Generator Installation Invoice Template — Free Download (2026)
Standby generator installation is one of the most technically complex home improvement projects — it touches electrical, gas, mechanical, and concrete work, requires permits from multiple authorities, and involves coordination with the utility company. A generator installation invoice needs to document every system component: the generator make, model, and rated kW; the transfer switch type; the load calculation basis; the fuel source and gas line sizing; the electrical service connection; permit numbers; and manufacturer warranty registration. This guide covers what to include on a generator installation invoice for residential whole-house and critical-loads standby generators.
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Generator make, model, kW rating, and fuel type
The generator specification is the core of the invoice — and yet many generator installation invoices just say 'install 22kW Generac.' That's not enough. 'Generator: Generac Guardian 7043, 22kW standby, air-cooled, natural gas / LP propane bi-fuel capable. Installed as natural gas. Serial number: [SN]. Warranty registration number: [WRN] — registered with Generac by installer on date of completion. Engine: 999cc Generac G-Force I-Twin. Output: 22kW at 1.0 PF standby rating. Rated voltage: 240V/120V, single phase. Control panel: Generac Mobile Link remote monitoring installed — WiFi connected to home network [owner credential setup].' The serial number enables warranty lookups. The warranty registration date matters — if it's not registered by the installer, the owner should do it immediately (Generac and others require registration within a certain window). Mobile Link or equivalent remote monitoring is an important value-add to document: the owner should know it's installed and that they need to set up their account.
Transfer switch type, amperage, and circuits managed
The transfer switch is as important to document as the generator — it's the safety device that prevents backfeed to the utility (which can electrocute line workers), and its configuration determines what circuits the generator will power. Automatic transfer switch (ATS): 'Generac RXSC200A3 200A whole-house automatic transfer switch. Load-side connection to main electrical panel. Transfer time: <10 seconds from utility loss detection. Neutral switching: open transition (momentary dead-time between utility and generator power). Voltage and frequency monitoring before transfer back to utility.' Manual transfer switch: 'Manual transfer switch, 6-circuit, 30A. Covers: HVAC (circuit 1/2), refrigerator (circuit 14), sump pump (circuit 22), master bedroom (circuit 8), garage door (circuit 19). Transfer via manual lever — operator must throw switch during outage and return to utility when power restores.' Critical-loads panel (also called a generator-ready sub-panel): 'Critical loads panel: Generac RXSC100A3, 100A. Circuits transferred: see panel schedule attached. Non-critical circuits remain on utility power — generator does not power entire home.' Always document whether it's an ATS or manual — this is the most important thing a future emergency responder, electrician, or home buyer needs to know.
Load calculation — what the generator is sized to handle
A professional generator installation includes a load calculation that shows the installer selected a generator large enough for the intended loads. This protects both the installer and the customer. 'Load calculation basis: performed per NEC Article 220 and NFPA 37 guidelines. Peak connected load (selected circuits): HVAC (4-ton heat pump): 5,200W running / 14,000W starting (capacitor-assisted motor). Refrigerator: 1,200W. Well pump (1 HP): 750W running / 2,250W starting. Critical lighting and outlets: 3,500W. Sump pump (1/2 HP): 375W running. Electric range (not included — too large for generator scope). Total running load: 11,025W / Total starting load (largest motor started last): 19,900W peak. Generator selected: 22kW — provides 10%+ headroom above peak starting demand. Note: Electric vehicle charging, electric range, electric water heater, and hot tub are NOT included in this calculation and are NOT supported by this generator without load management or a larger generator.' Documenting what is NOT powered is as important as what is — it prevents the phone call when the homeowner tries to run the range on generator power and the generator trips.
Gas line: line pressure, pipe size, length, and meter capacity
Natural gas installations require specific documentation of the gas supply side. 'Gas supply: natural gas. Existing gas meter: 250,000 BTU/hr maximum capacity. Existing meter adequacy for generator: verified by contractor — meter capacity is adequate for generator plus existing appliances running simultaneously. Note: if meter is upgraded by utility company in the future, re-verification not required. Gas line: new 1-1/4" black iron pipe, 48 LF from existing gas tee at HVAC unit to generator. Gas pressure test: 1.5× working pressure for 15 minutes per IFGC 406 — passed. Operating pressure at generator: 6" WC (water column) measured with water manometer. Generator minimum inlet pressure requirement: 5" WC per Generac specification — met. Generator maximum inlet pressure: 14" WC — operating pressure within limits. Flexible gas connector: 36" stainless corrugated connector with shutoff valve at generator.' For propane: document tank size, fill level at installation, and vaporization rate adequacy. 'Propane: existing 250-gallon above-ground tank. Regulator output: 11" WC. Vaporization rate at minimum ambient temperature (10°F): 82,000 BTU/hr — insufficient for 22kW generator (130,000 BTU/hr) at temperatures below 10°F. Owner advised: in severe cold snaps the generator may not start or may run below rated output due to LP vaporization limits. Upgrade to 500-gallon tank recommended.'
Electrical service connection and utility coordination
The electrical service connection side of the invoice documents how the generator connects to the home's electrical system and what utility coordination was required. 'Electrical service: 200A overhead service, single-phase 240/120V. Transfer switch connected load-side of main breaker. Utility coordination: SouthWest Gas / [Utility name] notified per [utility] requirements. Utility permission or letter of non-objection received [date] — copy on file. Generator disconnect: 60A fusible disconnect switch installed at generator, labeled 'Generator Main Disconnect' per NEC 702.12. Anti-islanding: ATS open-transition design prevents parallel operation with utility — meets NEC 702 requirements. Service entrance cable: #2 AWG aluminum SER cable from ATS to main panel, length 22 ft.' Also document: where the disconnect switch is located (inspectors, fire departments, and future electricians need this). That it's a non-paralleling, open-transition transfer — this is the safety fact that matters for utility compliance. If the home has solar or battery storage: 'Note: home has 10kW solar array with 7.6kW grid-tied inverter. Solar is on utility-side of transfer switch — solar does not power generator loads and will not feed generator during an outage. Solar system protection: ATS disconnects generator loads from utility; solar inverter anti-islanding protection maintains grid-tie functionality independent of generator transfer.'
Permit numbers, inspections, and completed items
Generator installations typically require a building permit (mechanical), an electrical permit, and sometimes a gas permit — all separate. Document all of them. 'Permits obtained prior to installation: Building permit: [City] Building Department, permit #BP-2026-4419, issued June 2, 2026. Electrical permit: [City] Building Department, permit #EP-2026-2877, issued June 2, 2026. Gas permit: obtained by licensed gas contractor (subcontractor); permit #GP-2026-0618, issued June 3, 2026. Inspections: Building/mechanical inspection completed June 12, 2026 — passed. Electrical inspection scheduled June 15, 2026 — owner must be present. Gas inspection completed June 10, 2026 — passed. Final inspection: pending electrical inspection. Note: do not operate generator at full load until all inspections are complete and final permits are closed. Utility interconnection: notification letter sent to [Utility] June 9, 2026. No utility inspection required for standby-only (non-export) installation per [Utility] policy.' Listing inspections as pending on the invoice gives you a follow-up touchpoint and creates a record of what you were responsible for completing.
Generator installation invoice example
22kW whole-house standby — natural gas
INVOICE #GEN-2026-0147
Apex Power Solutions | (803) 555-0177 | EC Lic #: SC-E-2203 | Customer: R. & T. Wilkerson | 3814 Cedar Ridge Ln., Lexington, SC 29072 | Service: June 9–11, 2026 | Permits: BP-2026-4419, EP-2026-2877, GP-2026-0618
| Item | Amount |
|---|---|
| Load calculation — performed per NEC 220. Peak connected load: 18,900W running / 21,400W starting. 22kW generator selected with 3% headroom above peak starting demand. Loads NOT covered: electric range, EV charger, electric dryer (240V circuits that would exceed capacity). Circuits excluded documented on panel schedule. | $0 (included) |
| Generac Guardian 7043, 22kW standby generator, natural gas, air-cooled. Serial #: 8GH221220041. Installed on 36"×36" pre-cast concrete pad per Generac installation requirements. Setback: 18" from structure, 5 ft from windows and doors, 5 ft from gas meters — all met. Mobile Link remote monitoring configured and connected to home WiFi. Warranty registered with Generac on June 11, 2026, registration #10244893. | $7,200 |
| Generac RXSC200A3, 200A whole-house ATS — installed at main 200A panel. Load-side connection, open-transition, neutral switching. Transfer time ≤10 seconds. Utility-side voltage/frequency sensing before re-transfer. Generator disconnect: 60A fusible AC disconnect mounted at generator. | $2,100 |
| Gas line — 1-1/4" black iron pipe, 48 LF from existing tee at furnace to generator location. Manual shutoff valve at generator. 36" stainless flexible connector. Pressure test: 15 min at 9 psi per IFGC 406 — passed. Operating pressure at generator: 6.2" WC — within Generac spec (5"–14" WC). | $840 |
| Electrical wiring — #2 AWG aluminum SER cable, ATS to main panel (22 LF). 60A generator disconnect wiring. Control wiring: Generac Evolution controller to ATS (50 LF, 16AWG multiconductor). Mobile Link WiFi antenna installed inside garage. | $680 |
| Concrete pad — 36"×36"×4" pre-cast pad (supplied by contractor). Installed level, compacted gravel base. | $340 |
| Permits — building and electrical permits obtained by contractor. Gas permit obtained by sub. Inspections: building ✓, gas ✓, electrical scheduled June 15. | $385 |
| Commissioning — load test at 50% and 100% rated output, both passed. Transfer test (simulated utility loss): 8-second transfer time. Exercise schedule set: every Sunday 12:00pm, 20 minutes. Owner briefed on: exercise schedule, maintenance schedule (first oil change at 25 hours), fuel shutoff location, manual transfer procedure if ATS fails. | $0 (included) |
| Deposit paid June 6, 2026 (40%) | −$4,618 |
| Balance due on commissioning | $6,927 |
5 invoicing rules for generator installation contractors
Document what the generator does NOT power — it prevents the most common post-install complaint
The most predictable post-installation frustration for standby generator customers is discovering that the generator doesn't power something they assumed it would. Electric ranges, electric dryers, EV chargers, hot tubs, and whole-home air conditioning are all loads that commonly exceed what a 22kW generator can power when combined with other loads. If these loads are excluded from your load calculation and therefore not supported by the generator, put it in writing on the invoice: 'Excluded from generator support: electric range (10,000W — would exceed capacity when combined with HVAC), EV charger (7,200W), electric hot water heater. If owner wishes to run these loads on generator power, a larger generator (32kW+) or load management controller would be required.' This one note on the invoice will save you an angry phone call during the first power outage. The customer discovered they can't run the range; if you documented it, you can point to the invoice. If you didn't, you own the conversation.
List the transfer switch type and amperage every time
A transfer switch failure or miswiring can cause a fatality — a utility line worker working on what they believe is a dead line can be killed by backfeed from a generator that doesn't properly disconnect from the utility. Your invoice should specify what type of transfer switch was installed, its amperage, and that it prevents parallel operation. This isn't just documentation — it's a statement about the safety of the installation. 'ATS: open-transition, non-paralleling. Generator output is electrically isolated from utility at all times when generator is in use — no backfeed to utility grid.' This specification matters to future electricians who service the panel, home inspectors who evaluate the system, and insurance companies that may want to verify the installation. A transfer switch spec on the invoice is also what lets you prove you installed the right device if there's ever a dispute about what type of switch you quoted vs. what you installed.
Include the generator serial number and confirm warranty registration
Generator warranties (Generac, Briggs & Stratton, Kohler, Cummins) have specific registration requirements. Most require registration within 30 days of installation. If the warranty isn't registered, it may not be honored when the customer needs it — and they'll blame you. Put the serial number and warranty registration confirmation on your invoice: 'Generator serial number: 8GH221220041. Warranty registered with Generac on June 11, 2026 — registration #10244893. 5-year residential standby warranty.' This gives the customer the information they need to look up their warranty, creates a record that you completed the registration, and avoids the very unpleasant situation where a generator fails 3 years in and the manufacturer won't honor the warranty because nobody registered it.
Document the load calculation basis — even a simple one
Generator sizing is partly judgment, but it should be documented judgment. A load calculation doesn't need to be a 20-page engineering report — even a simple list of connected loads and their wattages, showing that the generator was appropriately sized for the intended circuits, protects you from the claim that you sold an undersized generator. Showing your work also differentiates you from competitors who quote generators by gut feel or by sales volume (Generac dealers have incentives to sell larger generators). If the customer requested a specific size over your recommendation, note that too: 'Customer elected 22kW generator. Contractor recommendation was 26kW based on load calculation. Customer decision accepted — generator will run at 97%+ capacity during simultaneous HVAC and well pump demand. Headroom is minimal; avoid adding loads.' This transfers the risk of the undersized choice to the customer's decision, where it belongs.
Note the permit status and when final inspection is expected
Generator installations require permits in virtually every jurisdiction — and the permit process almost always includes at least an electrical inspection. Many contractors get the permit, do the work, and move on without thinking about the final inspection. The problem: if the inspection is never completed, the permit is left open, which can cause problems at property sale. Some title companies flag open permits. 'Permits: Building #BP-2026-4419 (closed — inspection passed June 12). Electrical #EP-2026-2877 (open — inspection scheduled June 15). Gas #GP-2026-0618 (closed — inspection passed June 10). Final closeout: contractor will confirm electrical inspection passes and permit closes — customer will receive notification by June 17.' Documenting permit status also creates a checkpoint for following up. If you say the electrical inspection is scheduled for June 15, you'll actually schedule it. If it's not on your invoice, it's easy to forget. An open permit on a sold house causes headaches for everyone.
Frequently asked questions
How much does standby generator installation cost in 2026?↓
Standby generator installation costs in 2026: 10kW air-cooled, critical loads only: $5,000–$9,000 installed. 22kW air-cooled, whole-house: $9,000–$15,000 installed. 26kW air-cooled, whole-house with headroom: $12,000–$18,000 installed. 20–60kW liquid-cooled (commercial/large residential): $15,000–$50,000+ installed. Cost variables: Generator size (kW rating — biggest driver). Fuel type (natural gas is less expensive to install than propane if a gas line exists; propane requires a tank). Transfer switch type (ATS vs. manual; whole-house vs. critical loads panel). Distance from gas meter to generator location (longer gas line = more labor and materials). Electrical panel complexity. Concrete pad (some sites require a custom pour vs. pre-cast). Permit fees (vary by jurisdiction). Labor market. Brands: Generac is the largest market share. Kohler, Briggs & Stratton (Standby Power), and Cummins are strong alternatives — all comparable in quality at similar price points.
What size standby generator do I need?↓
Generator sizing depends on what loads you want to power during an outage. Rough guidelines: Critical loads only (sump pump, refrigerator, lights, some outlets): 10–13kW. Whole-house including HVAC (typical 3-ton system): 18–22kW. Whole-house including large HVAC (4+ tons): 22–26kW. Well pump included: add 2–4kW for sizing purposes. Electric range or EV charger: add 10–15kW for these loads alone. The motor starting load is the key sizing factor: HVAC compressors and well pumps have large starting currents (3–6× running current) that last for a fraction of a second but determine the peak demand the generator must meet without stalling. Your generator installer should perform a load calculation before recommending a size. Be skeptical of installers who recommend the same size to everyone without a calculation.
What is the difference between a standby generator and a portable generator?↓
Standby generators: Permanently installed, typically on a concrete pad outside the home. Automatically detect power loss and start within 10–30 seconds. Connected to the home's wiring via a transfer switch — powers circuits throughout the house. Run on natural gas or propane from an existing line or tank — you don't need to buy or store fuel. Sized for days or weeks of continuous operation. Cost: $5,000–$50,000+ installed. Portable generators: Freestanding, stored and brought out when needed. Must be manually started and connected with extension cords or a manual transfer switch/interlock. Run on gasoline — you must store fuel or source it during an emergency (when gas stations may be closed). Sized for hours to a day or two of operation. Must be operated outside away from doors and windows (carbon monoxide risk). Cost: $500–$5,000 for the unit; add cost of transfer switch or interlock kit if connecting to the panel. For a residential standby generator invoice, you're almost always billing for a permanent standby unit, not a portable — but it's worth noting the distinction because some customers confuse the two.
How often should a standby generator be tested?↓
Most standby generator manufacturers recommend weekly automatic exercise runs of 20–30 minutes under load. This keeps the engine oil warm, exercises the battery, keeps fuel lines and injectors clear, and verifies the system is operational. The generator's automatic exercise can be set to run on a specific day and time — set it to a time when the noise won't disturb your neighbors (weekdays around noon, not 6am Saturday). Beyond the weekly exercise: Annual maintenance: oil and filter change, spark plugs, air filter, battery check, transfer switch contacts inspection. Every 2 years or 200 hours: coolant (liquid-cooled units). Every 3–5 years: battery replacement. Oil change at 25 hours (first oil change after break-in period is important). Many generator contractors offer annual maintenance contracts — this is a good recurring revenue stream for installers and a good value for owners who don't want to track maintenance themselves.
Does a standby generator add value to a home?↓
Yes — standby generators typically add $10,000–$15,000+ to appraised value for a well-documented, permitted installation. Value drivers: Whole-house coverage is worth more than critical loads only. Permitted and inspected installation (vs. unpermitted). Recent installation with recent service records. Brand (Generac, Kohler, Cummins have strong recognition). Fuel source (natural gas is more convenient than propane for most buyers). Warranty remaining. Real estate agents in storm-prone markets (coastal areas, tornado corridors, ice storm regions) frequently cite standby generators as a meaningful selling point. In markets where power outages are common, buyers may factor the generator into their offer. Practical note: provide your customer with a copy of the final invoice, permit closeout documentation, warranty registration confirmation, and maintenance records — this package is what they'll need to demonstrate the value of the system at resale. A standby generator with no paperwork is worth much less to a buyer than one with a complete installation record.
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