How to Calculate Your Off-Grid Power Budget

To calculate your off-grid power budget, audit every appliance you plan to run, multiply each device's wattage by its daily hours of use to get watt-hours (Wh), sum the totals, and add a 25% safety margin. This daily Wh number drives every other decision: your battery bank should store 1.5-2x this number (for LiFePO4), your solar array should replenish it within one day of sunlight, and your inverter must handle your peak simultaneous load. A typical van build needs 1,000-1,500Wh/day, an RV needs 1,800-3,000Wh/day, and an off-grid cabin needs 3,500-6,000Wh/day.

Step 1: Audit Every Appliance

Walk through your setup (or planned setup) and write down every device that will draw power. Include things people commonly forget:

• • Wi-Fi router and cell booster (running 24/7)
• • Water pump (short runtime but high wattage)
• • Vent fans, diesel heater fans, and furnace blowers
• • Battery chargers for power tools, cameras, drones
• • CPAP or other medical devices (often running 8+ hours)
• • Parasitic loads from inverters, charge controllers, and BMS systems (5-30W always-on)

Find the wattage on each device's label, in its manual, or by measuring with a Kill-A-Watt meter. If a label shows only amps and volts, multiply them: Amps x Volts = Watts. For appliances with motors (fridge, pump), use the running wattage for daily calculations and note the surge wattage separately for inverter sizing.

Step 2: Calculate Daily Watt-Hours

For each appliance, multiply its wattage by its estimated daily hours of use. The formula is simple:

Important nuances: Refrigerators and freezers cycle on and off -- a fridge rated at 150W might only run its compressor 8 hours out of 24. A CPAP machine at its lowest pressure setting uses 30W but climbs to 60W with a heated humidifier. Be realistic about usage patterns, but err on the side of overestimating rather than underestimating.

Sum all the watt-hours to get your raw daily consumption. Then multiply by 1.25 to add the 25% safety margin, covering inverter efficiency losses (10-15%), wire losses, battery self-discharge, and real-world usage that inevitably exceeds estimates. For more detail on this calculation, see our power station sizing guide.

Example: RV Boondocking Power Audit

A typical RV boondocking setup with comfort-level amenities:

Example: Off-Grid Cabin Power Audit

A modest off-grid cabin with standard household amenities:

Example: Campervan Power Audit

A minimalist van build focused on essentials:

Step 3: Size Your Battery Bank

Your battery bank must store enough energy to power your system through periods without solar recharge (cloudy days, nighttime). The sizing formula depends on your battery chemistry:

To convert watt-hours to amp-hours (the rating on most batteries): divide Wh by your system voltage. For the RV example at 2219 Wh with 2 days of autonomy on a 12V LiFePO4 system: 2219 x 2 x 1.2 = 5325 Wh, divided by 12V = 444 Ah. You would need approximately 500Ah of 12V LiFePO4 battery storage. Learn about wiring batteries for higher voltage or capacity in our battery series vs parallel guide.

Step 4: Size Your Solar Array

Your solar array must replenish your daily consumption within the available sunlight hours. The key variable is peak sun hours (PSH) -- the number of hours per day that solar irradiance averages 1,000 W/m2 in your location. This ranges from 3-4 hours in northern/cloudy climates to 6-7 hours in the American Southwest.

The 0.8 factor accounts for real-world losses: panel temperature derating (~10%), wire losses (~3%), charge controller efficiency (~5%), and dust/soiling (~2%). These compound to roughly 20% total loss.

For the RV example with 2219 Wh daily need and 5 PSH: 2219 / 5 / 0.8 = 555W of solar panels. Rounding to available panel sizes, a 600W array (three 200W panels or two 300W panels) would comfortably replenish the battery bank daily. For panel wiring strategies, see our guide on solar panel wiring: series vs parallel.

Step 5: Choose Your Inverter

Your inverter converts DC battery power to AC household power. It must be sized for your peak simultaneous load, not your daily consumption. Two numbers matter:

• 1. Continuous wattage: Add up the wattage of every appliance that might run at the same time. In the cabin example: fridge (150W) + lights (80W) + router (12W) + fan (60W) + laptop (65W) = 367W running simultaneously. Add a 20% buffer: ~440W minimum continuous rating.
• 2. Surge wattage: Motor-driven appliances (fridge compressor, well pump, washing machine) draw 2-3x their running wattage for 1-2 seconds at startup. The cabin's well pump at 750W has a surge around 1,500-2,250W. Your inverter's surge rating must exceed this peak.

Always choose a pure sine wave inverter for off-grid systems. Modified sine wave inverters damage sensitive electronics and reduce motor efficiency. For the cabin scenario, a 3,000W continuous / 6,000W surge inverter handles all appliances comfortably, including simultaneous operation of the well pump and washing machine.

Sizing Summary by Scenario

Use this table as a starting point. Your actual numbers will vary based on specific appliances, climate, and lifestyle.

Want to skip the manual calculations? Try our interactive off-grid power calculator tool to size your system automatically.

Common Power Budget Mistakes to Avoid

• 1. Forgetting parasitic loads. Inverters, charge controllers, and BMS systems consume 5-30W continuously -- even when no appliances are running. Over 24 hours, a 15W parasitic load adds 360Wh to your daily budget.
• 2. Using manufacturer wattage ratings as gospel. A fridge rated at 150W does not draw 150W continuously. Measure actual consumption with a Kill-A-Watt meter for the most accurate budget.
• 3. Ignoring seasonal variation. Summer and winter power needs differ dramatically. AC units and heaters are power-hungry. Size your system for your worst-case season, or plan to supplement with a generator during peak demand.
• 4. Under-sizing the solar array for the location. Three peak sun hours in the Pacific Northwest is very different from six in Arizona. Use your location's actual PSH data, not a national average.
• 5. Confusing capacity with output. A portable power station with 2,000Wh of capacity but only 1,500W output cannot run a 1,800W coffee maker, regardless of how much energy is stored. See our power station sizing guide for more on this distinction.

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