left of the main breaker panel making is easy to monitor generator status.

Generator AC power enters the bottom left flows through the toroidal current sensing transformers and out to the generator back feed breaker in the main breaker panel.


Figure 7 Front Panel

Figure 8 Enclosure Wiring


Figure 9 Status Panel DC logic

Figure 10 Status Panel AC

Generator Wiring

The generator shed protects the generator when it is being stored. A flexible 4-conductor NEMA L14-30 twist lock power cord connects generator output to the house generator inlet. A GFCI protected convenience outlet powers the battery maintainer.The maintainer is wired to the generator battery, the door interlock switch, LED work light and back to the generator status panel.

Iíve installed thermal fire detectors at several locations within the house in addition to normal smoke and Carbon Monoxide alarms. I added one in the ceiling of the generator shed as protection against fire. We are storing 20 gallons of gasoline in the generator shed and it is hidden from casual view so I wanted to take every reasonable precaution.

Figure 11 Generator Wiring

Generator Shed

One of the reasons Iíve procrastinated getting a portable generator is figuring out where to store it and how to protect it from the elements when it is in use. I decided to add a small bump out to the rear of the house. It is 3 feet deep and 12 feet long. Half of it is used for the generator and the other half for general garden tool storage.

A rather unique feature is instead of hinging the generator door on the side opening like a normal door I hinged it on top. To access the generator need to lift up the door. Attached to the bottom of the door is a hinged section that becomes a leg at the far end of the door. Extending the leg turns the door in to a makeshift roof. Before I sided the door covered the plywood sheathing with Grace Ice and water shield. That way if the generator needs to be left out in the rain or snow the door becomes a pretty serviceable roof.In the case of windblown snow or rain the sides can be closed off with a snap on canvas covering.

The generator is pretty heavy so I built the floor of the generator shed as low to the ground as feasible. To move the generator in and out constructed a small detachable ramp.

The generator power inlet, battery maintainer and lighting are located in the shed as is a thermal fire sensor. When the door is opened the LED work light (powered by the generator battery) is automatically turned on. Nothing like working in the dark trying to start the generator. Once the generator is running the interlock switch has a nifty feature that pulling the plunger out turns the light off. The control panel in the house monitors the status of the light and flashes the indicator when the light it one. The battery is pretty small and would soon become discharged if the light is left on without the generator running.

Fire Extinguisher

I keep a 3A, 40B-C extinguisher handy in the generator shed as a precaution.

Gas Can Modification

Modern gas cans are designed to prevent gasoline vapor from adding to air borne pollution. Due to their unvented nature and the complex spout arrangement gas pours very slowly and is pretty easy to spill. The last thing I wanted was to be outside spilling gas all over a hot generator. I picked up a gas can modification kit on eBay. It consists of a replacement nozzle and a traditional gas can vent. I drilled a hole in each gas can and popped in the vent. This eases refueling, just attach the aftermarket nozzle and open the vent.


In writing this up the description it seems harder and more complex that it actually is in practice.

Power Outage

When power goes out:

1.       Open generator door and extend leg that turns it into a roof. The door interlock switch automatically turns on a small LED work light.

2.       Position the generator ramp.

3.       Slide the generator out.

4.       Connect the AC power cable; DC battery cable is always connected.

5.       Set the engine choke.

6.       Press the starter button. If that fails the generator has a recoil starter.

7.       Let the generator warm up and release the choke.

8.       Pull out the light interlock switch to turn off the work light.

9.       Go back into the nice warm house and make sure everything looks good on the status panel. Whenever the generator is running the status panel meters are active, even when the back feed breaker is turned off.

10.   Open the breaker cabinet and turn off the main breaker and branch circuits.

11.   Slide the generator interlock allowing the generator back feed breaker to be turned on.

12.   Flip the generator back feed breaker on

13.   Start turning on circuits starting with the highest power ones first. In our case that is the well pump and refrigerator.

14.   Bask in the luxury of electric living.

15.   Flip the alarm switch to utility. When power returns the audible alarm will sound.

16.   The generator has a 6 gallon fuel tank with a run time of about 12 hours at 50% of full load. Our intention is to run the generator a few hours in the morning and evening, not 24/7 during an outage. We keep four 5 gallon gas cans in the generator shed. As the gas gets old we recycle it into our vehicles and replace with fresh gas and stabilizer.

Generator Shut Down

Shutting down is pretty much the reverse. Turn off the branch circuits so there is no load on the generator and shut it off.

Utility Power Return

1.       Turn off utility power audible alarm.

2.       Wait a while to be sure utility power is stable.

3.       Turn off branch circuits.

4.       Turn off the generator back feed breaker.

5.       Position the generator interlock so main breaker can be turned back on.

6.       Turn on main breaker.

7.       Turn on branch circuits.

8.       Flip the alarm switch to battery. This will sound the audible alarm if battery voltage is too high or low.

9.       Shut off the generator.

10.   Drain any remaining fuel back into a gas can.

11.   Start the generator and run the carburetor dry to prevent stale fuel from gumming it up.

12.   Perform any indicated maintenance, such as oil change.

13.   Refill gas cans and add stabilizer

14.   Close generator shed door.

Long Term Generator Storage

The generator needs to run 15 minutes every three months to maintain the warranty. This is a good time to visually inspect it and recycle any old gas into the cars.

Finished Installation

The status panel is located to the left of the main service entrance panel. The generator interlock prevents the generator back feed breaker (top left) from being turned on if the main breaker is on.

I printed out circuit breaker directory and pasted it to the door. To keep things simple color coded the breaker number labels with a small colored dot. Green indicates breaker should be turned on when running on generator, red means leave off and no color is optional.The device located in slots 21/22 is a whole house surge protector.

The caution labels came with the interlock kit. The one on the back feed breakers warns not to turn the breaker on if the front panel is removed. The two above the main breaker to the right indicated where the generator is located and provides operating instruction.

On the generator panel all three indicators are green. The top indicates we have utility power. The middle that battery voltage is between 12.7 Ė 14.5 volts, exact voltage is available on the meter to the left. The bottom indicator is solid green indicating the generator work light is off. When the work light is on the indicator blinks. The AC meters at the top of the panel are off because the generator is not running.

Figure 12 Gen Status Panel and Main Breakers