Power and energy: kW and kWh

Power (watts, W, or kilowatts, kW) is how fast energy flows, like litres per minute from a tap. Energy (kilowatt-hours, kWh) is how much has flowed in total, like the number of litres in the bucket. Energy = power × time: a 2 kW kettle that is switched on for 3 minutes uses 2 × 0.05 hours = 0.1 kWh.

Going deeper

1 kWh = 1000 W × 3600 s = 3.6 million joules (3.6 MJ). On this site, power is shown in the charts on Today (kW), and energy in the daily and monthly totals (kWh). The area under a power curve is the energy itself: that is integration.

How do they work together?

At every moment, whatever comes into the house also goes out again. Electricity from the sun, from the grid and from the battery together is exactly what the house uses, plus what the battery charges and what is exported. The flow diagram on Now shows this live.

The trick is to make supply and demand coincide in time. The sun delivers in the middle of the day and mostly in summer. The house needs electricity in the evening and mostly in winter, when the heat pump is running. Each device helps in its own way:

• The car charges when there is surplus solar power or when electricity is cheap.
• The heat pump can heat the house or the hot-water cylinder a little earlier; the house then acts as a thermal buffer.
• The battery shifts electricity by a few hours, from the afternoon to the evening, or helps the grid when it is out of balance.

What the battery cannot do is store summer sunshine for the winter. Its storage capacity is hundreds of times too small for that.

Going deeper: self-consumption and self-sufficiency

Self-consumption is the share of the solar power that the house uses itself. Self-sufficiency is the share of consumption that comes from the house's own solar panels. On this site we only count solar power that was used within the same hour. That is because this house's battery also trades with the grid, and then it is no longer possible to say whether a kWh from the battery came from the sun or from the grid.

Calculating per hour gives a slightly too favourable picture. Within an hour, the sun may briefly deliver too little for the kettle, while the hourly total still adds up.

Solar panels

A solar panel converts light directly into direct current (DC). New panels reach an efficiency of about 21 to 24% (older panels often 16 to 20%): that is the percentage of the solar energy falling on the panel that becomes electricity. An inverter converts it into alternating current (AC) for the house and the grid, with a further loss of around 2 to 4%.

How much a panel yields depends on its orientation, its tilt, shading and the season. On a sunny summer day this house generates many times more than in December. Panels facing east and west yield less per year than south-facing panels, but spread their output better over the day. That is a better match with consumption.

Going deeper

The power on the nameplate (watt-peak, Wp) applies at 1000 W/m² of sunlight (AM1.5 spectrum) and a cell temperature of 25 °C. In practice the cells get much hotter in the sun, and for every degree above 25 °C a panel delivers about 0.25 to 0.4% less (newer types at the lower end of that range). That is why a cold, clear spring day sometimes gives more power than a hot summer day.

Home battery

A battery has two key figures: its capacity in kWh (how much it holds) and its power in kW (how fast it can charge and discharge). The state of charge (SoC) tells you how full it is, as a percentage.

Storage costs energy: of every kWh that goes in, roughly 85 to 92% comes back out under heavy use; less when the battery is doing little, because the electronics' own consumption then weighs relatively heavily. You can see this in the daily chart on Home battery: every day, more goes in than comes out. A battery therefore only makes sense if the stored electricity is worth more later on than the loss costs.

Going deeper

The losses occur in the inverter (converting between direct and alternating current, twice), in the internal resistance of the cells (heat, which increases with the square of the current) and in the consumption of the electronics. Home batteries usually use lithium iron phosphate (LFP) cells: considerably less energy per kilogram than the nickel-rich cells in many cars, but safer, and they last for more charge cycles. For a battery that stays in one place, weight is not a problem.

Heat pump

A heat pump works like a fridge in reverse. A refrigerant evaporates at a low temperature, absorbing heat from the outdoor air as it does so, even in frost. A compressor compresses the gas, which makes it hot. In the condenser it releases that heat to the heating water. The electricity mainly drives the compressor and the fan. The circulation pump and the controls also use electricity. An electric backup heater (for severe frost or the legionella flush of the hot-water cylinder) converts electricity into heat one-to-one.

So: heat = electricity + heat from the outdoor air. With a COP of 4, a quarter of the heat is electricity and three quarters comes from the outdoor air.

Going deeper: why the COP drops as it gets colder

Thermodynamics sets an upper limit, the Carnot COP: COP_max = T_hot / (T_hot − T_cold), with temperatures in kelvin (°C + 273.15). An example: heating water at 35 °C (308.15 K) and outdoor air at 7 °C (280.15 K) gives a COP_max of 308.15 / 28 ≈ 11. With water at 45 °C and −5 °C outside, that drops to 318.15 / 50 ≈ 6.4.

Real heat pumps achieve roughly 35 to 50% of the Carnot COP, towards the lower end of that range when the temperature difference is large. Part of the reason is that the refrigerant has to be colder than the air in the evaporator, and warmer than the water in the condenser. So every degree lower in water temperature helps. For the same reason, producing domestic hot water (50 to 55 °C) has a lower COP than heating the house: the temperature difference is larger. In addition, around freezing point, when the air is humid, the outdoor unit has to defrost from time to time, and that costs energy too.

Going deeper: heat pump versus gas boiler

1 m³ of natural gas contains 35.17 MJ = 9.77 kWh of energy, based on the gross (higher) calorific value (Groningen natural gas equivalent). Over a heating season, a condensing gas boiler converts about 85 to 95% of that into heat in the house; the lower the return temperature, the better. (The 107% of an HR107 boiler is based on the net (lower) calorific value; on the gross calorific value it is about 96%, and the boiler only achieves that under favourable conditions.) At 90%, 1 m³ of gas delivers about 8.8 kWh of heat. A heat pump with an SPF of 3.5 needs 8.8 / 3.5 ≈ 2.5 kWh of electricity for the same amount of heat. See What does it deliver? for this house's figures.

Electric car

Above all, an electric car is efficient: the drivetrain converts 85 to 90% of the energy in the battery into motion, whereas a petrol engine averages 20 to 30% in everyday traffic. When braking, the motor also recharges the battery a little. Charging at home is usually done at 3.7 to 11 kW. A full charge then takes hours, but the car is usually parked for much longer than that. This means it can charge at the best moment.

Going deeper

Euro95 petrol (E10, containing up to 10% bioethanol) contains about 8.7 kWh per litre (net calorific value; for gas above we use the gross calorific value, because a condensing boiler also recovers the heat of condensation). A car that uses 6 litres per 100 km therefore consumes around 52 kWh of energy per 100 km. An electric car does the same with 15 to 20 kWh, including charging losses. What does it deliver? shows what that means in euros.

The electricity grid and the markets

On the electricity grid, generation and consumption must be equal at every moment. If there is briefly too much electricity, the grid frequency rises above 50 Hz; if there is a shortage, it falls. The Netherlands shares this frequency with the whole of continental Europe. TenneT, the Dutch transmission system operator (TSO), makes sure the Netherlands keeps its own part in balance.

• Day-ahead market: every day around midday, the prices for the following day are set; since 1 October 2025 per quarter-hour (before that, per hour). With plenty of sun and wind, electricity is cheap, and sometimes the price is even negative. On cold, windless evenings it is expensive.
• Intraday: further trading on the day itself, as forecasts change.
• Imbalance: every connection falls under a balance responsible party (BRP, programmaverantwoordelijke): the supplier itself or a party that does this on its behalf. The BRP submits a schedule in advance. If reality deviates from it, TenneT settles the deviation per quarter-hour at the imbalance price, which is derived from the price of the balancing energy TenneT activated in that quarter-hour. Anyone who deviates in the direction that helps the grid gets a favourable price: when the grid is short, a high payment; when there is a surplus, a low or even negative price for the electricity they take. That is what the supplier tries to do with this battery.
• Reserve capacity (FCR, aFRR, mFRR): capacity that TenneT contracts in advance to restore the balance within seconds to minutes. FCR responds automatically to the grid frequency, aFRR to a signal from TenneT.

There is also grid congestion: in many places the cables are full at busy times, for example on sunny afternoons or on winter evenings. Flexible consumption, such as smart charging and a battery, can dampen those peaks, but only if it takes the local grid into account. If many batteries respond to the same national price at the same time, that can actually make a local peak bigger.

What does a kWh actually cost?

With a dynamic (hourly-price) contract, you pay the wholesale price per hour or per quarter-hour, plus a supplier markup, plus energy tax, and 21% VAT on top of all of it. In 2026 energy tax is €0.09161 per kWh excluding VAT, just over 11 cents including VAT (up to 10,000 kWh per year; above that the rate is lower). Because these fixed components weigh heavily, the price for a household fluctuates less in percentage terms than the wholesale price; in cents, the fluctuation is actually slightly larger because of the VAT.

Net metering (salderingsregeling): up to and including 31 December 2026 you may offset exported solar power against your import per billing period (usually a year), including the energy tax. From 1 January 2027 this ends. You will then only receive a feed-in compensation; from 2027 to 2030 this must by law be at least half of the supply tariff excluding taxes. Many suppliers also charge feed-in charges; these may only cover the actual costs of handling that electricity, and the ACM (Netherlands Authority for Consumers and Markets) supervises this. As a result, a kWh you use yourself becomes worth considerably more than an exported one. What does it deliver? shows the difference for this house.

Glossary

kW

Power: how fast energy flows.

kWh

Energy: 1 kW for 1 hour, equal to 3.6 MJ.

Wp, kWp

Watt-peak: the power of a solar panel under standard test conditions.

COP

Heat divided by electricity, at a single moment.

SPF

Seasonal performance factor, measured: all heat divided by all electricity over a season or a year.

SCOP

Seasonal COP calculated according to a standard (EN 14825), from the manufacturer's datasheet (the energy label only shows the class); in practice often higher than the measured SPF.

SoC

State of charge of a battery, as a percentage.

Efficiency

Useful energy out divided by energy in.

Day-ahead

Wholesale price, set one day in advance per quarter-hour.

Imbalance

Difference between planned and actual, settled per quarter-hour.

Net metering

Offsetting exports against imports; ends on 1 January 2027.

Grid congestion

The grid is full: no more electricity can flow through it.

Test yourself

Assignments for students

Use the files under Open data; they open directly in Excel. The column names are in Dutch.

1. For each month, calculate what share of the solar power was used directly (zon_direct_gebruikt_kwh / zon_kwh). Why does this differ between June and December?
2. For each month, calculate the SPF of the heat pump (warmte_kwh / warmte_bekend_stroom_kwh) and plot it against the average outdoor temperature. Does the relationship match the Carnot formula?
3. What is the efficiency of the home battery over a month (accu_ontladen_kwh / accu_laden_kwh)? Why is a single day too short for this?
4. The costs in the daily values are calculated without net metering. Work out for yourself what net metering is worth per calendar year: energy tax × the smaller of export and import. How much more is a directly used kWh worth than an exported one, with and without net metering?
5. Design a rule for a home energy management system (HEMS): when should the car charge? Test your rule using the hourly values.

About this site

The real electricity flows of a single home: what the solar panels generate, what the house uses, what comes from the grid or goes back to it, and what the home battery, the heat pump and the car are doing. The live view is updated every 15 seconds, today's charts every five minutes and the history every hour.

1. Meters and devices in the house send their readings to Home Assistant, the house's home automation software.
2. Home Assistant writes all readings to a time-series database (InfluxDB) on a self-hosted server.
3. Every 15 seconds, a small program calculates the figures for this site from that data, and publishes only those public figures.
4. This site reads those figures and draws the charts in your browser.

Everything runs in the house, on its own hardware. The site loads nothing from external sources: no trackers, no adverts, no cookies. We only show energy figures: no address, no cameras, nothing about who is at home or about daily routines.

Which markets does the battery trade on?

The energy supplier decides, moment by moment, what earns the most: responding to imbalance (what this battery does most), trading on the day-ahead market, or providing reserve capacity to TenneT. That is why the battery does not always follow the simple pattern of "charge when it is cheap".

How we calculate

• Consumption = solar + import − export + battery discharge − battery charge. Depending on where the meters are located (DC or AC side), this also includes the inverter's conversion losses.
• Energy is calculated per minute: Home Assistant only stores a reading when it changes, so each value counts until the next one. Import and export (and charging and discharging) are separated per minute, so that they do not cancel each other out within an hour. Energy counters are also read per minute; a counter that starts again from zero (daily counter, charging session, new meter) is detected. Home Assistant does not pass on to the database that a sensor has dropped out. We therefore carry a non-zero value forward for a limited time only: at most one hour for a grid meter, at most a few hours for other sensors. Anything missing after that counts as not measured (see the measurement coverage).
• Directly used solar power = per hour, the smaller of solar generation and consumption.
• Self-consumption = directly used solar power / solar power. Self-sufficiency = directly used solar power / consumption.
• Prices: on this site the day-ahead price is shown excluding VAT (wholesale). The all-in price = (day-ahead + supplier markup + energy tax) × 1.21, using the energy tax rate for the calendar year.
• Costs = import × all-in price − export × feed-in compensation, per quarter-hour: since October 2025 the day-ahead price applies per quarter-hour, so each kWh is counted at the price of the moment it flowed. Net metering is not included here. Net metering (energy tax on exports, per calendar year up to at most the amount imported) is settled per year on What does it deliver?. Fixed costs, the energy tax reduction and battery trading are not included.
• Heat = the heat pump's heat meter (often a value calculated by the controller, not a calibrated meter), or, per minute, the electricity consumption × the COP. The SPF = all heat divided by all electricity used by the heat pump, including standby.