Why a load list comes first
A load is anything that uses electricity: a light, a fridge, a pump, a phone charger. A load list is a table of all your loads and how much energy each one uses in a day.
Every solar, battery and generator size starts from this list. If the list is wrong, everything built on it is wrong too. Too small and you run out of power. Too big and you pay for panels and batteries you do not need.
Installers will ask for it. The DOE’s guide to planning a home renewable energy system starts with it, too.
The columns
Each row of a load list has the same columns:
- Load: what it is.
- Watts: its power while running, from the label or a plug-in meter.
- Hours per day: how long it actually runs.
- Days per week: for things you do not use daily, like a washing machine.
- Watt-hours per day: watts × hours × days per week ÷ 7.
Add up the last column. That total is your daily energy use, in Wh or kWh per day. It is the number solar and batteries must cover.
Text description
An example winter load list for a small off-grid cabin, with columns for watts, hours per day, days per week, and watt-hours per day, which is watts times hours times days per week, divided by 7. Fridge, which cycles on and off: 120 watts, about 8 hours of running a day, 960 watt-hours. Well pump: 1,000 watts, 1 hour, 1,000 watt-hours. Router and standby loads: 15 watts, 24 hours, 360 watt-hours. Microwave: 1,100 watts, a quarter hour, 275 watt-hours. LED lights: 50 watts, 5 hours, 250 watt-hours. Clothes washer: 500 watts, 1 hour, 3 days a week, about 214 watt-hours a day. Laptop and phones: 60 watts, 3 hours, 180 watt-hours. The daily total is about 3,240 watt-hours, or 3.2 kilowatt-hours. The fridge and the well pump together are more than half of it.
Running and starting watts
Things with motors, like pumps, fridges and power tools, draw a short burst of extra power as they start. This is the surge or starting watts. It is often two to four times the running watts, sometimes more, for a second or less.
Surge does not add much energy. But your inverter must handle it, or it trips when the pump starts. A load list notes surge watts for motor loads so the installer can size the inverter.
Text description
A sketch of a pump motor's power over a few seconds, not to scale. Before it starts, it draws nothing. As it starts, power jumps for a second or less to the surge, or starting watts, often 2 to 4 times the running watts, and sometimes more. Then it settles to the steady running watts. The shaded area under the line is the energy used; the surge spike adds only a sliver. Notes say the inverter must be able to handle the surge, and that a load list records surge watts for motor loads.
Some labels give amps instead of watts. Remember from lesson 21: watts are roughly volts × amps.
Build your load list
Add your loads one at a time. Start with the ones you know, then add the rest. The tool totals your daily watt-hours and shows which loads take the biggest share.
Do not worry about getting every phone charger. Small loads matter less than you think. Big loads matter more.
The big loads
On most homesteads a few loads decide everything. They are usually things that make heat or cold, or move water:
- Electric space heating and electric water heating.
- Air conditioning.
- Electric ranges, ovens and clothes dryers.
- Well pumps.
On EIA’s national survey of home energy use, space heating, water heating and cooling are among the largest uses in many homes. Off-grid homes often meet heat, hot water and cooking with other fuels, or with very efficient electric equipment, for that reason.
Big or small
Is each load likely to be a big share of an off-grid home’s daily energy, or a small one?
- LED lights for the evening
- Phone and laptop charging
- Electric resistance water heater
- Electric clothes dryer
- Well pump on a deep well
- Efficient fridge
- Electric space heater in winter
Seasons and standby
A load list is not the same all year. Winter adds lights for longer evenings and, sometimes, heating, fans for a wood stove, or heat cable. Summer adds fans, pumps for the garden and cooling.
Make a winter list and a summer list. They can differ a lot.
Also look for standby or phantom loads: things that draw power while switched off or idle. Examples include chargers, TVs, internet routers and appliances with clocks. Each is small, but they run 24 hours a day.
Check
Your load list totals 6 kWh a day in winter. A 1,500 W space heater running 2 hours makes up half of that. What is the best first step?
Summary
- A load list shows each load’s watts, hours and daily watt-hours. Its total is your daily energy use.
- Motor loads have a surge at start-up that sizes the inverter, not the battery.
- A few big loads, mostly heat, cooling and pumps, usually decide the system size.
- Make winter and summer lists, and do not forget phantom loads.
Next, you will see why cutting loads is cheaper than generating more power.
Go deeper: duty cycle
Many loads switch themselves on and off. A fridge compressor might run 30 to 50 percent of the time, depending on the room temperature and how often the door opens. The share of time a load is running is its duty cycle. To estimate daily energy, multiply running watts by 24 hours and by the duty cycle. The DOE suggests dividing by about three for a fridge if you only know its rated watts. A plug-in meter over a few days is much better.