Living with Solar: part 1

I’ve recently had a solar panel system with batteries installed — and I’m very pleased with it. In the following series of articles I’ll note some of the things I learned along the way.

How it works

There are now three sources of electricity for our house: the sun, energy stored in the batteries, and the incoming grid. These are automatically combined as required.

If solar power by itself is enough to cover the household usage, any excess is used to charge the batteries. And any excess after that (for example, if the batteries are already full) is exported to the grid, where it can be used to power neighbouring houses.

Conversely, when there’s insufficient solar generation to power the house, the remainder is made up by discharging the batteries, and if necessary by drawing from the grid.

With the system I have, the monitoring app shows in close to real time how the power is flowing. In the following diagram, it’s a bright day, and there’s plenty of excess power to charge the battery, as well as power the house:

Example graphic from Solis Cloud

This picture is available via an app or web page, and is updated every 5 minutes. Data is also stored for generating historical graphs.

Terminology: kW, kWh and kWp

When you have a system design drawn up for you, it’s very helpful to understand the terms being used.

Power: kW

Power is a measure of how quickly you are using or generating energy. The units are watts (W) or kilowatts (kW), where 1kW = 1,000W. An electric kettle, for example, might use 2kW of power while it is switched on; this is an indication of how quickly it is transferring energy into the water to heat it up. A modern TV might only use 50W, or 0.05kW, while it’s running, and only 1–2W on standby.

Energy: kWh

Energy is the total amount of work done by the electricity you have consumed or generated, and this is what you are actually charged for.

Technically, the standard unit of energy is the joule (J): one watt of power means one joule of energy per second.

However, joules are too small¹ to be conveniently used in household applications, so instead the utility companies measure energy in kilowatt-hours (kWh), often simply called “units”. This is the amount of energy which is used by an appliance drawing 1 kilowatt when left running for 1 hour. It’s the same as an appliance drawing 2kW for half an hour, or 0.1kW for 10 hours, and so on. You just multiply the power in kW by the time it is running in hours, to find the total energy consumed in kWh.

This means that a 2kW kettle, taking 2 minutes to boil, uses much less energy than a 2kW tumble drier running for 90 minutes to dry a load:

  • Boil a kettle: 2kW x (2/60) = 0.067 kWh of energy
  • Dry a load: 2kW x (90/60) = 3.000 kWh of energy

What if you had a kettle with a smaller heating element, say 1kW? It consumes energy at half the rate, but this also means it takes twice as long to heat the same amount of water — which would be 4 minutes in this case. So the total energy used is the same:

  • Less powerful kettle: 1kW x (4/60) = 0.067 kWh of energy

You also have to wait longer for your cup of tea. But if you put less water in either type of kettle, it will take less time to boil, and save you energy.

In short: energy (kWh) is what your electricity meter measures. Power (kW) is how quickly the meter is increasing.

Peak power: kWp

The power generated by solar panels varies depending on how much light is striking them. Obviously, there’s less light on cloudy days or in shade. Even on sunny days, it is affected by the angle at which the light hits the panels, which changes as the sun moves across the sky. When striking at an angle, the same amount of sunlight is spread over a larger area, so each square metre of panel receives less light. Furthermore, when the sun is low in the sky, more light is filtered out by the atmosphere.

Power output on 22nd June 2022, a totally cloudless day. System with 14 panels (5.18kWp), facing south-west. Times in UTC (local daylight savings time is UTC+1)

Therefore, the panels are rated by their peak output power, which is how much power they will give under optimal conditions²: the sun is shining brightly, completely perpendicular to the panels, and (perhaps surprisingly) when it is cool — solar panels work less efficiently when they get hot.

The instantaneous power generated is in kilowatts, so this maximum figure is kilowatts-peak (kWp). When completed, my system has 14 panels rated at 370Wp, so the total rating of the system is 5.18kWp. This means that in theory, my system could generate 5.18kW of power, when all the conditions are perfect. In practice, it will rarely reach that³.

Putting it together

Power (kW) measures, at a single instant in time:

  • The rate at which you are using energy in your house
  • The rate at which your panels are generating energy
  • The rate at which you are importing energy from or exporting to the grid
  • How quickly your battery is charging or discharging

Peak power (kWp) measures the instantaneous power which could be generated by your panels, in absolute best-case conditions.

Energy (kWh) measures, over a period of time:

  • The total your panels have generated — also called the yield.
  • The total you have consumed from the grid (and paid for)
  • What you have exported to the grid (and been paid for)

Energy also measures the storage capacity of your battery. A 7kWh battery could power 1kW for 7 hours — if it were able to discharge all the way from 100% to 0%.

Terminology: system components

These are some of the terms you may come across:

  • The device which converts the Direct Current (DC) from the panels into the Alternating Current (AC) required for your house and the grid, is called an inverter.
  • If the inverter also has a battery connection, so that it can charge and discharge the battery, it is a hybrid inverter. Otherwise you will need a separate battery charger.
  • Instead of one large central inverter, you may be offered separate micro-inverters which fit behind each panel.
  • With a central inverter, the photovoltaic (PV) panels are connected together in series, forming strings, like daisy chains.
  • The current flowing through a string is limited by the least-well-performing panel; if one panel is in shade, it reduces what the other panels can generate. You can fix this problem by fitting optimizers to the panels, or by using micro-inverters. (These add to the cost, of course!)
  • The device which distributes electricity around your house, which you may informally know as the “fuse box”, is the consumer unit. You will have one of these already, and the PV system will connect to it. At worst, if you don’t have any spare circuit breakers, it may need to be replaced.

In part 2, I’ll describe my experiences having the system designed and installed.

¹One kilowatt is 1000 joules per second, and an hour has 3600 seconds, so 1kWh of energy is equal to 3,600,000 joules, or 3.6 megajoules (MJ).

²More precisely, it’s the output under “standard test conditions” (STC): incident radiation of 1kW/m² and a cell temperature of 25°C.

³However there are phenomena which mean that the peak can be briefly exceeded — for example, if the sun is very close to the edge of a cloud, it may illuminate that cloud and increase the total light received.

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