Sizing a solar array or battery bank for an unmanned site begins with one honest number: how many watt-hours the site actually consumes in a day. A power budget worksheet is the disciplined way to arrive at that number by listing every load, recording its real current draw and how often it runs, and summing the results row by row. This is a fill-in-the-blanks exercise, not a guess, and getting it right is the difference between a site that rides through a cloudy week and one that goes dark.
Power Budget Worksheet in one line: A power budget worksheet is a row-by-row tally of every electrical load at a remote site, each multiplied by its supply voltage and its daily on-time to yield watt-hours per day. That daily total, plus an allowance for controller and wiring losses, is the figure that sizes the solar array and the battery bank.
The first column of the worksheet is a complete list of everything that draws current at the site, and the trap is stopping at the controller and radio. Start with the always-on devices: the RTU or flow computer, the cellular modem or radio in its idle state, and any transmitter that is continuously powered such as a pressure or level transmitter on a 4-20 mA loop. These loads run twenty-four hours a day, so even a modest draw of a few tens of milliamps adds up to a meaningful share of the daily budget.
Next come the intermittent and event-driven loads that are easy to forget. A radio or modem draws far more current when it transmits than when it listens, so the transmit burst needs its own row with a realistic count of transmissions per day. A solenoid or valve actuator might pull a large current but only for the seconds it takes to stroke, and it may cycle only a handful of times a day. Sample-conditioning heaters, analyzer ovens, and enclosure heaters are seasonal loads that can dominate the winter budget while contributing almost nothing in summer.
For each load, capture two values you can defend: the current it draws in the state being counted, and how long it stays in that state per day. Read the current from a clamp meter or the device datasheet rather than trusting a nameplate maximum, because nameplate figures are usually worst-case ceilings that overstate the real draw. If a device has multiple operating states, give each state its own row so the transmit burst and the idle listen are tallied separately.
With current and on-time captured, each row becomes a watt-hour figure through a short chain of arithmetic. Multiply the current in amps by the system voltage, usually 12 or 24 volts DC, to get watts for that load. Then multiply watts by the fraction of the day the load is active to get watt-hours per day. A transmitter drawing 25 milliamps continuously on a 12-volt system is 0.3 watts running all day, which is 7.2 watt-hours; a valve pulling 2 amps at 12 volts for a total of 30 seconds a day is 24 watts for 0.0083 of a day, or about 0.2 watt-hours.
Duty cycle is where careful accounting pays off. Express on-time consistently, either as hours per day or as a decimal fraction of the day, and do not mix the two in the same column. For a load that fires a fixed number of times a day, compute the total seconds of on-time as the count times the seconds per event, then convert to a fraction of the 86,400 seconds in a day. Summing the watt-hour column gives the raw daily load, the single most important output of the worksheet.
Two adjustments belong at the bottom of the sheet before the number is used to size hardware. Add a line for charge-controller and wiring losses, commonly a small percentage uplift, because not all energy harvested reaches the load. Then consider a headroom factor for load growth and measurement uncertainty, so the array and battery are not sized to a total with zero margin. The result is a conservative daily watt-hour figure that array sizing and days-of-autonomy calculations can build on.
A worksheet built at commissioning is a snapshot, and remote sites drift. A modem that was supposed to sleep may sit awake because a firmware update changed its power management; a heater thermostat may fail closed and run continuously; an operator may add a second transmitter without anyone revisiting the budget. Each of these silently pushes the real daily consumption above the number the array was sized for, and the first symptom is often a battery that no longer recovers over a sunny weekend.
When the site reports its own DC bus voltage, load current, and solar charge current to a cloud SCADA host, the worksheet stops being a one-time spreadsheet and becomes a living model. Trending the actual amp-hours consumed per day against the budgeted figure exposes the gap directly, and a growing gap is an early warning that a load has changed or a device is misbehaving long before the battery reaches a critical state of charge.
This is also how a power budget stays honest across seasons. A host that logs daily energy in and energy out lets an operator see the heater load appear in autumn and confirm the array still carries it through the shortest days of the year. Rather than rebuilding the worksheet from scratch at every visit, the field team refines the same rows against measured data, so the budget converges on what the site truly draws instead of what it drew on the day it was installed.
The most commonly missed loads are the transmit burst of a radio or cellular modem, which draws several times its idle current, and seasonal heaters for sample conditioning or enclosure freeze protection. Both can dominate the daily total yet are invisible if you only count the steady idle draw. Giving each operating state its own worksheet row prevents these from slipping through.
Use measured current from a clamp meter or a realistic datasheet figure for the specific operating state, not the nameplate maximum. Nameplate ratings are worst-case ceilings meant for wiring and fuse sizing, so budgeting to them can overstate consumption by a wide margin and force an oversized, expensive array. Measured values give a budget you can defend against real behavior.
The daily watt-hour total is the input that everything downstream depends on. Array sizing divides that daily energy by the usable sun-hours the site receives, adjusted for panel derating, to find the panel wattage needed. Battery sizing multiplies the daily figure by the days of autonomy you want and divides by the usable depth of discharge for the chosen chemistry.
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