Size a Home Backup Generator — Project Guide

🧑‍🔧 Size a Home Backup Generator

Field: Electrical

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Work out how many watts a backup generator needs to run and to start your loads, convert that to amps, then check the wiring between the generator and your panel.

Sizing a backup generator comes down to three separate questions: how many watts the circuits you want to back up need, both while running and at the moment the biggest motor starts; how much current that puts on the wiring at 240 V; and whether the cable between the generator (or transfer switch) and your panel can carry it over that distance without an excessive voltage drop. This guide treats your chosen circuits as one combined load and runs those checks in order.

A word on scope: sizing and installing a transfer switch or interlock is electrical work most jurisdictions require a licensed electrician (or at minimum a permit and inspection) for. Use this to understand the numbers going in, not as a substitute for that.

🧮 Inverter Sizing 🧮 Power Calculation 🧮 Voltage Drop 🧮 Cable Sizing

Watts, amps and the two ratings on a generator label

A generator's label carries two watt figures. The running (or rated) watts are what it can supply continuously. The starting (or surge) watts are what it can supply for a few seconds, and they exist because an electric motor draws several times its running current at the instant it starts. A sump pump that runs at 800 W can ask for 2,400 W for a second or two. Sizing to the running total alone is the classic way to buy a generator that stalls or trips the moment the pump kicks in.

Volts and amps come afterward. A North American house is normally split-phase: two 120 V legs that together make 240 V for the big loads. Watts divided by volts gives the current the wiring must carry, and it is the current, not the watts, that decides the conductor size and the breaker. That is why this project goes from watts to amps to voltage drop to conductor.

Making the load list

The list is the input that matters most, and it is the one nobody can do for you.

Write down what you want running during an outage, and read each appliance's nameplate: watts directly, or amps times volts.
Add the running watts of everything that could be on at once. Cycling loads such as a refrigerator rarely start together, but the total should still count each one.
Find the single largest motor and its starting surge. A pump or blower motor is commonly two to three times its running watts at start; a hard-starting air-conditioner compressor can be worse, and its nameplate's locked-rotor current gives the real figure.
Decide honestly about the big loads. An electric range, an electric water heater or central air conditioning can each be larger than everything else on the list, and the usual answer is to leave them off the backed-up circuits rather than to buy a far larger generator.

Steps

1🔌Inverter Sizing
Inverter Sizing does the same arithmetic a generator needs, in watts. Enter the total running watts of everything you want on at once, the running watts of the single largest motor, a starting-surge multiplier for that motor (nameplate "locked rotor" figures give the real number; two to three is common for a pump or blower), and a safety margin. It returns the continuous rating and the starting (surge) rating the generator must meet.
🧮 Open the Inverter Sizing calculator
2🔺Power Calculation
Pick a generator at or above those two ratings, then turn its running rating into current. Choose Single-phase, enter 240 V and a power factor (0.9 to 0.95 is a reasonable planning estimate for a mixed household load with a few motors). This calculator works from current to power, so work out the current by hand first (watts ÷ volts ÷ power factor) and enter it to confirm the kW and kVA. Hit "📋 Copy Values": Voltage Drop next needs the same current and system voltage.
🧮 Open the Power Calculation calculator
3🔋Voltage Drop
Next, check the run from the generator or transfer switch to your panel. "📥 Paste Values" fills in the current and system voltage from Step 2. Add the one-way length of the run and the resistance of your candidate conductor from a wire table (ohms per 1,000 ft), to see the actual voltage drop. Most guidance keeps it under about 3% for a single feeder run. Hit "📋 Copy Values" again: Cable Sizing needs the same current, length and voltage.
🧮 Open the Voltage Drop calculator
4🪢Cable Sizing
If Step 3's voltage drop came out too high, use this to size the conductor instead of guessing a bigger gauge. "📥 Paste Values" fills in the same current, run length and system voltage, so add your allowed voltage-drop percentage. It returns the conductor area the voltage-drop limit needs and the next standard size up. That is a voltage-drop answer only: the conductor's current-carrying rating (ampacity) is a separate code check, and the larger of the two sizes wins.
🧮 Open the Cable Sizing calculator

Worked example: backing up six circuits from a 5 kW portable generator

A house wants a furnace blower, a refrigerator, a chest freezer, a sump pump, lights and electronics, and one kitchen appliance at a time (the microwave or the coffee maker) on backup power. The nameplate running watts are 700, 200, 150, 800, 500 and 1,100, which add up to 3,450 W. The sump pump is the largest motor. The generator will live in a small shed at the back of the lot, 150 ft of cable from the panel.

Step 1: Running and starting watts

You enter	Value
Total Continuous Load	3,450 W
Largest Motor — Running Watts	800 W
Motor Starting Surge Multiplier	3
Safety Margin	20 %

The calculator returns	Value
Continuous Rating Needed	4,140 W
Surge Rating Needed	5,050 W

The calculator adds a 20 percent margin to the running load: 3,450 × 1.2 = 4,140 W. For the surge it takes the same load, swaps the sump pump's 800 W for its starting surge of 800 × 3 = 2,400 W, and keeps the rest: 2,650 + 2,400 = 5,050 W. No margin is applied to the surge figure, so a generator whose starting rating only just meets it has no cushion.

So the generator needs at least about 4,140 running watts and 5,050 starting watts. Portable units come in steps, and one rated around 5,000 running watts clears the first figure with about a fifth to spare. Check its listed starting watts against the second before you buy. We use 5,000 W below.

Step 2: Current at 240 V

You enter	Value
Line Voltage	240 V
Line Current	23.15 A
Power Factor	0.9
System	Single-phase (or split-phase, per-leg current)

The calculator returns	Value
Real Power (P)	5.00 kW
Reactive Power (Q)	2.42 kVAR
Apparent Power (S)	5.56 kVA

This calculator works from current to power, so first find the current that a full 5,000 W draws at 240 V and a power factor of 0.9: I = 5,000 ÷ (240 × 0.9) = 23.1 A. Entering that with Single-phase selected confirms it, with 5.00 kW of real power, 5.56 kVA apparent and 2.42 kVAR reactive.

Two numbers matter downstream. The current, 23.1 A, is what the wiring and breaker must carry. The apparent power, 5.6 kVA, is what a generator rated in kVA rather than watts would have to supply at this power factor. This also assumes the load is balanced across the two 120 V legs; if everything sits on one leg, that leg's breaker sees far more current than the average.

Step 3: Voltage drop with the cord you already own

You enter	Value
Current	23.15 A
Length	150 ft
Resistance	1.588 Ω/1000ft
System Voltage	240 V

The calculator returns	Value
Voltage Drop	11.03 V
Percent Drop	4.59 %
Within Limits	0 yes/no

Start with the 12 AWG cord in the garage: copper at 1.588 ohms per 1,000 ft (the standard wire-table figure at 20 °C). The calculator doubles the run for the return path, and over 150 ft the cord drops 11.0 V, which is 4.6 percent of 240 V. That is over the 3 percent guide, so it reports the run as out of limits. Lost volts turn into heat in the cable and low voltage at the panel, which motors dislike.

The 12 AWG cord has a second problem the calculator cannot see. At 23.1 A it is beyond the 20 A at which a 12 AWG conductor is normally protected, so it would overheat long before voltage drop became the complaint. Voltage drop and ampacity are separate checks, and a conductor has to pass both.

Step 4: Sizing the conductor properly

You enter	Value
Current	23.15 A
Allowed V Drop	3 %
Cable Length	150 ft
System Voltage	240 V

The calculator returns	Value
Required Area	10.00 kcmil
Next Standard Size	10.38 kcmil
Actual V Drop	2.89 %

The handoff carries the current, length and voltage across. For a 3 percent limit the copper area needed is 10.00 kcmil, and the next standard size is 10.38 kcmil, which is 10 AWG, with an actual drop of 2.89 percent. So 10 AWG just clears the voltage-drop limit for this run.

That margin is thin. Both calculators use copper at 20 °C, and a conductor carrying 23 A in a warm conduit runs hotter. Copper's resistance rises by about a fifth at 75 °C, which would push 2.9 percent to roughly 3.5 percent. On a run this long the next size up, 8 AWG, is cheap insurance.

The voltage-drop answer is also not the whole answer, because a conductor and its breaker are rated for current. Conventional practice for a generator feeder protected at 30 A is 10 AWG copper, so the two checks agree here, but only because this run is long enough for the voltage-drop rule to bite. On a 20 ft run the ampacity check would decide it. Confirm the actual conductor and breaker sizes with your code or your electrician.

What it adds up to

The chain settled three things: a generator of at least 4,140 running and 5,050 starting watts, in practice a 5,000 W class unit; about 23 A on the 240 V feeder at full load; and a conductor of at least 10 AWG, with 8 AWG the more comfortable choice, for a 150 ft run, where the 12 AWG cord we started with fails on both voltage drop and current.

The parts that decide whether the set-up is safe are not calculations at all: a transfer switch or interlock, a proper inlet, and a generator that runs outdoors and well away from windows. The safety notes below cover them.

Notes & limitations

This is a sizing sanity-check for planning purposes, not a permit-ready electrical design. Transfer switch installation, interlock kits and backfeed prevention are safety-critical and typically inspected work.
Starting watts for motor-driven loads (well pumps, air conditioners, refrigerator compressors) can run several times their running watts. The surge rating in Step 1 covers only the single largest motor starting while everything else is already running, so if two big motors could start together, size for that instead.

Safety rules no calculator covers

Backup power kills people through a few well-known mistakes, and none of them show up in a load calculation.

Run the generator outdoors only, well away from doors, windows and vents. Its exhaust carries carbon monoxide, which can kill within minutes, and a garage with the door open is not outdoors.
Never connect it to the house by plugging a double-ended cord into a wall outlet. That feeds power back onto the utility's wires and can electrocute the crews working on them, and it is illegal in many places. Use a transfer switch or a code-compliant interlock kit.
Let the engine cool before refueling, and store fuel away from the generator and from living space.
Ground and bond it as the manufacturer's manual and your code require, since the right arrangement depends on the transfer equipment you use.

When to bring in an electrician and your utility

Any permanent connection, meaning an inlet, a transfer switch or an interlock, is work most jurisdictions want done under a permit and inspected, and many require a licensed electrician. A permanent standby generator adds gas piping, and the utility may need to be told.

An electrician can also do a proper load calculation from your panel and service, which is more accurate than a nameplate list, and can tell you whether the panel's main breaker, bus and neutral are suited to the set-up you have in mind.

Frequently asked questions

What is the difference between running watts and starting watts?

Running watts are what the generator can supply continuously; starting watts are what it can supply for a few seconds, and they cover the surge a motor draws as it starts. A generator must meet both: the running watts for everything on together, and the starting watts for the biggest motor starting on top of the rest.

Why does the calculator add a margin to the running load but not to the surge?

The margin protects a generator that would otherwise be run flat out, so it applies to the continuous figure. The surge is a short peak worked directly from the motor multiplier. If you are unsure of the multiplier, raise it, which adds a cushion where you need it.

Can a 5 kW portable generator run my central air conditioner?

Usually not alongside anything else. A compressor's starting current is large, and the locked-rotor amps on its nameplate give the real number. A soft-start kit can cut the surge, but the honest choices are to size for the air conditioner or to leave it off the backed-up circuits.

Is 3 percent voltage drop a hard limit?

No. The figure is a recommendation, from informational notes in the electrical code and widely repeated, of about 3 percent for a feeder or branch circuit and 5 percent overall. Motors and electronics tolerate more, up to a point, but the loss is wasted fuel and heat, and long runs use up the margin fast.

Why does the calculator ask for a power factor?

Because the volts times amps a wire carries (apparent power) is larger than the useful watts when motors and other reactive loads are on. Sizing the wiring needs the current, which follows the apparent power, while a portable generator's label is in watts. Using 0.9 to 0.95 for a mixed household load is a reasonable planning estimate.