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Watts to kWh Calculator
Kilowatt-hours are watts × hours ÷ 1,000. Watts alone never give you an energy figure — and a nameplate that only runs part of the time is not the same as continuous draw.
kWh = watts × hours ÷ 1,000
kWh = watts × hours ÷ 1,000. A 1,500 W heater for 4 hours is 6 kWh; a 1,000 W load for 30 minutes is 0.5 kWh. From current on AC, compute the watts first — volts × amps × power factor (× 1.732 on three-phase line-to-line) — then multiply by time.
For loads that cycle, apply a duty cycle so the average watts, not the nameplate peak, drive the energy. Watts on their own cannot be converted.
What you know
Nameplate or measured power while the load is on. A 1.5 kW heater is 1,500 W.
For a daily projection, enter how many hours (or minutes) it runs in a day — for example 4 hours for an evening heater.
A refrigerator that runs one-third of the time is about 33 percent, not 100. A heater on a thermostat that is cycling is often 40–60. Leave at 100 if the load really runs continuously for the whole duration.
From your bill. Blank means no cost is shown — an empty field never invents a rate.
The answer
Energy used
6.000 kWh
6,000 Wh
1,500 W average × 4.000 h ÷ 1,000
If that runtime is a daily figure
- Per day: 6.000 kWh
- Per 30-day month: 180.00 kWh
- Per year: 2,190 kWh
What this does not tell you
What a battery will hold through an outage. Usable capacity, inverter continuous rating and which circuits stay on are separate questions — the critical-load planner is built for that list, not this conversion.
What size conductor or breaker to install. Energy over time is not a substitute for the continuous-load and ampacity rules that size a circuit. Those start from current, not from kilowatt-hours.
Informational use only, please verify before you rely on it
This reproduces the standard arithmetic relating power, time and energy for DC, single-phase and three-phase systems, with power factor applied when the starting figures are amps and volts, and with an optional duty cycle for loads that do not run continuously. It is a preparation aid, not a measurement and not a determination. It does not size conductors, overcurrent protection, batteries or generators. Confirm with a meter and the equipment nameplate.
This tool is provided for general informational and educational purposes only. Its output is an illustrative estimate generated from the values you enter and from general assumptions that will not match every deal, market, lender, or homeowner. It is not tax, legal, accounting, financial, or professional advice, and it is not a quote, an offer, a credit decision, or a guarantee of pricing, approval, timing, savings, or eligibility.
You are solely responsible for independently confirming all information presented here including any figures, rates, fees, margins, timelines, tax treatment, and federal, state, local, or utility incentives, with the applicable lender, authority having jurisdiction, and your own qualified tax, legal, and financial advisors before acting on it, relying on it, or presenting it to a homeowner or any third party. Incentive programs, lender terms, and permitting requirements change frequently and vary by jurisdiction.
Seamless Home is not a tax advisor, law firm, lender, or licensed installing contractor, and makes no representation or warranty as to the accuracy, completeness, or currency of the information produced by this tool. To the fullest extent permitted by law, Seamless Home accepts no liability for any decision made or action taken in reliance on it.
When a watts-to-kWh answer gets inflated
The arithmetic is short enough that most converters stop at watts × hours ÷ 1,000. The figure that actually shows up on a bill is usually lower, because refrigerators, heat pumps and thermostat-controlled heaters do not draw nameplate watts for every hour of the day. Entering a duty cycle — roughly one-third for many refrigerators — is what keeps a 150 W compressor from being reported as 3.6 kWh/day when it is closer to 1.2. For how much current that wattage implies on the circuit, use the watts to amps calculator, which carries the answer through to a standard breaker size.
Starting from amps instead of watts brings power factor in. On AC, energy follows the watts, so volts × amps without the power factor overstates what the meter records — the same gap power factor explains between watts and volt-amps. If the goal is deciding which circuits a home battery should keep alive, energy per circuit is only one input; the critical load panel planner puts running watts, surge and daily energy against the inverter and the battery's usable capacity. Coverage is confirmed per service area rather than promised as blanket availability.
Frequently Asked Questions
How do you convert watts to kWh?+
Multiply the watts by the hours the load runs, then divide by 1,000: kWh = watts × hours ÷ 1,000. A 1,500 W heater running 4 hours uses 6 kWh. Watts alone do not convert — without a duration there is no energy figure.
How many kWh is 1,000 watts?+
It depends on how long it runs. 1,000 W for 1 hour is 1 kWh; for 30 minutes it is 0.5 kWh; for 24 hours it is 24 kWh. The same wattage can produce any energy figure once the time changes.
What is the difference between watts and kilowatt-hours?+
Watts (and kilowatts) measure power — how hard something is working right now. Kilowatt-hours measure energy — how much work it did over time. A utility bill is in kWh because you pay for energy, not for the instantaneous wattage of each appliance.
Why does duty cycle change the answer?+
Because most loads do not draw their nameplate watts for every minute of the period you care about. A refrigerator rated 150 W that runs one-third of the day averages about 50 W, so over 24 hours it uses roughly 1.2 kWh, not 3.6. Treating the nameplate as continuous overstates the energy a cycling load actually uses.
Does power factor matter when converting to kWh?+
Yes, when you start from amps and volts on AC. Watts are volts × amps × power factor (and × √3 on three-phase line-to-line), and energy follows the watts. Leaving the power factor out reports volt-amp-hours dressed up as kilowatt-hours, which overstates what the meter records for a motor or electronic load.
How do I estimate the cost from watts?+
Convert to kWh first, then multiply by your utility rate in dollars per kWh. A 1,500 W heater for 4 hours is 6 kWh; at $0.16/kWh that run costs about $0.96. The rate is optional in this tool — leaving it blank shows energy only, and never invents a price.
Is this a determination?+
No. It reproduces the standard arithmetic relating power, time and energy as a preparation aid. It does not measure a circuit, does not size conductors or overcurrent protection, and does not size a battery or a generator. Confirm against a meter and the equipment nameplate.
Related resources
Watt Hours to Amp Hours Calculator
Battery amp-hours from energy at a stated pack voltage.
Learn more →Watts to Amps Calculator
Current from a wattage, carried through to a standard breaker size.
Learn more →Amps to kW Calculator
Kilowatts from current, with the 80 percent continuous-load figure for a breaker.
Learn more →Critical Load Panel Planner
Which circuits a home battery keeps running, against continuous, surge and daily energy.
Learn more →What Is Power Factor?
Why watts and volt-amps differ on AC — and why energy follows the watts.
Learn more →Energy use, planned before the hardware lands
On the home projects Seamless Home runs — battery backup, HVAC, water treatment — the daily energy each circuit will use is settled during design rather than discovered after the equipment is on the wall. Coverage is confirmed per service area rather than promised as blanket availability.
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