Off-Grid Load Audit Calculator

Frequently Asked Questions

How do I calculate daily watt-hours from an appliance list?

For each device, Wh/day = watts × hours used per day, then sum the column. A 60 W fan running 8 h is 480 Wh; a 1,500 W kettle for 0.2 h is 300 Wh.

Why add a 30% reserve to the audit total?

A 30% buffer covers cloudy stretches, seasonal load swings, future additions, and battery round-trip losses. Designing to 130% of measured load keeps the system from limping in week-long overcast.

How do I handle phantom (standby) loads?

Modems, chargers, TVs in standby, and smart speakers can total 30–80 W around the clock - 720–1,920 Wh/day. Either meter them with a Kill-A-Watt or assume 50 W × 24 h = 1.2 kWh/day as a placeholder.

What about appliances rated only in amps?

Convert with W = V × A using the local voltage (120 V in North America, 230 V in most of Europe). A 6 A microwave on 120 V draws 720 W, so 0.1 h of use is 72 Wh.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Every off-grid solar design begins with an honest load audit - a written list of every appliance, its wattage, and the hours per day it actually runs. Sum the watt-hours and you have the daily energy budget that the panels, batteries, and inverter must serve. Underestimate this number and the batteries will be flat by 2 a.m. every night; overestimate and you have paid for kilowatts you do not need. This tool accepts a simple name, watts, hours line per appliance, totals the daily watt-hours, identifies the largest single load (which sets minimum inverter surge), and adds a recommended reserve (default 30%) to cover phantom loads, cloudy days, and growth.

Formula / Method

For each appliance: daily watt-hours = wattage (W) × hours-per-day. Total Wh = Σ per-appliance Wh. Total kWh = total Wh ÷ 1000. Recommended kWh with reserve = total kWh × (1 + reserve % ÷ 100). The 30% default reserve absorbs phantom standby draws (TVs, chargers, modems draw 5–15 W even "off"), seasonal variation, cloud days that produce less than expected, and a couple of years of load growth. Peak single-appliance wattage is reported separately because it sets the inverter's continuous and surge ratings - a 60 Wh/day microwave that pulls 1200 W for 3 minutes still needs an inverter that can deliver 1200 W instantly.

Worked Example

Cabin loads: fridge 120 W × 8 h = 960 Wh; LED lights 40 W × 5 h = 200 Wh; laptop 60 W × 6 h = 360 Wh; well pump 800 W × 1 h = 800 Wh; TV 90 W × 4 h = 360 Wh; microwave 1200 W × 0.3 h = 360 Wh. Total ≈ 3,040 Wh = 3.04 kWh/day. With a 30% reserve, design for ≈ 3.95 kWh/day. Peak load 1200 W means the inverter should be rated at least 1500 W continuous (the well pump may also surge to 2,400 W on startup - check the LRA on its nameplate).

Common Mistakes

Using nameplate wattage for variable loads - a fridge cycles roughly 30–40% of the day, not 24 hours; use a Kill-A-Watt meter to measure actual draw.

Forgetting phantom/standby loads - routers, smart speakers, and TVs may total 100–300 Wh/day on their own.

Ignoring seasonal swings - heating, AC, and well-pump usage can triple from winter to summer.

Treating motor startup surge as the running load - pumps, compressors, and air conditioners draw 3–7× running watts for a fraction of a second.

Listing "TV: 90 W, 24 h" when really it runs 4 h evenings - aspirational hours blow up the budget by 6×.

Tips & FAQ

How accurate is a paper audit? Within ±20% if you measure with a plug-in meter; within ±50% if you trust nameplates and guess hours. A whole-house clamp meter or a Sense / Emporia monitor on the mains gives the truth. What reserve is right? 20% for a tightly metered weekend cabin, 30% for a normal off-grid home, 40–50% for poorly characterised loads or growing families. Should I include heating/AC? Yes - even briefly. Resistance heat is a non-starter off-grid (1500 W × 8 h = 12 kWh, more than most arrays produce); a mini-split heat pump cuts that 3–4×. What about EV charging? A Level 1 EV trickle charges at 1.4 kW × 12 h = 17 kWh - that single load typically exceeds an off-grid system's entire daily budget; charge at the grid or build a much larger array. Refrigerator energy: a typical Energy Star fridge uses 1.0–1.4 kWh/day; an old '90s fridge can use 3+ kWh/day - replacing it is often cheaper than expanding the array. Inductive loads: well pumps, deep freezers, and compressor fridges should be soft-start where possible to avoid inverter surge faults. Document the audit: keep the spreadsheet - revisit annually as appliances change. This audit is a planning estimate; measure real loads before final sizing.

Off-Grid Load Audit Calculator

Frequently Asked Questions

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