Load Calculator

Every electrical project starts with one simple question: how much power will everything draw when it runs together. Guess too low and breakers trip, cables run hot and generators stall. Guess too high and you overspend on a panel, a generator or a circuit that never needed to be that large. This Load Calculator removes the guessing by adding up the wattage of your appliances, tools, lights and equipment, then turning that figure into the numbers you actually plan with: connected load, demand load after diversity, current in amps, daily energy use and the running cost on your electricity bill.

The tool is built for homeowners planning a workshop or an extension, landlords checking a rental unit, small business owners fitting out a shop, and anyone sizing a backup generator or an inverter. You enter the quantity and wattage of up to five items, your supply voltage, a diversity factor, the hours each day things run and the price you pay per kilowatt hour. It handles the arithmetic instantly and consistently, so you can test several combinations in seconds and see which setup keeps you inside safe limits without wasting money.

What Electrical Load Means

Electrical load is the total power demanded by everything connected to a supply at a given moment, measured in watts. A single LED bulb might draw 10 watts, a kettle 2,000 watts and an industrial motor many thousands. The load on a circuit is simply the sum of everything running on that circuit at once. Electricians care about load because every wire, breaker and transformer has a rating, and staying inside that rating is what keeps an installation safe and reliable over years of daily use.

Load is not the same thing as energy. Power in watts describes the rate of consumption right now, while energy in kilowatt hours describes how much power flows over time. Your electricity bill is based on energy, which is why the calculator asks for running hours as well as watts. A 2,000 watt heater used for one hour consumes the same energy as a 200 watt load left running for ten hours, and both cost exactly the same to run even though their loads differ tenfold.

Connected Load Versus Demand Load

Connected load is the simple total of every nameplate wattage on a circuit or in a building, assuming everything is switched on at the same time at full power. It is the starting figure for any load study, and this calculator reports it first. Real life rarely matches that assumption. Lights, sockets, machines and appliances cycle on and off through the day, so the true peak, called the demand load, is normally well below the connected total.

The diversity factor bridges the gap between the two figures. It is the percentage of the connected load you realistically expect to run at the busiest moment. A home where everything genuinely could run together might use 90 percent, while a large office with dozens of machines that never all start at once might use 60 or 70 percent. Electrical codes set their own demand rules for specific installations, and a licensed electrician will apply those rules on regulated work. For planning and budgeting, a sensible diversity figure gives you a far more realistic current and cost estimate than the raw nameplate total.

How To Use This Load Calculator

The calculator accepts up to five items or circuits, which is enough for most single room, single panel or single generator checks. Follow these steps for a dependable result.

1. Gather the wattage of each appliance or machine from its nameplate label, manual or product listing, noting how many of each you will run.
2. Enter the quantity and the watts per item on each row, and set the rows you do not need to zero.
3. Enter your supply voltage, such as 120, 208, 230 or 240 volts, matching the supply the load will actually connect to.
4. Choose a diversity factor that reflects how much of the total realistically runs at the busiest moment.
5. Add the average hours per day the load runs and the price you pay per kilowatt hour from your latest bill.
6. Read the demand load, the amps, the daily and monthly energy use, and the daily, monthly and yearly running costs.

A Worked Load Example

Imagine a small workshop with ten LED fittings at 100 watts each and two bench machines at 1,500 watts each. The connected load is 1,000 watts of lighting plus 3,000 watts of machinery, a total of 4,000 watts. With a diversity factor of 80 percent, the demand load is 3,200 watts, because the machines and every light seldom peak together. On a 240 volt supply that demand draws about 13.3 amps, a comfortable figure for a 20 or 32 amp circuit.

Run that demand for eight hours a day and the workshop uses 25.6 kilowatt hours daily, or roughly 768 kilowatt hours in a 30 day month. At 15 cents per kilowatt hour the running cost works out to 3.84 dollars a day, 115.20 dollars a month and about 1,382 dollars a year. Numbers like these turn a vague worry about electricity bills into a concrete figure you can budget for, compare against a cheaper tariff, or use to judge whether more efficient machines would pay for themselves.

How Watts, Volts And Amps Fit Together

Three values drive every load calculation, and each one can be found from the other two. Power in watts equals voltage multiplied by current in amps for simple resistive loads. Current in amps equals watts divided by volts, which is the conversion this calculator uses to size circuits. Energy in kilowatt hours equals watts multiplied by hours, divided by one thousand. Knowing these relationships helps you sanity check any figure the tool produces and understand the limits printed on nameplates.

• Watts from volts and amps: multiply volts by amps, so 240 volts at 10 amps delivers 2,400 watts.
• Amps from watts and volts: divide watts by volts, so a 2,400 watt load on 240 volts draws 10 amps.
• Kilowatt hours: multiply watts by hours and divide by 1,000, so 500 watts for four hours uses 2 kilowatt hours.
• Running cost: multiply kilowatt hours by your price per kilowatt hour from your electricity bill.
• Motor loads: motors draw extra starting current, so always keep headroom instead of planning to the exact limit.

Planning A Circuit, Panel Or Generator

The amps figure is the one that decides hardware. Breakers and cables are rated in amps, and good practice keeps continuous loads at or below about 80 percent of the breaker rating. If your demand works out to 26 amps continuous, a 32 amp circuit is a sensible choice, while a 20 amp circuit would run hot and trip. For a whole panel, add the demand loads of each circuit, then check the total against the panel rating printed inside the door.

Generator and inverter sizing uses the demand load in watts, with one important extra: starting surge. Motors, compressors and pumps can draw two or three times their running wattage for a few seconds while starting. Add generous headroom above your demand figure, typically 20 to 30 percent, and avoid the temptation to run a generator flat out for hours, since engines last longer and run cleaner at a moderate share of their rating.

Common Load Calculation Mistakes

• Adding amps instead of watts: always total the watts first, then convert the combined figure to amps at your supply voltage.
• Forgetting standby draw: chargers, screens and appliances on standby add up across a whole building and run all day.
• Using the wrong voltage: the same 2,400 watt load draws 20 amps at 120 volts but only 10 amps at 240 volts.
• Ignoring starting surge: anything with a motor needs extra capacity at switch on, not just its running wattage.
• Assuming everything runs together: skipping the diversity factor oversizes equipment and inflates quotes.
• Planning to 100 percent of a breaker: continuous loads need headroom or the breaker will age, heat up and trip.

Cutting Running Costs Once You Know The Load

A load study often pays for itself the first time you read the monthly cost line. The biggest wins usually come from the items that run the longest hours rather than the items with the highest wattage. Swapping ten 100 watt fittings for efficient 40 watt equivalents cuts 600 watts of connected load and saves money every single hour they are on. Scheduling heavy machines outside peak tariff hours, fixing timers so nothing runs overnight by accident, and replacing one ageing motor can each trim the monthly figure in ways a simple tariff switch never will.

Revisit the calculation whenever you add equipment. Loads creep upward over time as extra machines, heaters and chargers join a circuit that was sized years earlier. A quick check with realistic quantities and hours keeps you ahead of nuisance trips and surprise bills, and gives you hard numbers to hand to an electrician when you are ready to upgrade.

Frequently Asked Questions

What is the difference between connected load and demand load?

Connected load is the sum of every nameplate wattage, assuming all items run at full power at once. Demand load is the realistic peak after applying a diversity factor, because in practice not everything runs together. Circuits, panels and bills are planned around demand load, while connected load is the honest starting point for the calculation.

How do I find the wattage of an appliance?

Look for the rating plate on the back, underside or inside the door of the appliance, or check the manual and the maker listing online. The plate usually shows watts directly, or shows volts and amps, which you multiply together to get watts. For small electronics, the power adapter label lists the maximum draw.

What diversity factor should I use?

Homes and workshops where most equipment can genuinely run together suit 80 to 100 percent. Offices, retail spaces and larger installations where loads cycle on and off often sit between 60 and 80 percent. When in doubt, choose the higher figure, because planning with a little spare capacity is far cheaper than upgrading later.

Why does the voltage change the amps so much?

Current equals watts divided by volts, so doubling the voltage halves the current for the same power. A 2,400 watt load draws 20 amps on a 120 volt supply and only 10 amps on a 240 volt supply. That is why large appliances use higher voltage circuits: the same power flows through thinner, cooler cables.

How much headroom should I leave on a breaker?

A widely used rule keeps continuous loads at or below 80 percent of the breaker rating, so a 20 amp breaker should carry no more than about 16 amps for long periods. Headroom covers warm rooms, ageing components and small measurement errors, and it stops the nuisance tripping that plagues circuits sized right at the limit.

Can this calculator size a generator for me?

It gives you the demand load in watts, which is the main input for generator sizing. Choose a generator rated comfortably above that figure, typically 20 to 30 percent higher, and remember that motors and compressors draw a starting surge of two or three times their running watts. Check the generator surge rating against your largest motor.

What is a kilowatt hour in simple terms?

A kilowatt hour is the energy used when a 1,000 watt load runs for one hour. A 100 watt bulb running for ten hours also uses one kilowatt hour. Your meter counts kilowatt hours and your tariff prices them, which is why the calculator combines watts with daily hours to produce your running costs.

Why does my bill differ from the monthly cost shown?

The calculator uses a simple 30 day month, your stated hours and one flat price per kilowatt hour. Real bills add standing charges, tiered rates, taxes and estimated readings, and your actual running hours will vary day to day. Treat the figure as a close planning estimate and compare the energy portion of your bill for the fairest match.

Do standby loads really matter?

Yes, because they run around the clock. Ten devices drawing 5 watts each on standby consume 1.2 kilowatt hours every day, which is more than 400 kilowatt hours a year. On their own they look trivial, but across a house, office or workshop the standby total is one of the easiest costs to cut with switched sockets and timers.

How do I calculate load for a whole house?

List the major items room by room: heating, cooling, water heating, cooking, laundry, lighting and the large plug in appliances. Enter grouped totals on the five rows, for example all lighting as one row, then apply a realistic diversity factor because a whole house never peaks on every circuit together. For a service upgrade, hand the totals to a licensed electrician.

What happens if I overload a circuit?

The breaker should trip and cut power, which is its safety job. If protection fails or is oversized, the cable itself becomes the weak point, heating up inside walls where the damage stays hidden. Repeated tripping is a warning sign, not an inconvenience, and it means the load needs reducing or the circuit needs upgrading by a professional.

Are three phase loads handled by this calculator?

The calculator works on a single phase basis, which covers most homes and small premises. Three phase current splits the power across three conductors, so the current per phase is lower for the same total watts. If you are planning three phase machinery, use the watts figure from here and ask an electrician to convert it for your supply.

How accurate are appliance nameplate watts?

Nameplate watts are the maximum the maker rates the appliance to draw, so real consumption is often a little lower, especially for electronics and inverter driven motors. Using the nameplate figure is the safe and standard approach for planning. If you need measured truth for one device, a plug in energy meter will show its actual draw over a day or a week.

Should lighting be counted at full wattage?

For connected load, yes, count every fitting at its rated watts. For demand load, modern LED lighting in a large space rarely runs every fitting at full brightness all day, so the diversity factor can reflect dimming and daylight. In homes it is simpler and safer to keep lighting at 100 percent in the calculation because the totals are small.

When should I call an electrician instead of calculating myself?

Call an electrician for anything involving a consumer unit or panel change, new fixed circuits, three phase work, or any installation that must be certified. This calculator is a planning tool that helps you understand loads, compare options and budget running costs. It does not replace code compliant design, inspection or the sign off that regulated electrical work requires.