Two of the most common mistakes in energy-system planning come from the same root confusion: treating power and energy as if they were the same thing. They are not. Power is how fast energy moves at a given moment; energy is how much has moved over a period of time. Almost every later decision — battery size, inverter choice, cable rating, how long your lights stay on — depends on keeping these two ideas apart.

This article gives you a durable mental model and a few worked examples you can reproduce. It is not about any particular product, and the numbers below are illustrative, chosen to make the relationships clear rather than to describe a specific device.

The short answer

  • Power is a rate: energy per unit of time. Its SI unit is the watt (W), which is exactly one joule per second (W = J/s).
  • Energy is an amount: power sustained over time. Its SI unit is the joule (J), but in energy systems we almost always use the watt-hour (Wh) and kilowatt-hour (kWh) because they map directly onto how we talk about equipment and bills.

These unit definitions come from the SI Brochure, the reference maintained by the Bureau International des Poids et Mesures (BIPM): the joule is the coherent SI unit of energy and work, and the watt is the coherent SI unit of power, equal to one joule per second.

The single relationship that ties them together is:

Energy = Power × Time

Everything else on this page is a consequence of that one equation.

What power measures

Power tells you the rate at which a device draws, delivers, or converts energy right now. A rating in watts answers questions like:

  • Can my inverter supply this appliance while it runs?
  • Is this cable or fuse being asked to carry more than it should at this instant?
  • Will these loads together exceed what my system can deliver at once?

Because power is a rate, it can change from second to second. A power drill draws very little while idle and a great deal the moment it bites into wood. When you see a single wattage on a label, treat it as one operating point — often the maximum, sometimes a typical value — not a promise that the device always draws exactly that much. Whether a figure is rated, continuous, peak, or measured changes what it means; we keep those apart in continuous power, peak power, and surge loads.

Common multiples:

Unit Symbol Equals Typical use
watt W 1 J/s Small loads: LED lights, phone chargers
kilowatt kW 1 000 W Large loads: kettles, motors, heaters

What energy measures

Energy tells you how much was used or stored over a stretch of time. A rating in watt-hours answers different questions:

  • How much of my battery will this load consume overnight?
  • How much energy does my system need to produce in a day?
  • Will my storage last through a period of poor charging?

You get energy by multiplying power by the time it flows. Run a 100 W load for one hour and you use 100 watt-hours. Run it for two hours and you use 200 Wh. The power never changed — the energy did, because time did.

Unit Symbol Equals Typical use
watt-hour Wh 3 600 J Small daily loads, device budgets
kilowatt-hour kWh 1 000 Wh Whole-system daily energy, utility bills

The exact conversions follow from the definitions: because one hour is 3 600 seconds, one watt-hour is one watt sustained for 3 600 seconds, which is 3 600 joules. A kilowatt-hour is therefore 3 600 000 joules, or 3.6 megajoules. You rarely need the joule figure in practice, but it is worth seeing once so the watt-hour stops feeling arbitrary — it is just a joule counted in more convenient units.

A map between the three quantities

Because the three quantities are locked together by one equation, knowing any two gives you the third:

                 Energy (Wh)
                /          \
      ÷ Time (h)            ÷ Power (W)
              /              \
     Power (W)  ←  × Time →  Time (h)


  Energy = Power × Time        →  Wh  = W  × h
  Power  = Energy ÷ Time       →  W   = Wh ÷ h
  Time   = Energy ÷ Power      →  h   = Wh ÷ W

Keep the units beside the numbers and the arithmetic checks itself. If you multiply watts by hours you get watt-hours; if a calculation hands you "watts per hour" or "watt-hours" where you expected "watts", a unit slipped and the result is not yet trustworthy.

Worked example 1: energy from a constant load

Question: How much energy does a 60 W device use if it runs for 5 hours?

  • Formula: Energy = Power × Time
  • Inputs: Power = 60 W, Time = 5 h (assumed constant power)
Energy = 60 W × 5 h = 300 Wh = 0.30 kWh

Check it in SI units: 5 hours is 18 000 seconds, so 60 J/s × 18 000 s = 1 080 000 J = 1.08 MJ. Dividing by 3.6 MJ per kWh gives 0.30 kWh — the same answer by a second path. The result is reported to two significant figures because the inputs justify no more.

This is the everyday shape of an energy calculation: a power, a duration, and the assumption that the power holds steady over that duration. Real devices rarely draw a perfectly flat wattage, which is exactly why measuring or estimating a realistic run time matters when you build a daily energy load profile.

Worked example 2: high power is not high energy

A frequent misconception is that a high-wattage appliance must be the biggest drain on a system. Power and energy are independent, so that is not necessarily true.

Question: Over one day, which uses more energy — a 2 000 W kettle boiled for 3 minutes, or a 10 W router left on continuously?

  • Kettle: 2 000 W × 0.05 h = 100 Wh (3 minutes is 0.05 hours)
  • Router: 10 W × 24 h = 240 Wh

The kettle draws 200 times more power at the moment it runs, yet over the day the modest router uses more than twice the energy, because it runs 480 times as long. Both facts matter, but for different decisions:

  • The kettle's power decides whether your inverter and wiring can supply it at all.
  • The router's energy quietly dominates the daily total your storage and charging have to cover.

Sizing a system means checking both — peak power and daily energy — as separate questions. Neither one predicts the other.

A common trap: amp-hours are not energy

Battery capacity is often printed in amp-hours (Ah), which measure electric charge, not energy. An amp-hour figure alone does not tell you how much energy a battery holds, because the same charge carries different energy at different voltages. To get energy you also need the voltage:

Energy (Wh) = Charge (Ah) × Voltage (V)

A 100 Ah battery at a nominal 12 V holds roughly 1 200 Wh of energy; the same 100 Ah at 24 V holds roughly 2 400 Wh. Comparing two batteries by amp-hours alone, without their voltages, compares the wrong quantity. We work through this carefully — including nominal versus usable capacity — in watt-hours vs. amp-hours.

Why this distinction runs through the whole system

Once you separate the two ideas, a lot of equipment language falls into place:

  • A battery stores energy, so its meaningful rating is in Wh (or Ah at a stated voltage) — not in watts.
  • An inverter delivers power, so its rating is in watts (or kVA) — it tells you what you can run at once, not for how long.
  • A solar panel or charger is rated in watts for its maximum output, but what fills your battery over a day is the energy it actually produces, which depends on how long and how well it operates.

Mixing these up leads to predictable errors: buying a large inverter and expecting long runtime, or a large battery and expecting it to power a load its inverter cannot supply. Power answers "can I run this?"; energy answers "for how long?" Both questions are real, and a sound design answers each on its own terms.

Key takeaways

  • Power is a rate (watts, joules per second); energy is an accumulated amount (watt-hours, kilowatt-hours, joules). Energy = Power × Time.
  • 1 Wh = 3 600 J, and 1 kWh = 3.6 MJ, because one hour is 3 600 seconds.
  • A high-power device can use little energy, and a low-power device can use a lot — check peak power and daily energy separately.
  • Amp-hours measure charge, not energy; converting to watt-hours needs a voltage.

Where to go next