Saved energy is free energy
Efficiency means getting the same result with less energy. An efficient bulb gives the same light with fewer watts. A tight house stays warm with less heat.
On the grid, efficiency lowers your bill. Off the grid, it does more. Every kilowatt-hour you cut is one you never need to make with panels, store in a battery, or fill in with a generator.
That is why off-grid guides, including the DOE’s, start with cutting loads before sizing anything.
A chain of savings
Energy passes through several steps on its way from a panel to a plug: wires, a charge controller, a battery and an inverter. Each step loses a little. By the time energy reaches the plug, often only about 70 to 85 percent of what the panels made is left.
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A band of energy flows from left to right through six stages: panels, wires, charge controller, battery, inverter and plug. The band starts at 100 percent at the panels and gets a little narrower at each step, because each step loses some energy, mostly as heat, shown by small arrows rising off the band. At the plug, often only about 70 to 85 percent of what the panels made is left; the end of the band shows that range. A note says that saving 1 kilowatt-hour at the plug saves more than 1 kilowatt-hour at the panels.
So saving 1 kWh at the plug saves more than 1 kWh at the panels. It also means a smaller battery, a smaller inverter if it is a big load, and fewer generator hours in winter.
A dollar spent on efficiency often saves several dollars of solar and battery. The exact ratio depends on your system and prices.
Small loads that never sleep
Phantom loads, or standby loads, draw power while a device looks off. Think of chargers, TVs, game consoles, routers and appliances with displays.
One phantom load is small. But it runs every hour of every day. On an off-grid system it also runs through the night, straight from the battery.
Set the standby draw of the things that never switch off. How much solar does it take just to feed them?
0.6 kWh every day, 219 kWh a year.
Off-grid, that is roughly 170 to 370 W of extra solar panel just to feed it (2 to 4.5 sun hours).
Assumptions: Assumes 80% system efficiency from panel to plug.
A switched power strip, or simply unplugging what you are not using, cuts most of it.
Lighting
Lighting is the easiest win. The DOE reports that LED bulbs use at least 75 percent less energy than old incandescent bulbs and last many times longer.
A 60 W incandescent bulb and a 9 W LED give about the same light. Over 5 hours a night, that is 300 Wh against 45 Wh. For a whole house of bulbs, the gap is large.
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Two area charts for bulbs that give about the same light, each on for 5 hours a night. The incandescent bulb draws 60 watts, so its rectangle is tall: 60 watts times 5 hours is 300 watt-hours. The LED bulb draws 9 watts, so its rectangle is short: 9 watts times 5 hours is 45 watt-hours. The LED uses about 85 percent less energy for the same light.
Light is measured in lumens, not watts. When you compare bulbs, compare lumens for brightness and watts for energy.
The big swaps
The biggest savings come from the big loads in your load list: heat, hot water and cooling.
- A heat pump moves heat instead of making it. DOE says a modern heat pump can cut the electricity used for heating by up to 75 percent, compared with electric resistance heat such as baseboard heaters.
- A heat pump water heater works the same way. DOE says it can be two to three times as efficient as a standard electric water heater.
- An efficient fridge can use much less than an old one. Old second fridges and freezers are often big hidden loads.
- A tight, well-insulated home needs less heat and cooling in the first place (lesson 9).
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Two energy-flow sketches. Left, an electric resistance heater, such as a baseboard heater, with a coefficient of performance, or COP, of 1: one unit of electricity goes in and one unit of heat comes out. Right, a heat pump with a COP of 3: one unit of electricity goes in, two more units of heat are moved in from the outside air, and three units of heat come out into the house. A note says a heat pump's COP falls as the outdoor air gets colder, and cold-climate models are designed to keep a useful COP at lower temperatures.
Climate changes the order
Which swap pays off first depends on your climate zone.
In a cold zone, heating is usually the biggest load, so the envelope and heating choice come first. In a hot zone, cooling and shading come first. In a mild zone, hot water and the fridge may be the biggest loads left.
Look at the biggest lines in your own load list. Start there, not with a list someone else made.
Habits count too
Some savings cost nothing:
- Run the washer, pump and power tools in the middle of a sunny day, when the panels are making power.
- Wash clothes in cold water. Most of a washer’s energy goes to heating water.
- Dry clothes on a line when the weather allows.
- Turn off what is not in use, and keep fridge and freezer doors shut.
Check
An off-grid cabin uses 5 kWh a day. An electric water heater uses 2 kWh of that. Which change will shrink the solar and battery the most?
Summary
- Efficiency shrinks every part of the system: panels, battery, inverter and generator hours.
- Energy is lost between panel and plug, so each kWh saved at the plug saves more at the panels.
- Phantom loads are small but run all day. LEDs are an easy win.
- The big swaps are for heat, hot water and cooling. Your climate zone sets the order.
Next, you will see how solar panels turn sunlight into the energy you just learned to save.
Go deeper: coefficient of performance
Heat pump efficiency is often given as a coefficient of performance, or COP. A COP of 3 means the heat pump delivers 3 units of heat for every 1 unit of electricity it uses. Electric resistance heat has a COP of 1. A heat pump’s COP falls as the outdoor air gets colder, because there is less heat to move. Cold-climate models are designed to keep a useful COP at lower temperatures. Manufacturers publish COP at set test temperatures, so compare models at the temperatures you actually see.