Engines Generators And House Power

---
name: engines-generators-and-house-power
description: "Use when sizing, buying, building or wiring a generator or whole-house/off-grid power system — doing a load audit with motor starting surges, matching engine RPM to a 2-pole or 4-pole generator head, choosing a store-bought genset vs built-from-components vs inverter generator, picking diesel vs petrol vs propane, sizing a battery/hybrid bank, or arranging transfer switching, grounding and neutral bonding. Also covers the underlying machine theory: Faraday's law, the EMF equation and the RPM-to-Hz rule, back-EMF and speed droop, the loss mechanisms, and the four generator types. Part 4 of the Engines, Generators and Fuel Sources reference."
---

# Electric Generators and House Power Systems

> **Part 4 of 7** of the *Engines, Generators, and Fuel Sources* reference (plugin
> `engines-generators-and-fuels`), covering §12–§14 — Faraday to a wired house: generator theory, the four machine types, and designing or buying a house system. Sibling skills:
> `engines-thermodynamics-and-the-carnot-ceiling` (§1–§3 — the four laws, the Carnot ceiling, working fluids and phase behaviour),
> `engines-rankine-steam-engines-and-turbines` (§4–§6 — the Rankine cycle and its five improvements, reciprocating engines and turbines, and what you would actually build),
> `engines-otto-diesel-brayton-stirling-and-combined-cycles` (§7–§11 — internal combustion, gas turbines, the Stirling engine, and the combined cycle),
> `engines-fuels-and-combustion` (§15–§17 — every fuel that can be burned, transformed or harvested, with energy densities and what engine each pairs with),
> `engines-rebuilding-engines-materials-and-tolerances` (§18–§20 — engine anatomy and the rebuild process, engine management and forced induction, and the materials and tolerance thinking that makes parts real),
> `engines-safety-and-reference` (§21–§23 — the six things that kill, the glossary, and the books that actually teach this),
>
> Section numbers are **shared across the whole set**: a reference written as §N → `skill` points
> into that sibling skill. Everything here is durable engineering and physics — the laws, cycles and
> equations do not expire; efficiency figures for current plant and practice do drift.

Every engine produces mechanical work — usually rotation at a shaft. To make electricity you need a
generator: the physical implementation of Faraday's law of induction.

## §12 Generator theory

### Faraday's law and the minus sign

    EMF = −dΦ/dt

A changing magnetic flux Φ through a loop induces a voltage. The minus sign (Lenz's law) means the induced current **opposes** the change — conservation of energy in disguise. If it reinforced instead, you would have a runaway energy source.

**Three ways to change the flux:** change field strength, change loop area, or change loop orientation relative to the field. All three are used in real machines; **the most common is changing orientation** — rotating a coil in a magnetic field.

### The EMF equation and the RPM → Hz consequence

    EMF = N · B · A · ω · sin(ωt)

N = turns, B = field strength, A = coil area, ω = angular velocity. Output is sinusoidal AC. Frequency f = ω/(2π), so a 2-pole machine at **3,600 RPM produces 60 Hz**; at **3,000 RPM, 50 Hz**. This single relationship is why the whole of §14 keeps returning to shaft speed.

### Back-EMF and why speed droops under load

A motor's back-EMF opposes its supply voltage:

    current drawn = (V_supply − back-EMF) / winding_resistance

A generator runs the other way round: the induced EMF drives current **into the load**, and that
current in the machine's own field produces an **electromagnetic torque opposing the prime mover**.
That opposing torque is the load, mechanically speaking — it is how electrical power out becomes
shaft power in. Draw more current and the opposing torque rises, so the prime mover slows and
frequency falls with it, unless the governor (or an electronic inverter) puts in more input power to
hold speed. (Governor and droop: §22 → `engines-safety-and-reference`.)

### The four real loss mechanisms

Four categories; core loss splits into two sub-mechanisms.

| Loss | What it is |
|---|---|
| Copper losses | I²R in the windings |
| Core loss — hysteresis | Energy lost per magnetization cycle, proportional to the area of the B-H loop |
| Core loss — eddy currents | Circulating currents induced in the core iron, **suppressed by laminating the core** |
| Friction and windage | Mechanical drag on the rotating assembly |
| Excitation losses | Power consumed producing the magnetic field in wound-field machines |

Generator efficiency is typically **90–98% above a few kW**.

## §13 Generator types, and AC vs DC generation

| Type | How the field is produced | Typical use | Notes |
|---|---|---|---|
| Permanent Magnet (PMA) | Permanent magnets on rotor | Small wind, hydro, modified automotive alternators, portable generators | Simple, efficient, no excitation power. Voltage varies with speed — needs rectification and/or regulation. |
| Wound-Field Synchronous | DC through rotor windings via slip rings or brushless exciter | Power plants, large standby generators | Constant voltage at constant speed; field current controls output voltage; island mode or grid-parallel. |
| Induction (asynchronous) | Field induced in rotor by stator field | Wind turbines, micro-hydro | Simple, rugged, no slip rings. Needs grid or capacitor excitation for reactive power. Cannot black-start on its own. |
| Automotive alternator | Wound rotor with slip rings, field regulated by voltage regulator | Car electrical systems | Produces DC via built-in rectifier. Cheap and available. Low efficiency (50–65%) at part load. Can be modified for higher output or used for small wind/hydro. |

### AC vs DC generation

Most generators inherently produce AC. Grid-connected or whole-house generation needs AC at the correct voltage and frequency (120/240 V 60 Hz, or 230 V 50 Hz). Battery charging or direct DC loads need rectification.

PMAs produce variable-frequency AC tracking engine speed. To get stable 60 Hz you have exactly two options: **run the engine at fixed speed** (wasteful at part load), or **use an inverter** to convert variable-frequency AC → DC → clean 60 Hz AC.

**Why inverter generators win at part load:** inverter generators (e.g. Honda EU series) do exactly the second, and achieve much better part-load efficiency: engine speed varies with load, so at low load it slows, saving fuel and reducing noise. Architecture detail in §14, Option 3.

## §14 House power systems

### System architecture (complete house system)

Energy source (engine + fuel, or renewable) → generator → power conditioning (regulation, rectification, inversion) → energy storage (batteries for off-grid or backup) → distribution (panel, breakers, wiring) → control (monitoring, load management, transfer switching). **Each component must be matched to the others.**

### SAFETY — NEVER BACKFEED THROUGH A WALL OUTLET

> **NEVER BACKFEED THROUGH A WALL OUTLET.** Connecting a generator by plugging it into a wall outlet
> (backfeeding) is extremely dangerous and illegal. It energizes utility lines from your house and
> can kill a utility worker who thinks the line is dead. A proper transfer switch — manual or
> automatic — isolates the generator from the utility grid. **Non-negotiable.**
>
> A transfer switch selects between utility and generator power and makes it physically impossible to
> connect both simultaneously. An automatic transfer switch (ATS) monitors utility power, starts the
> generator when it fails, switches the load, and reverses when utility returns.

> **NEUTRAL BONDING IS CONDITIONAL — IT DEPENDS ON THE TRANSFER SWITCH.** Any one system gets its
> neutral bonded to ground at exactly **one** point. *Which* point, and whether the generator needs a
> grounding electrode of its own, depends on whether the generator is a **separately derived system**
> — and that is decided by whether the transfer switch switches the neutral.
>
> - **The transfer switch switches the neutral** (the neutral opens along with the hots): the
>   generator is separately derived. The bond belongs at the generator, and a permanently installed
>   separately derived system normally also needs its own grounding-electrode connection.
> - **The transfer switch does not switch the neutral** (generator neutral stays tied to the service
>   neutral): the generator is *not* separately derived. The single bond stays at the service
>   equipment and the generator must be **unbonded** — this is the difference between a
>   "bonded-neutral" and a "floating-neutral" machine, and many portable generators ship bonded.
>
> Get this wrong in either direction and it bites: two bonds put neutral current onto equipment
> grounding conductors and metal that is not meant to carry it, while no bond anywhere leaves fault
> current without a low-impedance return path, so protective devices may not trip. Cord-and-plug
> portable use is a separate case again.
>
> **This is not a rule you pick from a reference.** Follow the generator and transfer-switch
> manufacturer's installation instructions, and have a licensed electrician establish which case your
> installation is, to your local electrical code, and inspect the result.

Generator output is mains voltage and mains voltage kills; every generator also makes carbon monoxide. Read §21 → `engines-safety-and-reference` before running anything.

### The three options

| | Option 1: store-bought genset | Option 2: built from components | Option 3: inverter generator |
|---|---|---|---|
| What it is | Engine directly coupled to a generator, with fuel system, cooling system and control panel | Engine, generator head, coupling, control system, fuel system, cooling system, enclosure | Engine drives a PMA at variable speed; AC rectified to DC; inverter produces clean 60 Hz AC |
| Who it is for | **Correct for 99% of homeowners** | Anyone matching engine, head and drive themselves | Variable load; all premium portable generators |
| Why | The engineering is done for you, including safety interlocks, grounding, transfer switching and emissions compliance | You choose each component, at the cost of owning the speed-matching problem below | **The most efficient architecture for variable load** |

**Option 1 — store-bought genset.**
- *Sizing:* a typical home needs **5–20 kW** for whole-house backup. Do an energy audit (sum the wattages of loads you want to run simultaneously, including motor starting surges which can be **3–6× running wattage**). Over-sizing is wasteful (engines are less efficient at low load); under-sizing causes voltage drops that damage electronics and motors.
- *Fuel choice:* petrol (convenient, limited shelf life, dangerous to store in quantity); diesel (excellent for frequent/long run times, longer shelf life, more efficient, no spark ignition to fail); propane/natural gas (infinite shelf life for propane, clean-burning, can connect to utility gas for automatic operation, lower energy density); or dual-/tri-fuel for flexibility. Energy densities: §15 → `engines-fuels-and-combustion`.

**Option 2 — building from components.** Engine (small diesel or petrol, **5–20 HP**), generator head (PMA or wound-field alternator sized to the engine), coupling (direct shaft, belt, or chain with the right speed ratio), control system (voltage regulator, frequency meter, load management), fuel system, cooling system, enclosure.

*The critical matching is engine speed to generator speed.*

| Generator poles | Shaft speed for 60 Hz | Coupling |
|---|---|---|
| 2-pole | 3,600 RPM | Direct coupling works — most small engines run at 3,600 RPM |
| 4-pole | 1,800 RPM | Needs a 2:1 reduction (belt or gearbox) |

Belt drives absorb **2–5%** and need tensioning, but allow component flexibility and absorb shock loads.

*Power balance — worked through:* **1 HP ≈ 746 W**. A 10 HP engine can theoretically produce **7.46 kW**, but after generator efficiency (~90%) and drive losses (~5%) expect about **6.3 kW**. Size the engine with margin — **80% load is efficient and durable; 100% shortens engine life dramatically**.

**Option 3 — inverter generator.** Engine speed varies with load — at low load it slows, saving fuel and reducing noise. The architecture of all premium portable generators and the principle behind variable-speed standby units. The inverter also produces cleaner power (low THD), which matters for sensitive electronics.

### Sizing: the load audit

Start with a load audit: every appliance, its running wattage, and its **starting wattage** (critical for motors — well pumps, air conditioners, refrigerators need **3–6× running wattage for a few seconds**). **Size for the largest simultaneous load, not the sum of everything.**

| Scope | Typical size |
|---|---|
| Essential loads | 7–15 kW |
| Whole house including air conditioning | 15–25 kW |

### Engine selection for a house generator

| Fuel | Where it belongs, and why |
|---|---|
| **Diesel** | **Best for frequent or long-duration running** — more fuel-efficient, more durable, and diesel stores better than petrol. A small single-cylinder diesel (Lister-type or Chinese diesel) at **5–10 HP** runs for **thousands of hours** with basic maintenance, at **0.5–1.5 L/hr at rated load**. |
| **Petrol** | Cheaper, lighter and more available, but less durable at sustained high load and degrades in storage — **suitable for occasional backup**. |
| **Natural gas / propane** | Clean-burning; the fuel stores indefinitely (propane) or never needs storage (piped natural gas). Conversions of petrol engines to propane/natural gas are straightforward and common. |

Cycle background for these engines: §7–§8 → `engines-otto-diesel-brayton-stirling-and-combined-cycles`.

### Generator head selection

For 60 Hz, **2-pole needs 3,600 RPM, 4-pole needs 1,800 RPM**. Match the generator to engine speed; with a belt drive the pulley ratio sets the relationship. The generator must be rated for the engine's output with margin:

| Pairing | Result |
|---|---|
| 10 kW generator on a 10 kW engine | Fine |
| 10 kW generator on a 5 kW engine | Will only produce 5 kW |
| 5 kW generator on a 10 kW engine | A bottleneck — and the engine needs a governor to avoid overspeeding if the generator cannot absorb the power |

For an inverter system, **the inverter must be sized for peak load including motor starting surges, not just running load**.

### Battery storage and hybrid systems

A generator running 24/7 at low load is wasteful and noisy. A hybrid system runs the generator at high load for a few hours to charge a battery bank, then batteries power an inverter for the rest of the day: more efficient (the engine runs at its best BSFC point — see §22 → `engines-safety-and-reference`), quieter, and longer engine life. Size the bank for the energy needed between generator runs.

- **Lead-acid** (flooded, AGM, gel) — traditional.
- **Lithium iron phosphate (LiFePO₄)** — the modern choice: higher cycle life, higher depth of discharge, no maintenance, falling prices.

### Fuel supply and storage

| Fuel | Storage practice |
|---|---|
| Diesel | Store in steel or HDPE containers, add biocide and stabilizer for long-term storage, keep water out (condensation is the enemy — **keep tanks full to minimize airspace**) |
| Propane | Stores **indefinitely** in pressurized tanks with no degradation — an advantage for standby systems that sit unused for months |
| Natural gas | No storage needed if piped, but depends on utility reliability |
| Petrol | Use within **3–6 months**, add stabilizer, store in approved containers **away from living spaces** |

Fuel fire and vapour hazards — petrol's −40°C flash point, propane pooling below grade — are in §21 → `engines-safety-and-reference`.