How to Calculate Your Solar Load
- Your daily load in watt-hours (Wh) is the foundation of every sizing decision.
- Formula: Watts × Hours/day = Wh/day per appliance. Sum all appliances for total daily load.
- Add 20–25% for system losses (wiring, inverter, controller inefficiency).
- A typical off-grid cabin uses 1,000–3,000 Wh/day. A full home uses 5,000–15,000 Wh/day.
- Always calculate load before buying any components — oversizing and undersizing are both expensive mistakes.
Why load calculation comes first
Every component in your off-grid system — panels, batteries, charge controller, inverter — is sized based on your daily energy consumption. Get this wrong and your system will either run out of power or cost you thousands more than necessary.
The load calculation takes 15–30 minutes and can save you thousands. Do it before buying anything.
Step-by-step load calculation
- List every appliance you want to runGo room by room. Include lights, fridge, laptop, phone charger, water pump, fans — everything. Don't forget occasional-use items like a washing machine or power tools.
- Find the wattage of each applianceCheck the label on the device or its power adapter. For LED lights, check the bulb packaging. If you can't find it, use our reference table below.
- Estimate hours of use per dayBe honest and realistic. A fridge runs 24 hours but the compressor only cycles ~8–12 hours. A laptop might run 4–6 hours. Lights typically 3–5 hours.
- Calculate Wh/day for each applianceMultiply watts × hours/day. A 50W fridge running 10 hours = 500 Wh/day.
- Sum all appliances for total daily loadAdd up all Wh/day values. This is your raw daily load.
- Add 20–25% for system lossesMultiply your total by 1.25. This accounts for inverter inefficiency (~5–10%), wiring losses (~3–5%) and battery charge/discharge losses (~5–10%).
Interactive load calculator
| Appliance | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 100 | |||
| 1080 | |||
| 260 | |||
| 30 | |||
| 60 |
Common appliance wattages
| Appliance | Typical watts | Notes |
|---|---|---|
| LED bulb (per bulb) | 8–12W | Replace incandescents before sizing |
| 12V compressor fridge (small) | 30–60W | Runs ~8–12h/day equivalent |
| Full-size AC fridge | 100–200W | High draw — consider 12V fridge |
| Laptop | 45–90W | Check charger brick label |
| Phone charger | 10–20W | USB-C fast charge is higher |
| TV (32") | 30–60W | LED TVs are much more efficient |
| Ceiling fan | 15–75W | Varies by speed setting |
| Water pump (12V) | 30–80W | Only runs when pumping |
| Electric kettle | 1,000–1,500W | High draw — use gas kettle if possible |
| Microwave | 700–1,200W | Needs large inverter; limit use |
| Washing machine | 500–1,000W | Cold wash only; limit to 2–3x/week |
| Router/modem | 10–20W | Runs 24h — counts up |
Typical loads by system type
| Setup | Typical daily load | Example appliances |
|---|---|---|
| Minimal van/cabin | 300–800 Wh | Lights, phone, laptop, small fan |
| Comfortable cabin | 1,000–2,500 Wh | Above + 12V fridge, water pump, TV |
| Full-time RV | 2,000–4,000 Wh | Above + washing machine, air circulation |
| Small off-grid home | 4,000–8,000 Wh | Full appliances, no heavy loads |
| Full family home | 8,000–15,000 Wh | All appliances including AC/heat pump |
What to do with your load number
Once you have your adjusted daily load (with 25% losses added), you can size every component:
- Solar panels: Daily load (Wh) ÷ peak sun hours = minimum panel watts. See our panel sizing guide.
- Battery bank: Daily load × days of autonomy ÷ depth of discharge ÷ voltage. See our battery sizing guide.
- Charge controller: Sized to your panel array. See our MPPT controller guide.
- Inverter: Sized to your peak simultaneous wattage. See our inverter sizing guide.
Frequently asked questions
Should I calculate load for summer or winter?
Calculate for your worst case — typically winter, when you use more power (longer nights, more heating) and generate less (shorter days, lower sun angle). A system sized for winter will have surplus in summer.
How accurate does my load estimate need to be?
Within 20–30% is fine. The 25% system loss factor provides a buffer, and you can always add panels later more easily than replacing an undersized battery bank. Err on the side of overestimating loads.
What's the most common mistake in load calculations?
Forgetting continuous loads — particularly the fridge, router and any 24/7 devices. These small continuous draws add up dramatically over a full day.