How to Build a Daily Energy Budget, Step by Step

A daily energy budget is a list. Each row is one appliance, and each row ends in a number of watt-hours. Add the rows and you have your day. Nothing fancy.

If watts and watt-hours still feel slippery, read Watts, Amps and Watt-Hours in Plain English first.

Step 1: List everything that plugs in or wires in

Walk through the cabin, van or camp and write down every device you'll run. Lights, phone chargers, a laptop, a fan, a fridge, a water pump. Include the small stuff. Small loads that run all day add up.

Leave out anything you won't power from the battery. A gas stove isn't on the list.

Step 2: Find the watts for each one

The wattage is usually stamped on the bottom or back of the appliance, or on its nameplate¹. That figure is a starting point only. Many appliances have settings, such as a fan speed or a volume dial, and the power they actually draw depends on the setting in use¹.

If there's no wattage on the label, there may be an amp figure. Multiply the current in amps by the voltage the appliance uses to get watts¹.

To find a real figure instead of a label figure, see Measuring What an Appliance Really Uses. For rough numbers before you've measured anything, use Typical Power Use of Common Cabin and Camp Loads.

Step 3: Estimate hours of use per day

Be honest here. Guessing too low is easy to do. A light that's "on in the evening" might be four hours or six. Write the number you'd defend, then round up a little.

Some loads cycle. A fridge is plugged in all day but doesn't run its compressor all day. One guide suggests dividing the total plugged-in time by three to estimate the hours it runs at full wattage¹. Treat that as a rough guess, and check it with a meter if you can.

Step 4: Multiply

The formula is watts × hours used per day, and then you divide by 1,000 if you want kilowatt-hours¹. For a battery plan, stay in watt-hours and skip the division.

The basic idea is the same one behind a 100 W bulb running for an hour using 100 Wh².

Step 5: Add the rows

Here's a small example. The wattages are from the extension guide's ranges and one lamp example, and the hours are made up for the demonstration.

The laptop at 50 W and the fan's lowest listed rating of 55 W come from the guide¹. The 12 W lamp comes from a 12-watt LED lamp example³. Multiplying each row gives 60 + 200 + 165 = 425 Wh¹.

Step 6: Don't forget the quiet drains

Many appliances keep drawing a little power when they're switched off. The extension guide calls these phantom loads, and says they can be avoided by unplugging the appliance or using a power strip with a switch¹. Another source says each watt of continuous standby uses about 9 kWh of electricity per year⁴.

Inverters have their own losses, which sit on top of your appliance numbers. They get a full page: Inverter Losses and Standby Draw.

Two budgets side by side

Start with the quick one. Then replace each row with a measured one as you go. Once the total feels solid, move on to turning it into battery and panel sizes.