Generator Load Analysis

A generator load analysis is the engineering work that determines what size standby generator your house or building actually needs, based on what has to run, what it draws while running, and what it draws in the instant it starts. It is the step that gets skipped most often, and it is the reason so many South Florida homeowners end up with a machine that either cannot start the air conditioning or cost far more than the job required.

We do this analysis before we quote a standby generator, and we do it for people who already own a unit and want to know honestly what it will and will not carry. The output is a document, not an opinion: a list of loads, running and starting current for each, the resulting continuous and surge demand, the effect of any load management or soft start equipment, and the generator capacity that follows from all of it.

Why square footage charts get it wrong

The rule of thumb that circulates in showrooms and online configurators sizes a generator by the square footage of the house. It is fast, it is easy to sell against, and it has almost no relationship to electrical demand.

Consider two houses of identical size a mile apart in Plantation. One has gas cooking, a gas water heater, a single four ton air conditioning system, and no pool. The other has an electric range, an electric tankless water heater, two air conditioning systems, a pool with a heat pump, an irrigation well pump, and a car charging in the garage every night. The second house can easily demand more than double the first. Square footage cannot see any of that.

The chart also cannot see the thing that actually kills generators, which is not the total load at all. It is the instantaneous surge when a compressor starts. A house with a modest running load and one large single stage condenser can be a harder sizing problem than a larger house with two small variable speed systems. There is no shortcut around looking at the equipment.

Two different calculations, and why the job needs both

The dwelling load calculation

The first calculation is the standardized method used to size electrical services, and it is the one the building department expects to see with a permit. For a dwelling it works from a general load figured at three volt-amperes per square foot of area, plus fixed allowances for the small appliance branch circuits and the laundry circuit, plus the nameplate ratings of the fastened-in-place appliances. That subtotal is then diversified: the first ten kilovolt-amperes are counted in full and everything above that is counted at forty percent, on the reasonable assumption that a household never runs all of it simultaneously. The larger of the heating or the air conditioning load is then added at full value.

This method is well proven for its purpose, which is sizing conductors and service equipment fed from a utility. Its diversity assumptions are appropriate for a source that has effectively unlimited capacity and does not care about a momentary surge.

The running and starting load analysis

A generator is a different animal. It is a finite engine turning a finite alternator, and it has to do two things the utility never has to think about: carry the actual simultaneous load without dropping voltage or frequency, and survive the starting surge of the largest motor without stalling.

So the second calculation is built differently. We list every load that will be connected during an outage, assign it a running current from its nameplate rather than an allowance table, decide honestly which of them can run at the same time, and then identify the worst case starting event, which is normally the largest air conditioning compressor starting while everything else is already running. The generator has to handle that combination.

Both calculations get done. The dwelling calculation supports the permit and sizes the transfer switch and feeders. The running and starting analysis sizes the machine. They frequently produce different numbers, and treating either one as the whole answer is how systems end up wrong.

Starting current is where generators actually fail

An induction motor at the moment of energization is very close to a short circuit. Until the rotor turns, there is no back electromotive force limiting current, so the motor draws its locked rotor current, commonly four to seven times its running current, for a fraction of a second.

Air conditioning condensers are the problem load in every South Florida house. Look at the data plate on the outdoor unit and you will find two numbers that matter: rated load amps, which is what it draws running, and locked rotor amps, which is what it demands at start. It is normal for a residential condenser's locked rotor figure to be five or six times its running figure. Those are the numbers we use, not a table.

Three things make this worse than it sounds:

• The surge has to be supplied at voltage. If the generator sags too far, the compressor cannot come up to speed, current stays at locked rotor levels, and the internal overload eventually opens. The machine did not trip a breaker; it just failed to start the air conditioning, which is the one thing you bought it for.
• Frequency dips too. The engine is momentarily loaded hard and the governor takes a moment to recover. Motors, clocks and some electronics care about that.
• Everything else is already running. The compressor does not start into an empty generator. It starts on top of the refrigerator, the pool pump, the lights and the well pump.

Well and pump loads deserve their own mention. Irrigation wells and lawn pumps are extremely common across Broward and Palm Beach County, and a submersible pump starts against the head of a column of water, which makes its start harder than its horsepower suggests. Pool pumps, booster pumps and septic lift pumps all belong in the analysis for the same reason. Modern variable speed pool pumps behave much better, which is worth knowing if yours is due for replacement anyway.

Soft starters are usually the cheapest capacity you can buy

A soft starter is an electronic device installed in the condenser's control circuit that ramps the compressor up rather than throwing it across the line. A well matched unit can cut starting current dramatically, in many cases bringing the surge down close to running current.

The consequence for generator sizing is significant. When the worst case starting event is what forces you up a size or two in generator capacity, removing that surge can let a smaller machine do the same job, with a smaller gas supply, a smaller pad, and lower running cost. It also stops the light flicker you may already be seeing on utility power when the condenser kicks on.

Some caveats we always cover with customers. The device has to be compatible with the specific compressor, and scroll compressors, reciprocating compressors and two stage units are not all handled the same way. Variable speed and inverter driven condensers already ramp electronically and generally should not have one added. Installation goes inside the condenser, which means coordinating with your air conditioning contractor so nobody has an argument about equipment warranty later. When it fits, it is one of the highest value changes in the whole project.

Load management as an alternative to a bigger machine

The other way to close a gap between what a house demands and what a generator produces is to stop the house from demanding all of it at once. Load management modules shed selected large loads when the generator approaches capacity and restore them when there is headroom.

Typical shed candidates are the electric water heater, the dryer, the range, the pool heater, a second or third air conditioning system, and an EV charger. Air conditioning management modules can arbitrate between two systems so only one compressor runs at a time, which in a two system house is often the single change that makes a mid sized generator viable.

The tradeoff has to be stated in plain language before anyone signs anything. With load management, you are choosing a generator that runs your house comfortably but not simultaneously. Laundry may wait until the afternoon is over. The guest wing may be warm while the main house is cool. Most families find that entirely acceptable when they understand it in advance and unacceptable when they discover it on day three of an outage. We put the shed priorities in writing as part of the generator load analysis. The equipment side of this is covered further on our generator installation page and in the transfer switch article.

Fuel supply is part of the sizing problem, not a detail afterward

A generator can only produce what its fuel system can deliver. In South Florida this is not a footnote, it is frequently the binding constraint on the whole design.

Natural gas

Where a gas main runs down the street, natural gas is the most convenient fuel: no tank, no refills, and no running out during a long outage. Two things limit it.

The first is availability. Large areas of Broward County have no gas distribution at all, and whether your street has a main is a question with a yes or no answer that has to be settled before anything else is decided.

The second is capacity. Natural gas has lower energy content per cubic foot than propane, so an engine produces somewhat less output on natural gas than the same engine on propane, and the manufacturer publishes both ratings. More importantly, the gas has to arrive in volume. A residential meter sized for a range, a dryer and a water heater often cannot supply a standby generator on top of those appliances, and the gas utility has to evaluate and frequently upsize the meter and sometimes the service line. Then the piping from the meter to the unit has to be sized for that flow at the required pressure over the actual run length. Undersized pipe does not announce itself at commissioning; it shows up as an engine that stumbles and drops load when it is asked to work.

Meter upgrades have lead times. Starting that conversation in May for a system you want running in hurricane season is very different from starting it in August.

Propane

Propane works anywhere, and the engine makes its full rated output on it. The constraint moves to the tank.

Runtime is a straight calculation of consumption against tank capacity, and consumption depends on load, so a generator carrying a whole house at high demand empties a tank far faster than the same generator loafing. Sizing the tank for a realistic multi day outage, not for a four hour thunderstorm, is the whole point. The other half of that is delivery: after a serious storm, roads are blocked, depots are empty, and refill trucks are working a queue. The tank you have is the fuel you have.

Siting a tank in South Florida brings its own conditions. Above ground tanks need setbacks from the building and from openings, and they need anchoring for wind. Buried tanks avoid the visual problem but cost more and have to be dealt with carefully in our high water table, where an empty or near empty tank in saturated ground can float if it is not properly ballasted and anchored. Both need to be considered against flood elevation.

Diesel

Diesel shows up on larger residential systems and on most commercial standby installations. It brings excellent load response and a fuel you can store on site, and it also brings tank permitting, containment requirements, and the reality that stored diesel degrades and grows biological contamination in a humid climate, so it needs treatment and periodic polishing. It is a good answer for the right project and an unnecessary complication for a typical house.

What an undersized generator actually does

Undersizing does not politely deliver a little less power. It produces a set of symptoms that homeowners usually misread as the generator being broken:

• The unit shuts down on overload, or trips its output breaker, typically within seconds of the air conditioning calling for cooling.
• The compressor will not start at all. You have lights and a refrigerator and a house that is climbing past eighty five degrees with the generator running fine.
• Lights dim and pulse every time a motor cycles, and the dimming is severe enough to be alarming.
• The engine hunts, surging up and down as the governor chases a load it cannot hold.
• Motors run hot. This is the expensive one. A motor operated at depressed voltage draws more current to make the same torque, and sustained undervoltage operation cooks compressor and pump windings. An undersized generator can destroy the air conditioning system it was bought to run.
• Sensitive electronics misbehave when frequency and voltage wander outside their tolerance.

None of this is repairable by the generator technician, because nothing is broken. The machine is doing what it was sized to do.

What an oversized generator costs you

Oversizing is the safer error, but it is not free, and the salespeople who default to it are not doing you a favor.

The obvious costs are the unit itself, the larger pad, the heavier feeders and transfer switch, and possibly a gas meter and service upgrade you would not otherwise have needed. Then the ongoing costs: a larger engine consumes more fuel at every load point, including during every exercise cycle for the rest of its life.

The less obvious cost is what chronic light loading does to an engine. On diesel units this is the well known wet stacking problem: run a diesel at very low load for extended periods and combustion temperatures stay too low to burn fuel completely, so unburned fuel and lubricating oil accumulate in the exhaust system, carbon glazes the cylinder walls, and performance degrades. Correcting it requires running the unit under real load, often with a load bank. On air cooled natural gas and propane units the consequence is milder but the principle holds: an engine that never works is not being maintained by its exercise cycle in the way the manufacturer intended, and carbon accumulation and fouling follow.

The right target is a generator that is comfortably loaded during a real outage, with margin for the starting surge, rather than one that idles at a small fraction of capacity forever.

Documenting the calculation for the permit

The analysis is not just for choosing equipment. It is part of the permit package, and a clean, legible calculation is one of the things that keeps a plan review moving.

What we typically submit alongside the application:

• The dwelling or building load calculation, showing the method used and the resulting demand.
• The standby load schedule, listing what is served by the generator, whether the arrangement is whole house or an essential loads panel, and the running and starting figures for the motor loads.
• The generator nameplate data, including its rating on the actual fuel being used, since the natural gas and propane ratings differ.
• The load management arrangement, identifying the listed equipment and how the shed priorities are configured, because a calculation that relies on load management has to show the equipment that enforces it.
• Conductor and overcurrent protection sizing for the generator feeder and the transfer switch, and the transfer switch rating.
• Grounding and bonding arrangement, including whether the generator is treated as a separately derived system.
• Site plan and anchoring detail, covering setbacks, clearances to openings, flood elevation, and wind load attachment.

Some jurisdictions accept the manufacturer's sizing report as supporting documentation. None of them accept a square footage estimate. If a contractor cannot produce the calculation, that is a meaningful signal about the rest of the installation.

What we need from you to run the generator load analysis

Most of this you can gather in fifteen minutes with a phone camera, and it saves a great deal of back and forth:

• Data plate photos of every air conditioning condenser, readable, showing the running and locked rotor current. This is the single most useful thing you can send.
• A photo of the panel with the door open, plus the circuit directory.
• Whether the range, dryer, water heater and pool heater are electric or gas. An electric tankless water heater in particular changes the answer more than anything else in the house.
• Pumps: irrigation or lawn well, pool pump and whether it is variable speed, booster pump, septic lift pump.
• EV charging, present or planned, and at what amperage. We can often manage this load rather than size for it, which is worth discussing along with our EV charger installation work.
• Medical equipment that has to stay running, which changes the priority list and sometimes the whole approach.
• Your honest expectation for an outage. Running everything normally, or keeping the house livable while accepting some limits. Those are different machines.
• Whether natural gas is available at the property, or whether we are planning around propane.

Commercial and multi-tenant buildings

The same discipline applies with more variables. Commercial standby sizing has to account for three phase motor loads and their starting characteristics, elevator recall and operation, fire alarm and emergency lighting requirements, refrigeration in a restaurant or grocery, server and network equipment with its own uninterruptible power supplies, and the legal distinction between emergency systems, legally required standby systems, and optional standby systems, each of which carries different rules about what may share a source.

Motor starting on three phase equipment often calls for reduced voltage starting or variable frequency drives rather than a larger generator, and the analysis has to weigh those against machine capacity. Property managers who want to understand how backup power fits into a building's overall electrical picture can start with our property management electrical page.

Begin with the numbers

Start with the generator load analysis, before the equipment. A load analysis costs a small fraction of a standby system and it determines whether the rest of the money is well spent. It also tells you things worth knowing regardless: whether your service has capacity, whether a soft starter would improve your life on utility power, and whether your panel is ready for what you are planning to add.

If you already own a generator and are not sure it will do the job, we can run a generator load analysis on the equipment you already have and tell you plainly whether it is adequate, whether load management or a soft starter would close the gap, or whether it is undersized for the house. Getting that answer in the spring is a great deal better than getting it during a named storm, and our storm preparation guide covers the rest of the seasonal checklist.

Call (954) 602-0050 to get started, or send your condenser and panel photos over and we will begin the numbers before we come out. We work across Broward County and the Miami-Dade and Palm Beach communities listed on our service area page, and our residential electrician team handles the whole project from the calculation through final inspection. If you are further along and just want a second opinion on a proposal you have already received, call (954) 602-0050 and we will read it with you.