Generator Size Calculator
Add up running and starting watts for the appliances you want to back up, then size a generator in watts, kW and kVA and estimate runtime on one tank of fuel.
Appliances to run
Columns: appliance, quantity, running watts, starting (surge) watts. Resistive loads such as lights, kettles and heaters have the same figure in both columns.
The peak figure assumes everything else is already running when Sump pump (1/2 hp) starts, which is the worst realistic case.
Runtime on one tank
Peak load = total running watts + the single largest extra surge (starting watts − running watts), because only one motor starts at a time. kVA = kW ÷ power factor, and running current = watts ÷ volts. Runtime scales fuel burn linearly between the no-load figure and the full-load figure you enter — real consumption depends on the engine, altitude and temperature, so treat it as an estimate. Sizing a standby or transfer-switch installation is electrician's work; this is a planning tool, not a design.
What is the Generator Size Calculator?
The ByteTools Generator Size Calculator builds your load from an editable appliance table. Enter each item's quantity, running watts and starting (surge) watts, and it totals the running load, finds the single largest extra surge, and adds the two together to get the realistic peak the generator has to swallow.
- Editable appliance table with quantity, running and starting watts
- Peak load = total running watts plus the single largest extra surge
- Recommended size with an adjustable headroom percentage
- kW, kVA at your power factor, and running amps at 120 / 230 / 240 V
- Runtime per tank from tank size, full-load burn and a no-load allowance
- Copy the whole load list as CSV; everything stays in your browser
How to use the Generator Size Calculator
- 1
List the appliances you want to run, filling in quantity, running watts and starting (surge) watts for each row.
- 2
Add or remove rows as needed — resistive loads such as lights and kettles take the same figure in both watt columns.
- 3
Set the power factor, the sizing headroom you want and the supply voltage.
- 4
Read total running watts, the largest extra surge, peak starting watts and the recommended generator size.
- 5
In the runtime panel, enter the generator rating, tank size and fuel burn at full load to see the load factor and hours per tank.
About the Generator Size Calculator
The ByteTools Generator Size Calculator builds your load from an editable appliance table. Enter each item's quantity, running watts and starting (surge) watts, and it totals the running load, finds the single largest extra surge, and adds the two together to get the realistic peak the generator has to swallow.
From that peak it reports the recommended set size with your chosen headroom, the apparent power in kVA at a given power factor, and the running current at 120 V, 230 V or 240 V. A second panel estimates runtime by scaling fuel burn between the no-load and full-load figures from your generator's spec sheet.
Every calculation happens in your browser, so nothing about your home or your appliances is uploaded. Sizing is a planning exercise here, not a design — a permanently installed standby set with a transfer switch needs an electrician, and the nameplate watts on an appliance are usually its maximum rather than its typical draw.
Frequently asked questions
What size generator do I need to run my house?
It depends entirely on which circuits you want live. Backing up a fridge, sump pump, furnace blower, lights and a microwave usually lands between 5,000 and 7,500 running watts once the largest motor's surge is added. List your own appliances above to get a figure you can shop with.
What is the difference between running watts and starting watts?
Running watts are the steady draw once an appliance is up to speed. Starting or surge watts are the much larger, brief draw as an electric motor gets moving — often two to three times the running figure. Generators quote both, and both have to be satisfied.
Do I add up all the starting watts?
No, and doing so wildly oversizes the set. Motors start one at a time, so the realistic peak is the total running load plus the largest single extra surge. That is exactly what this calculator does, and it is why the peak figure is far lower than the sum of every surge column.
How do I convert watts to kVA for a generator?
kVA = kW ÷ power factor. Generators are usually rated at a 0.8 power factor, so a 6 kW load is 7.5 kVA. Purely resistive loads such as heaters run at a power factor of 1.0, where kW and kVA are the same number.
How long will a generator run on a tank of gas?
Runtime is usable tank size ÷ fuel burn per hour, and burn depends heavily on load. A set at half load might use 55 to 60 per cent of its full-load fuel rate. Enter your spec-sheet figures and the tool scales between the no-load and full-load rates for you.
Should I size a generator with extra capacity?
Yes — 20 per cent headroom is a common rule. It keeps the engine off its limit, leaves room for a load you forgot, and generally means quieter, more efficient running. The headroom percentage is editable if your situation calls for more or less.
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