What are power and energy?
Two concepts that determine the sizing of an off-grid system – and one phenomenon that is often overlooked.
⚡ Power
How much electricity the appliance needs right now (W).
🔋 Energy
How much electricity is consumed over time (kWh).
🚀 Start-up spike
Many devices need several times more power at a time.
Why is this important off-grid?
In an off-grid system, it is not enough to know how much electricity is consumed per day.
It is equally important to understand how much instantaneous power the equipment needs.
Home electrical appliances have two characteristics that are most important for off-grid use: power and energy.
Once you understand these two things, you will know how to choose the right equipment and size your system correctly.
It is therefore important to distinguish between:
– instantaneous power (W)
– energy accumulated over time (kWh)
Electricity
The unit of electrical power is watt (W). It tells you how much electricity the device is using at that moment.
This information can usually be found on the bottom of the device, in the user manual or in the product information.
Power (W) = Voltage (V) × Current (A)
Think of electricity as water
One of the easiest ways to understand power is to compare electricity to a water pipe.
JÄNNITE (V)
Water pressure
The higher the pressure, the more water is pushed forward in the pipe.
VIRTA (A)
Flow rate
How much water flows in a pipe in a given time.
TEHO (W)
Combined effect
The combination of pressure and flow – how much energy is carried by flowing water.
If your garden hose has high pressure and a lot of flow, water will come in strongly.
The same applies to electricity: voltage and current together make up power.
Power can vary drastically during use
Many devices do not use the same power all the time. This is an important consideration when making equipment choices.
Note: Start-up spike
For many compressor units, pumps and motors, the instantaneous starting power can be
compared to normal starting power.
Practical examples
Moccamaster coffee maker: ~1500 W for making coffee | less than 100 W for keeping warm
Refrigerator: instantaneous power typically 50-150 W | much higher at start-up
Induction cookers: a low power setting can mean short periods of full power, where the instantaneous power can be much higher than the set power level.
Practical advice:
If several high-power devices can be started at the same time,
equipment selection and power management play an important role in an off-grid system.
Energy consumption
The unit of electrical energy is kilowatt hour (kWh).
It tells you how much electricity an appliance has used over a given period of time.
Energy (kWh) = Power (kW) × Time (h)
The battery is like a tank of water
The energy storage in the battery can be thought of as a water tank.
SOLAR PANELS
Energy in
Solar panels produce energy and fill the tank.
ACCU
Warehouse
The battery stores energy for later use.
DEVICES
Energy out
The use of equipment drains the tank.
Examples: power vs. consumption
A high-powered appliance does not necessarily consume a lot of energy – it’s the uptime that counts.
| Device | Power | Operating time | Consumption |
|---|---|---|---|
| Coffee maker | 1500 W | ~4 min | ~0,1 kWh |
| Induction cooker | 2000 W | ~15-20 min | ~0,3 kWh |
| LED light bulb | 10 W | 24 h | 0,24 kWh/day |
| Fridge (modern) | instantaneous power ~50-150 W | 24 h | about 0.3-0.8 kWh/day |
A low-power device can still consume a lot of energy
If a 10 W bulb is on 24 hours a day, consumption is about 0.24 kWh per day.
In a year, this means almost 90 kWh of energy.
The main lessons
1. Power (W) tells you how much electricity the appliance needs at the moment.
It affects the sizing of the inverter and the electrical system.
2. Energy (kWh) is the total consumption over time.
It determines the battery and solar panel requirements.
3. Start-up spikes can be multiple
Compressors, pumps and motors can require much more power at times than their normal operation requires.
4. Even a small amount of continuous consumption accumulates
Always-on appliances and standby consumption can make up a significant part of daily energy consumption.
A common misconception
Many people think that if the battery has enough power, all devices will work.
But this is not always the case.
The battery may have enough energy, but the inverter may still not be able to supply the instantaneous power or start-up peak required by the
device.
Therefore, in an off-grid system, both the amount of energy and the instantaneous power must be considered.
Want to know what solution is right for your cottage?
Tell us briefly about your cottage’s electricity needs, and we’ll give you a realistic estimate of what kind of solution will work in practice.
