What can your battery actually run?

Check off the appliances you want to run, enter your battery or power station capacity, and see your total load, inverter headroom, and estimated runtime.

This off-grid load calculator adds up the running and starting wattage of any appliances you select, compares that against your inverter’s continuous and surge ratings, and estimates how long your battery or power station will last with nothing recharging it. It’s built for RV, van, cabin, and backup-power setups where you need to know what a given battery can realistically handle before you plug things in.

Your power source

Battery / power station capacity

Usable capacity

Inverter continuous rating (W)

Inverter surge rating (W)

Capacity is in watt-hours (Wh). If your power station lists amp-hours (Ah), multiply Ah × voltage to get Wh (e.g. 100Ah × 12V = 1,200Wh).

Appliances to run

Estimated runtime

—

Select appliances to see an estimate

Running load0 W

Peak surge load0 W

Want it to last a specific number of hours?

Nothing selected yet

Wattages shown are typical values and vary by model — check your appliance’s nameplate or manual for exact figures. This is a planning estimate, not a substitute for consulting your inverter and battery manufacturer’s specs.

How this calculator works

Every selected appliance contributes its running watts to a running total. Peak surge load is calculated as total running watts − running watts of your highest-surge appliance + that appliance’s starting watts — since in practice, one motor starting up spikes on top of everything else already running, not all of them at once.

Estimated runtime is capacity (Wh) × usable capacity % × 0.85 inverter efficiency ÷ total running watts. The 0.85 factor accounts for typical DC-to-AC inverter conversion losses.

Worked example

A 500Wh lithium power station (100% usable) running a mini fridge, LED TV, laptop, WiFi router, and three LED bulbs:

Appliance	Running W	Starting W
Mini fridge	70 W	300 W
LED TV	100 W	100 W
Laptop	60 W	60 W
WiFi router	15 W	15 W
3× LED bulb	30 W	30 W
Total running load	275 W	—
Peak surge (fridge starting)	—	505 W
Estimated runtime	~1 hour 33 minutes (500Wh × 0.85 ÷ 275W)

Things that trip people up

Starting watts sneak up on you

A compressor or motor can need 2–3x its running wattage just to start. An inverter that handles the running load fine can still fail to start a fridge or pump.

Rated Ah isn’t usable Ah

A lead-acid battery’s full amp-hour rating isn’t safe to fully discharge — draining past 50% shortens its life significantly. LiFePO4 tolerates much deeper discharge.

Cycling appliances use less than their rating

Fridges and AC units cycle on and off rather than running continuously, so real-world runtime is often longer than a worst-case, always-on estimate.

Common questions

Running (or rated) watts are what an appliance draws continuously. Starting (or surge) watts are the brief extra power motors need to start up — often 2–3x the running wattage for compressors and pumps. Your inverter and battery need to handle both.

Runtime is your battery capacity in watt-hours, multiplied by usable capacity percentage and typical inverter efficiency (about 85%), divided by your total running watts of everything switched on at once.

Most portable power stations already report usable watt-hours. DIY battery banks don’t — a lead-acid battery is typically only safe to discharge to 50% of its rated capacity, while LiFePO4 batteries can safely use 80–100%, so raw Wh overstates what you can actually use.

If total running watts exceed your inverter’s continuous rating, it will shut off or trip a breaker. If the surge from starting a motor exceeds the inverter’s surge rating, that appliance may fail to start even if the inverter can handle it once running.

This calculator estimates runtime with no recharging, which is the worst case (overnight or on a cloudy day). Any solar input during the day extends runtime beyond this estimate.

A mini fridge draws roughly 70W continuously but cycles on and off, averaging less. A 500Wh power station typically runs one for 6–10 hours depending on ambient temperature and door use.

More free tools

Repair Cost Estimator See what a repair typically costs before you call anyone. Symptom Checker Something acting up right now? Find the likely cause in a few clicks. Error Code Tool Look up what that blinking code on the display actually means. Maintenance Reminder Tool Track filters, coils, and gaskets across every appliance you own. AC Filter Reminder Never forget your AC filter again. Water Filter Reminder Never run out of clean water again. Mini-Split BTU Calculator Find the right size mini-split for any room. Water Heater Sizing Calculator Get the right tank size or tankless GPM rating. Water Softener Sizing Calculator Get the right grain capacity for your home.