Every quarter, a large share of portable power station buyers makes the same two mistakes: they buy a station that's too small for their load, or they buy a 1,500 Wh unit when 1,000 Wh would have saved them $300. Both mistakes come from the same root — nobody taught them to do the math first.

This guide fixes that. You'll get the exact formula, a step-by-step worked example, a device wattage cheat sheet, a second "surge" check that most articles skip, and a ready-to-embed interactive Wh calculator you can drop straight into your blog or product page.

TL;DR — the whole method in three lines:

1. Daily Wh = each device's watts × hours you use it, added up

2. Required capacity = Daily Wh ÷ 0.85 (efficiency) × 1.2 (20% buffer)

3. Output check = simultaneous watts ≤ station's rated output, AND startup surges ≤ station's surge rating

Step 0: Know the Three Numbers

A 65W laptop running for 3 hours uses 65 × 3 = 195 Wh. That single multiplication is the entire art of sizing.

Step 1: List Your Devices and Their Watts

Find each device's wattage on its label or in the manual. No wattage listed? Two quick rules: US wall-plug devices rated in amps → multiply amps × 120V (a "10A" circuit-rated device maxes at 1,200W, though its actual draw is usually far lower); USB / DC adapters → use the printed wattage directly (5–30W for phones, 45–100W for laptops). When in doubt, the cheat sheet below covers the common cases.

Step 2: Decide Realistic Hours

Be honest with yourself. "Laptop 3 h, lights 6 h, fan 8 h" beats "everything 24 h." Note that fridges and ACs are cyclical — a mini fridge runs its compressor only ~1/3 of the time, so a 100W fridge averages ~33W.

Step 3: Apply the Formula

Daily Wh = Σ (watts × hours) Required capacity = Daily Wh ÷ 0.85 × 1.2

● ÷ 0.85 — inverter and conversion losses (real efficiency is 80–90%)

● × 1.2 — 20% buffer so you rarely drain below 20% (deep discharge ages the battery)

A Complete Worked Example (Camping + Backup Kit)

● Now the formula: 1,195 ÷ 0.85 =1,406 Wh

● 1,406 × 1.2 =1,687 Wh→buy a 1,500–2,000 Wh station

Same kit but without the fridge? 775 ÷ 0.85 × 1.2 = 1,094 Wh → a 1,000 Wh station is the floor, 1,500 Wh is comfortable. This is exactly why the same lifestyle can need two very different stations — the fridge is the swing factor.

Device Wattage Cheat Sheet

Step 4: The Output Check (Most People Skip This)

Capacity says how long — but rated output says whether it runs at all. Run the second check:

Sum of everything running AT THE SAME TIME ≤ station's rated watts Biggest motor startup ≤ station's surge watts

Example: a fridge (100W) + microwave (800W) used together = 900W → needs a 1,000W-rated station. The fridge's compressor surge (~300–500W) on top needs the station's surge spec to cover it — a 1,800W-surge unit does; a 1,200W-surge unit may hiccup.

Rule of thumb for surge: if a device has a motor (fridge, AC, drill, pump), budget 2–3× its running watts at startup.

Free Wh Calculator (Embeddable)

Use the manual math above, or drop this calculator into a Shopify/WordPress page — it's self-contained HTML+JS, no dependencies:

<div id="wh-calc" style="max-width:520px;margin:0 auto;font-family:inherit">
  <style>#wh-calc{border:1px solid #ddd;border-radius:12px;padding:20px}
  #wh-calc input,#wh-calc select{width:100%;padding:8px;margin:4px 0 12px;box-sizing:border-box}
  #wh-calc .row{display:flex;gap:8px}#wh-calc .row>div{flex:1}
  #wh-calc .out{font-size:15px;line-height:1.6}
  #wh-calc button{width:100%;padding:10px;border:0;border-radius:8px;background:#1a7f37;color:#fff;font-size:15px;cursor:pointer}
  #wh-calc .big{font-size:20px;font-weight:700}</style>
  <h3 style="margin-top:0">Portable Power Station Size Calculator</h3>
  <div class="row"><div><label>Device</label><input id="dev" placeholder="e.g. Laptop"></div>
  <div><label>Watts</label><input id="w" type="number" value="65"></div>
  <div><label>Hours/day</label><input id="h" type="number" value="3"></div></div>
  <label>Quick add (typical watts)</label>
  <select id="preset"><option value="">—</option><option value="10">Phone (10W)</option>
  <option value="20">LED lights (20W)</option><option value="15">Router (15W)</option>
  <option value="65">Laptop (65W)</option><option value="40">Fan (40W)</option>
  <option value="100">Mini fridge (100W)</option><option value="100">TV (100W)</option>
  <option value="30">CPAP (30W)</option></select>
  <button onclick="whCalc.add()">Add device</button>
  <div id="list" class="out" style="margin-top:12px"></div>
  <button onclick="whCalc.run()" style="margin-top:8px;background:#0b5ed7">Calculate size</button>
  <div id="res" class="out" style="margin-top:12px"></div>
</div>
<script>
const whCalc={items:[],
 add(){const d=document.getElementById('dev').value||'Device',
  w=+document.getElementById('w').value||0,h=+document.getElementById('h').value||0;
  if(w>0&&h>0){this.items.push({d,w,h});this.render();}},
 render(){const l=document.getElementById('list');
  l.innerHTML=this.items.map((x,i)=>`<div style="display:flex;justify-content:space-between">
  <span>${x.d} (${x.w}W × ${x.h}h = ${x.w*x.h} Wh)</span>
  <span style="cursor:pointer;color:#c0392b" onclick="whCalc.del(${i})">✕</span></div>`).join('')||'<em>No devices yet</em>';},
 del(i){this.items.splice(i,1);this.render();},
 run(){const raw=this.items.reduce((s,x)=>s+x.w*x.h,0);
  if(!raw){document.getElementById('res').innerHTML='<em>Add at least one device first.</em>';return;}
  const eff=raw/0.85, buf=eff*1.2, maxW=Math.max(...this.items.map(x=>x.w));
  const size=buf<=450?'~300–500 Wh':buf<=1200?'~1,000 Wh':buf<=2400?'~1,500–2,000 Wh':'2,000 Wh or larger';
  document.getElementById('res').innerHTML=
   `<div><b>Daily load:</b> ${raw} Wh</div><div><b>With efficiency & 20% buffer:</b> ${Math.round(buf)} Wh</div>
    <div class="big">→ Recommended: ${size}</div>
    ${maxW>1000?`<div style="color:#c0392b">⚠️ A device draws ${maxW}W — check the station's rated/surge output (1,000W units top out at ~1,000W continuous).</div>`:
    `<div style="color:#1a7f37">✓ All devices fit a 1,000W-rated station's output.</div>`}`;}};
</script>

5 Mistakes That Break the Math

1. Using peak instead of average watts for cyclical devices. A fridge's "100W" label means compressor-on draw; its average is ~1/3 of that.

2. Forgetting the 0.85 efficiency factor. A "1,000 Wh" station delivers ~850 Wh to the wall, not 1,000.

3. Skipping the surge check. Capacity alone is why a 1,000W station occasionally refuses a perfectly "small" fridge.

4. Sizing for a single night, not three. Outages and multi-day camps compound: multiply daily Wh by the number of days you want coverage.

5. Ignoring cold weather. Winter cuts usable capacity 10–20% — add it to your buffer if the station lives in an unheated shed.

Scaling Up: When the Number Says "Bigger"

If your required capacity lands in a range that's awkward:

●1,600–2,400 Wh → a 2,000 Wh station, or a 1,000 Wh unit with a 200–400W solar panel for daily replenishment

●3,000 Wh+ → look at expandable stations or two units on parallel AC

●Home-wide → this math stops being "portable" territory; you're sizing a stationary home battery (same formula, 10× the numbers)

Frequently Asked Questions

What size power station do I need for a weekend camping trip? Add your device-hours (laptop 3h + lights 8h + fan 10h + phones 2h ≈ 775 Wh in our example), divide by 0.85, add 20% → about 1,100 Wh. A 1,000 Wh station is the floor; 1,500 Wh is comfortable for two days.

How do I calculate how many days a power station will last? Station Wh × 0.85 ÷ your daily Wh. A 1,000 Wh station with a 500 Wh/day load lasts 1,000 × 0.85 ÷ 500 ≈ 1.7 days.

Why is there a 0.85 in the formula? It's real-world inverter and conversion efficiency — stations deliver 80–90% of stored energy to devices, with the rest lost as heat in conversion.

Do I need to calculate surge power for every device? Only for anything with a motor or compressor (fridge, AC, drill, pump, sump). Budget 2–3× running watts at startup and check it against the station's surge rating.

Can I use this formula for solar sizing too? Yes, in reverse: your daily Wh tells you the solar panel wattage needed to recharge it. As a rule, a panel roughly equal to a third of the station's capacity replaces about one full charge per day of good sun (e.g., ~300W panel for a 1,000 Wh station).

Final Thoughts

Sizing a portable power station isn't guesswork — it's one addition, one division, and one multiplication, plus a second check that catches motor startups. Do it once, honestly, with real device hours, and you'll buy the right size the first time: not too small, not too big, and not overpaying for watt-hours you'll never draw.

[Optional CTA: Not sure what your total adds up to? Use the calculator above, or see our 1,000 Wh and 2,000 Wh stations — sized to fit the most common load profiles → NEJoye D Series]