Electricity cost tools
How many watts does it take to run a house?
How many watts does a house need?
There is no single number, because three different watt figures answer three different questions. For the default selection here — refrigerator, router, always-on electronics and whole-house LED lighting — the running watts are 405 W if everything draws power at the same instant, the average across 24 hours is 148 W, and the startup peak is 1,055 W for a fraction of a second when the compressor starts. That average works out at 3.54 kWh per day. Add electric heating, electric water heating, air conditioning or EV charging and every one of those figures changes by more than all the small loads combined — so tick your own list rather than reading a generic range.
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
405 W
running watts — everything you ticked, drawing power at the same instant
148 W average across 24 hours, or 3.54 kWh per day
- Running watts
- 405 W
- Average watts
- 148 W
- Startup peak
- 1,055 W
- Energy per day
- 3.54 kWh
Three different watt figures, three different questions
Running watts assume every ticked item draws its full power at the same moment, with no diversity and no staggering. Average watts discount each item by how much of the day it actually draws power — that is the figure that turns into kilowatt-hours. Startup peak is a fraction of a second, not a load you run continuously. Pages that publish a single "a house needs 5,000–7,500 W" range are mixing all three.
What sets the startup peak
The Refrigerator sets it. One unit of it starting while everything else is already running gives 405 W − 150 W + 800 W = 1,055 W. Two identical motors are not assumed to start in the same instant.
| Load | Watts × qty | Share of day | Average draw | Start surge (each) |
|---|---|---|---|---|
| Refrigerator | 150 W × 1 | 35% | 53 W | 800 W |
| Router, modem and hub | 15 W × 1 | 100% | 15 W | — |
| Standby and always-on electronics | 40 W × 1 | 100% | 40 W | — |
| LED lighting (whole house) | 200 W × 1 | 20% | 40 W | — |
Method
- Running watts = Σ (watts × quantity) = 405 W
- Average watts = Σ (watts × quantity × share of day) = 148 W
- Energy per day = 148 W ÷ 1000 × 24 h = 3.54 kWh
- Startup peak = running watts − one unit's running draw + that unit's startup surge = 1,055 W
This page sizes the load only. It does not recommend a generator, inverter or battery rating: those depend on voltage, phase, surge duration, power factor and manufacturer specifications, which this tool does not model.
Assumptions
- Running watts assume no diversity: every selected item is drawing power at the same instant.
- Share of day is a 24-hour duty cycle, not the number of hours the appliance is switched on at the wall.
- Only one unit of one motor load is assumed to start at a time; the surge replaces that unit's running draw rather than stacking on top of it.
- Nameplate ratings are typical examples from the house-load dataset and are editable on every row.
- All figures come from summariseBackupLoads() in batteryEngine-1.2.0 — this page adds no arithmetic of its own.
What can change the result?
- Which loads you genuinely run at the same time, rather than everything the house owns
- Duty cycle: a thermostatic load such as a water heater or air conditioner swings with weather and setpoint
- Electric heating, electric water heating and EV charging each move the result more than every small load combined
- Motor startup surges vary widely between soft-start and direct-on-line equipment
- Quantity — two fridges, two pumps or a second AC unit change the profile, not just the total
Running, average and startup are not interchangeable
Most pages answering this question publish a single range, typically 5,000 to 7,500 W. That number is a running-watt estimate for a particular American house with central air conditioning, and it is presented without saying which of the three quantities it is. It cannot be used to estimate a bill, because running watts are not what you consume; and it cannot be used on its own to size equipment, because it says nothing about startup.
Running watts are the sum of every selected item at its rated power, all at once. Nothing is staggered and no diversity is assumed, which makes it deliberately conservative. It is the figure a power source has to sustain if everything happens to be on together.
Average watts weight each item by the share of the day it actually draws power. A fridge compressor runs roughly a third of the time; lighting is an evening load; a water heater is at full power only while it reheats. Average watts are what turn into kilowatt-hours and therefore into money.
The startup peak lasts a fraction of a second. It is what a motor demands as it comes up to speed. The tool takes the worst realistic case: one single unit of one motor load starting while everything else is already running. That unit's normal running draw is replaced by its startup surge, rather than the surge being stacked on top of it, and two identical motors are not assumed to start in the same instant.
Where the daily energy figure comes from
Energy per day is the average watts divided by 1,000 and multiplied by 24 hours. For the default selection that is 148 W ÷ 1000 × 24 = 3.54 kWh. Everything on this page comes from the same load-summary function used by the battery backup tools, so the two cannot produce different running, average or surge figures for the same list of loads.
What this page will not do is tell you what to buy. Converting a load profile into a generator, inverter or battery rating brings in voltage and phase, surge duration, power factor and manufacturer specifications. That is a separate question, on a separate page.
Your next decision
- Check which generator ratings cover this loadRuns your running watts and startup peak against common generator size classes as two separate checks.
- See how long a battery would run these loadsSame engine, but answers the energy question: runtime across 5, 10, 13.5 and 20 kWh batteries.
- Turn these watts into a cost per monthAverage watts and your own rate give cost per hour, day, month and year.
- Why nameplate watts overstate real consumptionThe duty-cycle problem behind the difference between running and average watts.
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.
- Generator sizeContinuous and surge checked separately, with the limiting load named.
- 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?