Electricity cost tools
What size generator do I need?
How is generator size actually decided?
By two independent constraints, not by one number. The continuous rating has to cover your running watts — every load that can be on at the same instant — plus whatever headroom you choose. The surge rating has to cover the startup peak, which is set by one motor load starting while everything else already runs. For the default selection here, that is 405 W running, 486 W after a 20% headroom allowance, and 1,055 W at startup — so the smallest listed class passing both checks is the 2,000 W inverter. A pass here means the size class clears these electrical load checks. It is not a recommendation to buy: voltage requirements, 120/240 V circuits, how long the generator can hold its surge rating, motor power factor and the manufacturer's own specifications all affect the real choice and are not modelled here.
Tick everything you want counted
Always on
Compressor cycles roughly a third of the day.
Cycles slightly more than a fridge. 120 W · 40% of the day
Small load that never switches off.
TVs, set-top boxes, chargers and smart devices idling.
Evening use, LED fixtures throughout.
Kitchen
Brief but high power — it sizes power capacity, not energy. 1,200 W · 2% of the day
Element cycles once the cavity is up to temperature. 2,400 W · 8% of the day
One zone in use, not the whole appliance at maximum. 2,000 W · 5% of the day
Most of the draw is the heating element, not the pump. 1,200 W · 5% of the day
A few minutes a day at full power. 2,000 W · 1% of the day
Laundry and water
Motor plus heater; the heater dominates on hot cycles. 500 W · 6% of the day
One of the largest single loads in most homes. 3,000 W · 6% of the day
Thermostatic: full power while reheating, nothing in between. 4,500 W · 10% of the day
Large start surge; a common reason sizing goes wrong. 900 W · 3% of the day
Low energy, high surge. 800 W · 3% of the day
Heating and cooling
Dominates any load list it is added to. 1,800 W · 40% of the day
Keeps a gas heating system usable. 500 W · 30% of the day
Cycling depends heavily on outdoor temperature. 3,500 W · 35% of the day
One room, moderate surge. 900 W · 50% of the day
Resistance heating: no surge, but sustained full power. 1,500 W · 50% of the day
Comfort and everything else
Screen size drives the draw. 100 W · 20% of the day
Machine plus monitors and peripherals. 200 W · 30% of the day
Charger draw, not battery capacity. 65 W · 40% of the day
Seconds a day, but a real motor start. 550 W · 1% of the day
Constant power while charging; schedule matters more than size. 7,400 W · 15% of the day
486 W
continuous capability your selected load asks for
Plus a separate short-duration requirement of 1,055 W at startup
- Running watts
- 405 W
- + 20% headroom
- 486 W
- Startup peak
- 1,055 W
- Energy per day
- 3.54 kWh
Headroom
A SnapEnergyLab default of 20% spare continuous capacity above the running load. It is a convention, not a standard — set it to 0 to see the raw comparison.
Which load sets the surge requirement
The Refrigerator. One unit starting while everything else is already running gives 405 W − 150 W + 800 W = 1,055 W. Remove or soft-start that one load and the surge column changes far more than the running column does.
Which ratings pass both electrical load checks
Two independent tests. A size passes only if its continuous rating covers your running watts plus headroom and its surge rating covers the computed startup peak. "Passes these electrical load checks" is exactly what a pass means here — see the scope note below.
| Size class | Continuous | Surge | Continuous check | Surge check | Result |
|---|---|---|---|---|---|
| 2,000 W inverter | 1,600 W | 2,000 W | ✅ 1,114 W spare | ✅ 945 W spare | Passes these electrical load checks |
| 3,500 W portable | 3,000 W | 3,500 W | ✅ 2,514 W spare | ✅ 2,445 W spare | Passes these electrical load checks |
| 5,000 W portable | 4,500 W | 5,500 W | ✅ 4,014 W spare | ✅ 4,445 W spare | Passes these electrical load checks |
| 7,500 W portable | 7,500 W | 9,500 W | ✅ 7,014 W spare | ✅ 8,445 W spare | Passes these electrical load checks |
| 9,500 W standby | 9,500 W | 12,000 W | ✅ 9,014 W spare | ✅ 10,945 W spare | Passes these electrical load checks |
| 12,000 W standby | 12,000 W | 16,000 W | ✅ 11,514 W spare | ✅ 14,945 W spare | Passes these electrical load checks |
| 22,000 W standby | 22,000 W | 26,000 W | ✅ 21,514 W spare | ✅ 24,945 W spare | Passes these electrical load checks |
The smallest listed class that passes both checks for this selection is the 2,000 W inverter. That is an arithmetic result about power capability, not advice to buy one.
What a pass does not tell you
These are electrical load checks only. A real generator choice also depends on voltage and whether you need 120 V and 240 V circuits, how long the generator can actually hold its surge rating, the power factor and starting characteristics of your motor loads, altitude and temperature derating, fuel supply and run time per tank, transfer-switch and grounding requirements, and the manufacturer's own specifications. None of that is modelled here. This page also says nothing about runtime, fuel cost or emissions.
Method
- Running watts = Σ (watts × quantity) = 405 W
- Required continuous = 405 W × (1 + 20/100) = 486 W
- Required surge = running watts − one unit's running draw + that unit's startup surge = 1,055 W
- Pass = continuous rating ≥ required continuous AND surge rating ≥ required surge
Energy and power are separate questions. This page only tests power capability; for how long a battery or generator can sustain the load, use the battery backup tools.
Assumptions
- Running watts assume no diversity — everything selected draws power at the same instant.
- Only one motor is assumed to start at a time, and its surge replaces its own running draw rather than adding to it.
- Headroom is a disclosed SnapEnergyLab default of 20% and is editable.
- Size classes and their surge allowances are generic market-common ratings from generatorSizes-1.0.0, with no vendor performance claim attached.
- The load profile is produced by summariseBackupLoads() in batteryEngine-1.2.0 — the same engine as the battery backup tools, so the two pages cannot disagree.
What can change the result?
- Whether large loads can be staggered instead of started together — that changes the surge requirement, not the energy
- Soft starters on air conditioners and pumps, which can cut the startup peak substantially
- Electric water heating, electric ranges and EV charging, which dominate the continuous requirement
- How much of the house you actually intend to run during an outage
- The headroom convention you choose
Why "add up the watts and add 20%" fails
The standard advice collapses two different constraints into one. Adding a flat percentage to a running total protects against being slightly wrong about the running load. It does nothing about startup, which is where undersized generators actually fail: the well pump tries to start, the voltage sags, and either the pump does not come up to speed or the generator trips.
Startup is dominated by a single item, not by the sum. In most houses one motor — a well pump, a sump pump, a central air conditioner or a dryer — has a surge several times its own running draw, and everything else is irrelevant to the surge question. That is why this page names the load that sets the requirement instead of only printing a number: removing that one load, or fitting a soft starter to it, changes the surge column far more than it changes the running column.
The two checks are also asymmetric in what they cost you. Failing the continuous check means the generator cannot run the house at all. Failing the surge check means it runs everything right up until one specific appliance tries to start.
Scope of this calculator
This page answers the power-capacity question only. It does not model runtime, fuel consumption, fuel cost or emissions; it does not decide between portable, inverter and standby generators; and it does not address transfer switches, grounding, permits or local electrical code. The size classes in the table are market-common nominal ratings used as reference points, not products, and no manufacturer performance claim is attached to them.
The load profile itself is produced by the same summary function used by our battery backup tools, so a load list that gives 405 W running and 1,055 W surge here gives the same figures there. Anything about how long a power source lasts is an energy question, and it belongs to the battery pages rather than to this one.
Your next decision
- Start from the load insteadRunning, average and startup watts for your own house, with no equipment recommendation attached.
- Compare against a battery instead of a generatorSame load profile, but answers how many hours a 5, 10, 13.5 or 20 kWh battery would cover.
- Size a battery for a target number of hoursTurns your load and a runtime target into required nominal capacity.
- Why capacity and power are separate limitsThe same distinction that splits this page's two checks, explained from first principles.
Electricity cost tools
- Electricity cost calculatorWatts and hours in, cost per day, month and year out.
- Cost per kWh explainedThree different costs per kWh on one bill, and which one belongs in a calculator.
- Appliance wattage calculatorA configurable appliance list: what this combination draws, and what it costs.
- Watts to run a houseRunning, average and startup watts — separated, not averaged into one range.
- kWh a house usesBenchmark, bill-derived estimate and bottom-up model, kept apart.
- Normal electricity billDiagnose the bill: is it the kWh or the rate that is unusual?