Electrical Loads Guide

An electrical load is any device that consumes electricity. Every load draws power measured in watts (W). Your total loads determine how large your solar system needs to be — more loads mean more solar panels, more battery capacity, and bigger wires.

Loads come in two flavours. Resistive loads (heaters, incandescent lights, toasters) draw a steady, predictable current. Inductive loads (fridge compressors, water pumps, fans with motors) draw a surge of current at startup — often 3 to 7 times their running watts — before settling down.

Duty cycles matter too. A fridge rated at 150W does not run 24 hours a day. Its compressor cycles on and off, typically running about 8 hours out of 24. If you size your system assuming 150W continuously, you will overbuild by three times.

Phantom loads sneak up on you. TVs on standby, chargers left plugged in, and WiFi routers all draw small amounts of power around the clock. Individually they seem trivial, but added together they can consume a surprising amount of energy over 24 hours.

DC loads run directly from your battery bank at the system voltage (12V, 24V, or 48V). They are more efficient because no energy conversion is needed. Examples include 12V LED lights, USB phone chargers, DC water pumps, and 12V fridges. If you can run a device on DC, you should — it saves money on both the inverter and the battery.

AC loads require an inverter to convert battery DC power into mains-style AC power (120V or 230V). The inverter is not perfectly efficient — typically 90-95% — so every AC load costs you roughly 10% more battery capacity than the same device running on DC. Examples: microwave, TV, laptop charger, washing machine.

The practical takeaway: minimise your AC loads. Every AC device you can replace with a DC equivalent shrinks the inverter you need (inverters are priced by wattage) and reduces your daily battery consumption. A 12V DC fridge uses far less total system energy than a 150W AC fridge running through an inverter.

The formula is straightforward: Daily Wh = Watts x Hours per day. Multiply each appliance's wattage by how many hours it runs in a typical day, then add everything up.

Worked example: 5x LED lights at 5W each running 5 hours (5 x 5W x 5h = 125Wh) + DC fridge at 60W running 8 hours (60W x 8h = 480Wh) + laptop at 65W via inverter for 4 hours (65W x 4h x 1.1 inverter loss = 286Wh). Total = 125 + 480 + 286 = 891Wh/day.

To convert watt-hours to amp-hours (which is how batteries are rated), divide by your system voltage: 891Wh / 12V = 74.25Ah. This is the minimum daily energy your battery bank must deliver. Your actual battery needs to be larger because you never drain it to zero — a typical lead-acid battery should not go below 50% state of charge, and lithium batteries around 20%.

This daily Wh figure is the foundation of your entire system design. Solar panel sizing, battery capacity, controller rating, and inverter size all flow from this number.

Use the table below as a starting point. These are typical wattage values and daily run-times for common off-grid appliances. Your actual consumption may differ — always check the label on your specific device.

Once you know your daily watt-hour consumption, you can work backwards to size every part of your system.

Solar needed = daily Wh / peak sun hours. If you consume 1000Wh/day and get 5 peak sun hours, you need at least 200W of solar panels (1000 / 5 = 200W). Add 20-30% extra for cloudy days and panel degradation over time.

Battery needed = daily Wh x autonomy days / depth of discharge / system voltage. For 1 day of autonomy at 12V with 80% DoD: 1000 x 1 / 0.8 / 12 = 104Ah minimum. For 2 days of autonomy: 208Ah. Most off-grid systems aim for 2-3 days of autonomy.

Charge controller = total panel watts / system voltage gives you the minimum current rating. A 400W array on a 12V system produces about 33A — you need a controller rated for at least that. Inverter = must handle the peak simultaneous AC load. If your microwave (1000W) and TV (80W) might run at the same time, you need at least a 1200W inverter.

Solar Planner's built-in load calculator handles all of this automatically. Enter your appliances, set the hours, and it computes the recommended solar, battery, controller, and inverter sizes for you.

Underestimating surge watts: Motors in fridges, pumps, and air conditioners draw 3-7x their rated running watts for a fraction of a second at startup. Your inverter must handle this surge or the device will not start. Check the surge rating on your inverter, not just the continuous rating.

Ignoring inverter efficiency loss: That 10% conversion loss adds up. A 1000Wh AC load actually drains about 1111Wh from your battery. Always factor inverter efficiency into your daily consumption total.

Forgetting standby and phantom loads: A WiFi router at 10W running 24/7 consumes 240Wh/day — that is a quarter of a small battery. Phone chargers left plugged in, TVs on standby, and LED indicators all add up over 24 hours.

Not accounting for seasonal variation: Winter means shorter days (less solar), longer nights (more lighting), and often more indoor time (more device charging). Your summer consumption can look very different from winter. Design for your worst month, not your average month.

Mixing up watts and watt-hours: Watts measure power (how much a device draws at any instant). Watt-hours measure energy (total consumption over time). A 100W light bulb running for 10 hours uses 1000Wh. A 1000W microwave running for 6 minutes also uses 100Wh. The wattage tells you about wire and inverter sizing; the watt-hours tell you about battery and solar sizing.