How to Calculate Your Home's Energy Load for an Off-Grid System

Three numbers, three different jobs

Before any arithmetic, separate the quantities. They are measured differently, they behave differently, and each one sizes a different part of the system.

Energy is power multiplied by time, in watt-hours (Wh) or kilowatt-hours (kWh). It is the area under the day's curve, and it sizes the battery and the array. Continuous power is the highest level the house sustains for minutes at a time, in watts (W), and it sizes the inverter. Surge is the momentary demand when a motor starts — potentially several times its running power for a fraction of a second — and it is measured against the inverter's short-term overload rating rather than its continuous one.

A house with modest daily energy use can still have a punishing surge, and one with large daily use can have a gentle one. Neither number predicts the other.

Building the load table

The load table is the whole exercise. Every device, what it draws, and how long it actually runs.

Anything with a thermostat or a motor spends much of its life switched off, so the duty cycle is what matters: a compressor running a third of the time contributes roughly a third of its running power on average. Wrong in the optimistic direction, the battery empties overnight; wrong the other way, you have sized for a house nobody lives in.

Two rows deserve comment. Standby is the line that surprises people: a handful of watts drawn continuously by devices that appear to be off becomes a meaningful share of a small system's daily total, because it runs every hour of every day. And peak continuous is not the sum of everything in the table — it is the largest combination that realistically runs at the same moment, which takes judgement rather than addition.

Measure rather than declare

The single most valuable step in this whole process costs nothing but patience: measure the devices instead of reading their labels.

A plug-in energy meter left in place for several days gives an energy total that already includes duty cycling, standby and the way the household actually behaves — three things arithmetic tends to get wrong in the same direction. Where metering is impractical, err towards the nameplate for resistive loads and towards measurement-based estimates for everything with a motor or a thermostat.

From daily energy to a battery

Here is where most sizing goes astray: the battery's nominal capacity is not the energy available to the load. Several things take a share first.

Start with daily energy and multiply by days of autonomy — how long the pack must carry the house with no meaningful generation. Then divide by the usable depth of discharge, because taking a pack to empty is how it is destroyed rather than how it is used; the trade between depth and cycle life is the subject of depth of discharge and it is a design decision, not a constant. Divide again by round-trip efficiency, since more energy goes in than comes back out. Finally apply a temperature derating if the pack lives somewhere cold, because capacity falls when it is cold — the effect described in how temperature affects battery performance.

The pack also needs to satisfy the *power* question, not just the energy one: it must be able to deliver the peak continuous draw within the limits its battery management system enforces. A pack big enough in kilowatt-hours can still be current-limited at exactly the wrong moment.

From daily energy to an array

The array calculation has the same shape and one crucial difference: it is sized for the worst month rather than the average one.

Take the daily energy, divide by the peak sun hours available at the site in the worst month, and then divide again by the cumulative system losses between the array and the load — soiling and temperature at the modules, wiring, the charge controller, the battery's round trip, and the inverter's conversion. Each stage takes a percentage, and they multiply rather than add.

Peak sun hours vary enormously with latitude, climate and shading; they are site data rather than something to assume, and irradiance data sets published by bodies such as NREL exist for exactly this. What is reliable is the shape: winter days are shorter, the sun is lower and cloud more persistent, so the same array yields far less — for the reasons set out in how panels behave under cloud.

Where the estimate goes wrong

Four failure modes account for most disappointed off-grid systems, and all of them are errors of method rather than arithmetic.

Growth. The table describes the house on the day it was written. Households acquire devices, and a system sized exactly to today's table has no room for next year's freezer.

Simultaneity. Adding every device's running power gives a peak that will never occur; assuming nothing coincides gives one that certainly will be exceeded. Judgement, not addition.

Forgotten continuous loads. Standby, a router, a controller, a fan — small, invisible, running every hour of the year.

Margin stacked on margin. Rounding up at each stage compounds into a system that spends its life lightly loaded, where its own standby consumption becomes a large share of what it actually delivers.

The discipline that avoids all four is the same: write the table honestly, measure what you can, size each stage against the number that governs it, and put the margin where you chose to put it.