Every gear guide on this site says the same thing: size your backup power to what you actually run. This calculator makes that a 30-second job. Add your devices (we’ve pre-loaded typical wattages and duty cycles), pick a station size, and see your runtime — or flip it around and see what capacity your target outage-duration requires.
Power Station Runtime Calculator
Add your devices and pick a station size — we’ll estimate how long it runs.
Estimates use 85% usable capacity (inverter + battery overhead) and your duty-cycle inputs. Fridge/freezer watts below are cycling averages — set duty cycle ~35% for a modern fridge. Real runtimes vary with temperature, battery age, and model efficiency.
‘+
”+opts+”+
‘ W‘+
‘% on‘+
‘
‘;
}
function addRow(presetIdx){
var p = PRESETS[presetIdx !== undefined ? presetIdx : 0];
var div = document.createElement(‘div’);
div.innerHTML = rowHTML(0, presetIdx !== undefined ? presetIdx : 0, p.w, p.d);
box.appendChild(div.firstChild);
}
function dailyWh(){
var total = 0;
document.querySelectorAll(‘#lsh-devs .lsh-row’).forEach(function(r){
var w = parseFloat(r.querySelector(‘.lsh-w’).value)||0;
var d = (parseFloat(r.querySelector(‘.lsh-d’).value)||0)/100;
total += w*d*24;
});
return total;
}
function capWh(){
var sel = document.getElementById(‘lsh-cap’).value;
if(sel===’custom’) return parseFloat(document.getElementById(‘lsh-custom’).value)||1000;
return parseFloat(sel);
}
function fmt(h){
if(h>=48) return (h/24).toFixed(1)+’ days’;
if(h>=1) return h.toFixed(1)+’ hours’;
return (h*60).toFixed(0)+’ minutes’;
}
function render(){
var daily = dailyWh(), cap = capWh(), usable = cap*USABLE;
var need = parseFloat(document.getElementById(‘lsh-hours’).value)||24;
var out = document.getElementById(‘lsh-out’);
if(daily= need;
var color = ok ? ‘#00e676’ : ‘#ff7a18’;
var msg = ok
? ‘✓ Runs your load for ‘+fmt(runtime)+’ — covers your ‘+need+’h need with margin.’
: ‘✗ Runs your load for ‘+fmt(runtime)+’ — short of your ‘+need+’h need.’;
var rec = Math.ceil(required/500)*500;
out.innerHTML =
‘‘+
‘
‘;
}
document.getElementById(‘lsh-add’).addEventListener(‘click’, function(){ addRow(PRESETS.length-1); render(); });
box.addEventListener(‘input’, render);
box.addEventListener(‘change’, function(e){
if(e.target.classList.contains(‘lsh-pre’)){
var p = PRESETS[+e.target.value];
var r = e.target.closest(‘.lsh-row’);
r.querySelector(‘.lsh-w’).value = p.w;
r.querySelector(‘.lsh-d’).value = p.d;
}
render();
});
box.addEventListener(‘click’, function(e){
if(e.target.classList.contains(‘lsh-del’)){ e.target.closest(‘.lsh-row’).remove(); render(); }
});
[‘lsh-cap’,’lsh-custom’,’lsh-hours’].forEach(function(id){
document.getElementById(id).addEventListener(‘input’, render);
document.getElementById(id).addEventListener(‘change’, render);
});
document.getElementById(‘lsh-cap’).addEventListener(‘change’, function(){
document.getElementById(‘lsh-custom-wrap’).style.display = this.value===’custom’ ? ‘block’ : ‘none’;
});
addRow(0); addRow(3); addRow(5); render();
})();
‘+fmt(runtime)+’
‘+
‘‘+msg+’
‘+
‘‘+
‘Daily energy draw: ‘+Math.round(daily).toLocaleString()+’ Wh/day
‘+ ‘Usable capacity (‘+Math.round(USABLE*100)+’% of ‘+Math.round(cap).toLocaleString()+’ Wh): ‘+Math.round(usable).toLocaleString()+’ Wh
‘+ ‘For ‘+need+’h autonomy you need roughly a ‘+rec.toLocaleString()+’ Wh station
‘+ ‘Usable capacity (‘+Math.round(USABLE*100)+’% of ‘+Math.round(cap).toLocaleString()+’ Wh): ‘+Math.round(usable).toLocaleString()+’ Wh
‘+ ‘For ‘+need+’h autonomy you need roughly a ‘+rec.toLocaleString()+’ Wh station
How the math works
Runtime = usable watt-hours ÷ average hourly draw. Three things trip people up:
- Usable capacity is less than advertised. Inverters and battery management eat 10–20%; we use 85% of nameplate Wh. A “1,024 Wh” station gives you ~870.
- Duty cycle dominates. A fridge rated 150W doesn’t draw 150W for 24 hours — the compressor runs ~35% of the time in a cool kitchen. That’s why it lasts ~7 hours per 1,000 Wh, not 6.
- Surge vs continuous. The calculator handles runtime; but your station’s continuous output must still exceed each device’s startup spike (fridges spike 800–1,200W for a fraction of a second). Every station we recommend covers this.
Rules of thumb, if you hate calculators
- Phones + router + lights for a day: ~300 Wh
- Add a fridge overnight: ~1,000 Wh
- Fridge + freezer + medical gear for 24h: ~2,000 Wh
- Multi-day outage with solar recharge: 2,000 Wh + 200W panel
What to buy with these numbers
Matched picks at every capacity tier are in our best portable power stations guide. If you’re sizing for a specific scenario — hurricane, winter storm, or California’s PSPS shutoffs — those guides have scenario-specific advice: hurricane prep, winter storm prep, PSPS guide.