What Size Generator Do I Need? A Step-by-Step Sizing Guide
The honest answer to what size generator do I need is: big enough to cover the largest combination of things you will run at the same time, plus the brief power surge when motors start. That number comes from adding up your loads, not from the size of your house. This guide walks through the process step by step, with a sample worksheet using typical appliance ranges, so you can arrive at a realistic starting point before you talk to a professional.
In this guide
- Sizing basics: watts, kilowatts and why surge matters
- Step 1: List what you want to power
- Step 2: Find running and starting watts
- Step 3: Account for HVAC and motor surge
- Step 4: Choose essential circuits or whole-home
- Sample sizing worksheet (approximate values)
- How load management changes the answer
- Why a professional load calculation matters
- When to call a professional
Sizing basics: watts, kilowatts and why surge matters
Generators are rated by how much electrical power they can deliver:
- Watts (W) measure power. Most portable generators list their output in watts.
- Kilowatts (kW) are thousands of watts. Standby generators are usually rated in kW, so a 22 kW unit is rated around 22,000 watts.
- Running (rated) watts are what an appliance uses once it is going.
- Starting (surge) watts are the short burst some appliances need to get going, especially anything with a motor or compressor.
A generator usually lists both a continuous rating and a higher peak or surge rating. Your total running load must fit within the continuous rating, and your biggest single start-up surge, added to everything already running, must fit within what the generator can deliver at that moment. Many professionals also recommend not running a generator near 100% of its rating for long periods; leaving some headroom is easier on the engine and gives room for surges.
Fuel type can matter too. Some standby generators produce slightly less power on natural gas than on propane, and manufacturers list both ratings on the spec sheet.
Step 1: List what you want to power
Walk through your home or building and write down what you would truly want running during a multi-day outage. In Houston, that list is shaped by the season: in summer, cooling is usually the biggest decision, while in a rare freeze like February 2021, heating and protecting pipes take priority.
Common categories:
- Must-haves: refrigerator and freezer, some lighting, internet and phones, medical equipment, sump or sewage pump, garage door opener.
- Comfort: central air or window units, ceiling fans, furnace blower, TV.
- Heavy hitters: electric water heater, electric range or oven, electric dryer, pool pump, well pump, EV charger.
Be realistic. You probably do not need the dryer, oven and EV charger running during a storm, and leaving them off the list can cut your required size significantly.
Step 2: Find running and starting watts
The best source for each appliance is its nameplate – the sticker or plate listing volts, amps and sometimes watts. If only volts and amps are shown, you can estimate running watts with:
Watts ≈ Volts × Amps
So a 120-volt appliance drawing 5 amps uses about 600 watts. For 240-volt appliances such as an air conditioner condenser or water heater, use 240 in the formula.
Starting watts are harder to find on a label. Motor-driven appliances often surge to two or three times their running watts, and air conditioner compressors can go higher. Air conditioners may list LRA (locked rotor amps), which hints at how big that starting surge can be.
Step 3: Account for HVAC and motor surge
The single biggest sizing challenge in most Houston homes is the central air conditioner. A typical system has a compressor and condenser fan outside plus a blower inside. The compressor’s starting surge is short but large, and if the generator cannot supply it, voltage dips, lights flicker, and the generator may trip its breaker or shut down.
Two rules of thumb help:
- Size around your largest motor. Add the starting watts of your biggest motor load to the running watts of everything else that will be on at the same time.
- Do not assume motors start one at a time. Refrigerators, freezers, sump pumps and AC units cycle on their own schedules. Plan for at least the largest surge occurring while other loads are already running.
A soft-start kit on an air conditioner can noticeably reduce its starting surge, which sometimes allows a smaller generator to start it reliably. Whether that applies to your unit is something to confirm with an HVAC or generator professional.
Step 4: Choose essential circuits or whole-home
You have two broad paths:
- Essential circuits: a transfer switch feeds a subpanel holding only the circuits you selected. This keeps the generator smaller and fuel use lower, but anything not on that subpanel stays dark.
- Whole-home: a service-rated transfer switch backs up the entire panel. It is more convenient – the house works almost normally – but requires a larger generator or load management to avoid overloads when big appliances run together.
Portable generators almost always fall into the essential-circuits approach, connected through a manual transfer switch or interlock. Standby generators can go either way.
Sample sizing worksheet (approximate values)
The figures below are typical approximate ranges for illustration only. Real values vary widely by model, age and efficiency. Always use your own nameplate data or a professional measurement.
| Appliance | Approx. running watts | Approx. starting watts |
|---|---|---|
| Refrigerator / freezer | 600–800 | 1,200–2,200 |
| LED lighting (10–15 bulbs) | 100–200 | – |
| Internet router, modem, phone chargers | 50–100 | – |
| Television | 100–300 | – |
| Microwave | 1,000–1,500 | – |
| Sump pump (about 1/2 hp) | 800–1,050 | 1,500–2,500 |
| Furnace blower (gas furnace) | 500–900 | 1,000–2,000 |
| Window AC (around 10,000 BTU) | 900–1,500 | 2,000–3,500 |
| Central AC (around 3 tons) | 3,000–4,000 | Can be several times running watts |
| Electric water heater | 4,000–5,500 | – |
Example: essential-circuits household (summer, window AC). Refrigerator (700), lights (150), internet and chargers (100), TV (200), sump pump (1,000), and two window AC units (1,200 each = 2,400) gives roughly 4,550 running watts. The largest single surge might come from a window unit, adding around 1,500–2,000 watts above its running draw at start-up. A generator with a continuous rating in the 6,000–7,500 watt range would be a reasonable starting point for this approximate example, before a professional confirms it.
Example: whole-home with central air. Add a 3-ton central AC (around 3,500 running) plus its starting surge, an electric water heater and normal household loads, and the requirement climbs quickly into the standby range measured in kilowatts. This is exactly where a load calculation and possibly load management become essential.
How load management changes the answer
Load management lets a generator serve more circuits than it could carry all at once. Load shedding modules temporarily switch off selected heavy appliances – an electric dryer, water heater or second AC – when the generator nears capacity, then restore them when power is available. Smart circuit managers can prioritize loads so essentials always stay on.
The result is that a whole-home transfer switch can sometimes pair with a smaller generator than a simple sum of every appliance would suggest. The trade-off is that some large loads will pause during the outage. Our guide on how emergency generator systems work explains where load shedding fits into the start-and-transfer sequence.
Why a professional load calculation matters
The worksheet above is a planning tool, not a final answer. A professional load calculation accounts for things a homeowner estimate tends to miss:
- the actual electrical service size and panel layout;
- measured starting current on your specific air conditioner and pumps;
- fuel type and how it affects the generator’s rated output;
- future additions such as a pool, workshop, EV charger or home office;
- local permitting and installation requirements;
- placement, clearances and fuel supply sizing.
Getting the size right protects your investment both ways: an undersized generator struggles or shuts down when you need it most, and an oversized one costs more to buy and to fuel. For businesses, the stakes are higher because motor loads, three-phase power and critical equipment all change the calculation.
When to call a professional
If you already own a generator and the lights flicker or it shuts down when the AC starts, it may be undersized for your current loads or have a fault – our generator diagnostics service can tell the difference. If you are planning a new system, JaWilly Services handles residential generator installation and commercial generator services across Houston, Katy, Sugar Land and nearby communities, with upfront quotes and no hidden fees.
Before storm season, run the load test in our hurricane season generator checklist to confirm your generator can carry what you need. Call or text 832-418-7218 or contact us to talk through your loads with a certified technician.