Power versus energy: watts and kilowatt-hours
Watts and kilowatt-hours measure two different things, and almost every sizing mistake starts by confusing them. Power, measured in watts, is the rate at which a device uses electricity right now. Energy, measured in kilowatt-hours, is the total amount used over time.
The simplest way to keep them apart is a car analogy. Watts is your speed, how fast you are going at this instant. A kilowatt-hour is the distance you cover, the result of that speed applied over time. A device with high wattage that runs for seconds can use less energy than a low-wattage device left on all day.
This is why the calculator asks two things for each appliance, its wattage and the hours you use it. Wattage alone tells you the load an inverter must carry in the moment. Wattage multiplied by hours tells you the energy a battery or a solar system must supply across the day.
How to read an appliance's wattage
You do not have to guess. Nearly every appliance carries a rating plate, usually a sticker or a stamped panel on the back or base, that lists its electrical demand.
Some plates print the wattage directly, for example "1500W". Others give you voltage and current instead, such as "220V" and "5A". For a simple resistive load you multiply them: Volts × Amps = Watts. So 220 volts at 5 amps is 1,100 watts.
Watch the gap between running watts and peak watts too. A washing machine might list a high rating it only reaches during the spin cycle or when its internal heater runs. For daily energy the running figure matters most, with the brief startup surge handled separately.
Motor surge, duty cycle, and standby power
Not every watt behaves the same way over a day, and three effects change how much energy an appliance really uses.
The first is motor surge. Anything with a compressor or a motor, a fridge, a water pump, an air conditioner, pulls two to five times its running watts for a second or two at startup. That spike is critical when you size an inverter, but it barely touches daily energy because it lasts moments. The inverter size calculator accounts for it.
The second is duty cycle. A fridge is plugged in all day, but its compressor only runs part of the time, cycling on and off to hold temperature. Its nameplate might say 200 watts, yet across 24 hours it may only run the equivalent of 8 full hours.
The third is standby power, sometimes called phantom or vampire load. Televisions, chargers, set-top boxes, and routers draw a trickle of power around the clock, even when off or idle. Each one is tiny, but a house full of them can quietly burn several kilowatt-hours a week.
How to calculate your daily energy
The method is simple and the calculator above does it for you, but it helps to see the working.
For each appliance, multiply its watts by the hours you use it in a day, then divide by 1,000 to get kilowatt-hours. Add up every appliance and you have your daily energy.
Take a modest home through an evening. Five 10 watt LED bulbs for 5 hours is 0.25 kWh. Three 75 watt fans for 8 hours is 1.8 kWh. A 90 watt TV for 5 hours is 0.45 kWh. A fridge at 200 watts for 8 effective hours is 1.6 kWh. Add a laptop and the WiFi and the total lands near 4.5 to 5 kWh for the day. That single figure is the foundation of any solar or battery plan.
The biggest energy users in your home
Not all appliances are equal. Energy is watts multiplied by hours, so the appliances that are both high-powered and run for a long time dominate everything else.
The pattern is always the same. Anything that makes heat or cold, an air conditioner, an electric water heater, an iron, a kettle, uses far more energy than lights and electronics ever will. An air conditioner is the worst of both worlds, high wattage and long hours, which is why a single AC can use more energy than the rest of the house combined.
This tells you where to focus. Shaving power off a 10 watt bulb is pointless. Cutting the hours on an air conditioner, or switching to an efficient inverter model, changes your bill and your backup needs dramatically.
From wattage to inverter, battery, and solar
Once you have your numbers, here is exactly where each one goes.
Your total load in watts sizes the inverter or generator. It has to supply everything running at once, plus the startup surge of the largest motor. Feed your load into the inverter size calculator to get the rating and the battery voltage to shop for.
Your daily energy in kilowatt-hours sizes the solar array and the battery. Divide daily energy by your local peak sun hours and add for losses to get panel wattage. Our guide on how much solar you need walks through it step by step.
For the battery, remember you cannot use all of its rated capacity. Lead-acid and tubular batteries give you about half, lithium gives you most of it. So a night that needs 3 kWh of energy needs roughly a 6 kWh tubular bank, or a 3.5 kWh lithium one. Our guide on making a battery last longer explains why.
Reading your electricity bill
Your monthly kilowatt-hours is not just a planning number. It is the thing your utility charges you for.
Electricity is billed per unit, and one unit is one kilowatt-hour. Your bill is, roughly, your monthly kWh multiplied by the tariff per unit. If your appliances add up to 250 kWh a month, that figure multiplied by your rate per unit is your bill before taxes and fixed charges.
Many countries use slab or tiered tariffs, where the price per unit rises as you cross usage thresholds. Under that system the last units you use are the most expensive, so trimming a heavy user like an air conditioner can save more than the raw kilowatt-hours suggest, because it pulls you back out of a higher-priced slab.
How to cut your energy use
Once you can see where your energy goes, cutting it is straightforward. Focus on the big users and the long hours, not the small stuff.
- Attack heat and cold first. An air conditioner, water heater, or iron uses more in an hour than a fan or a bulb uses in a day. A ceiling fan draws roughly a tenth of the power of an AC, so use fans first and the AC only when you must.
- Replace old, inefficient hardware. An old fridge or a conventional AC runs for hours every day, so an efficient inverter model pays back its higher price through lower bills and smaller backup needs.
- Kill standby loads. Switch televisions, chargers, and set-top boxes off at the wall rather than leaving them idle. A switched power strip turns that into a one-tap habit.
- Swap every bulb for LED. They use a fraction of the power of older bulbs for the same light, and because lights run for many hours the saving is real.
- Match the appliance to the job. A microwave uses far less energy than an oven for a small meal, and a laptop uses a fraction of a desktop computer.
Frequently asked questions
What is the difference between watts and kWh?
Watts is how fast you use power right now. A kilowatt-hour is how much energy you use over time. A 1,000 watt appliance running for one hour uses one kilowatt-hour.
How do I find the wattage of an appliance?
Check the rating plate, usually a sticker or metal panel on the back or base. It either prints the watts directly or gives volts and amps. For a simple resistive device, volts times amps equals watts.
Which number do I use to size an inverter?
Your total load in watts, plus the startup surge of the largest motor. The inverter must supply everything that is on at once, in the moment.
Which number sizes solar panels and batteries?
Your daily energy in kilowatt-hours. Panels are sized around how much energy you use in a day, and the battery around how much of that you need after dark.
Why is my fridge counted at 8 hours if it is always plugged in?
A fridge cycles its compressor on and off to hold temperature. It is not drawing full power the whole time, so a few effective hours is a fair daily estimate.
How much electricity does an air conditioner use?
A lot. A 1 ton unit draws around 1,000 to 1,200 watts and often runs for hours, so it can use over 1 kWh every hour. A single AC can use more energy than the rest of a small home combined.
Does switching things off at the wall really save power?
Yes, a little. Televisions, chargers, and set-top boxes draw a trickle of standby power around the clock. Each is small, but together they can add several kWh a week.
Knowing your numbers turns guesswork into planning. Your total load sizes your backup, your daily energy sizes your solar and battery, and your monthly kilowatt-hours explains your bill. Add your appliances at the top of the page and you have all three.
