The short answer
Most restaurants land between 50 and 150 kW. Where you fall in that band depends on three things: square footage, how much cooking is electric versus gas, and how many motors (refrigeration, HVAC, hood fans) can start at the same time. This guide walks the same math a sizing engineer would — then our selector wizard turns your answers into a specific model.
Running watts vs starting watts, in plain terms
Every load in your building has two numbers:
- Running watts — what it draws while operating steadily.
- Starting (surge) watts — the brief spike a motor draws while spinning up. For compressors and fans this is typically 2–3× the running watts for a few seconds.
Resistive loads — ovens, fryers, heat lamps, lighting — have no meaningful surge: starting equals running. Motor loads — walk-in compressors, rooftop HVAC units, exhaust and make-up air fans, ice machines — are where sizing goes wrong. A generator must handle both the steady total and the worst-case moment when a compressor kicks on. Whichever number is bigger sets the size.
Method 1: the square-footage estimate
For food service and retail with 24-hour refrigeration, the standard planning formula is:
Generator kW ≈ 50 kW + 10 W per square foot
| Restaurant size | Formula result | Typical band (critical loads only) |
|---|---|---|
| 1,500 sq ft counter-service | 65 kW | 25–40 kW |
| 3,000 sq ft casual dining | 80 kW | 40–60 kW |
| 5,000 sq ft full-service | 100 kW | 60–100 kW |
| 8,000 sq ft large / multi-unit | 130 kW | 100–150 kW |
Read the two columns as a range, not a contradiction. The formula assumes you back up everything — HVAC, all cooking, all lighting. The right-hand band is what restaurants actually install when they protect the loads that make money and prevent losses: refrigeration, some cooking, POS, lighting, and partial HVAC. Deciding what to back up is the biggest cost lever you have.
Method 2: the worked example — a 100-seat restaurant
Industry sizing worksheets put a typical 100-seat, roughly 3,000–4,000 sq ft restaurant at about 35 kW of running load for the essentials:
| Load | Running kW | Starting kW (surge) |
|---|---|---|
| Walk-in cooler + freezer compressors | 6 | 15 (≈2.5×) |
| Reach-ins, ice machine | 4 | 9 |
| HVAC (one 10-ton rooftop unit) | 11 | 28 (≈2.5×) |
| Hood exhaust + make-up air fans | 4 | 9 |
| Lighting, POS, network, signage | 5 | 5 (no surge) |
| Gas cooking line (igniters, controls) | 2 | 2 |
| Water heater controls, misc. | 3 | 3 |
| Total | ≈35 kW | worst moment ≈ 50–60 kW |
The starting column is why 35 kW of running load does not mean a 35 kW generator. If the HVAC compressor starts while everything else is running, the system sees a momentary demand in the 50–60 kW range. That is the real requirement — hence the common recommendation of a 50–60 kW unit for this profile. If the restaurant cooks on electric ranges instead of gas, add 15–30 kW and the answer moves into the 80–100 kW class.
Two ways to shave that surge:
- Stagger the motors. If controls delay the HVAC start until refrigeration has recovered, only the largest single surge counts — and a smaller generator works.
- Soft starters or VFDs on the biggest compressors cut inrush substantially versus across-the-line starting.
Why you add 25% reserve
Never plan to run a generator at 100% of nameplate. Standard practice is to add a 20–25% reserve margin on top of your governing load:
Generator size = governing load × 1.25
The reserve covers three real things: loads you forgot or will add later (a second walk-in, EV charger, patio heaters), voltage stability when motors start, and engine longevity — a generator loafing at 60–80% load lives longer and runs cleaner than one pinned at its limit. It also keeps you above the 30% minimum monthly test load required by NFPA 110 without oversizing so far that you create a load-bank problem.
Single-phase or three-phase?
The generator must match the building's electrical service — this is a hard constraint, not a preference:
- 120/240 V single-phase: small storefronts and food-truck-scale operations, typically under ~50 kW of demand.
- 120/208 V three-phase: the standard service in most commercial buildings and strip malls. Most restaurants are here.
- 277/480 V three-phase: larger buildings and heavy kitchen equipment; less common for standalone restaurants.
Check your main panel or ask your electrician. A 208 V three-phase building needs a three-phase generator; a single-phase unit cannot serve it. Commercial standby lines such as the Cat Olympian series (20–200 kW, gas and diesel) come configured for each service type.
Before you get quotes: your sizing checklist
- Square footage and gas vs electric cooking line
- Nameplate data (running amps + LRA) for walk-in compressors and HVAC units
- Which loads are critical vs nice-to-have (refrigeration and POS first)
- Building service: voltage and phase from the main panel
- Whether large motors can be staggered on startup
- Fuel available at the site — pipeline natural gas or on-site diesel (compare them here)
- 25% reserve applied to the final number
- Load calculation confirmed by a licensed electrician before purchase
Frequently asked questions
What size generator do I need to run a whole restaurant?
Use 50 kW + 10 W per square foot as the ceiling: about 90 kW for 4,000 sq ft, 100 kW for 5,000 sq ft. Backing up only critical loads typically cuts that by 30–50%.
What size generator do I need for a restaurant walk-in cooler?
A walk-in cooler compressor typically runs at 1.5–3 kW but surges to 2–3× that at startup. A small dedicated unit should be 2.5–3× the compressor's running watts — but in practice the walk-in is one line item in a whole-building calculation, not a standalone purchase.
What size generator do I need for a 200-amp service?
A fully loaded 200 A, 120/240 V single-phase service is 48 kW; on 208 V three-phase it is about 72 kVA. A generator sized to the service (not the actual load) is usually oversized — size to the measured or calculated demand plus 25% instead.
Do I need a three-phase generator for my restaurant?
Only if your building has three-phase service — most commercial spaces do. Match the generator to the service voltage and phase exactly; your main panel label or electrician can confirm in minutes.
Get a specific model, not a range
The math above gets you to a band. Our generator selector applies the same rules — square footage, motor loads, staggering, fuel, voltage, reserve — and returns a specific commercial standby model for your restaurant. It takes about two minutes and costs nothing.