How Our Calculator Picks Your Recommendation

Sizing backup power is not hard, but it is tedious. Enough variables are involved — wattage, runtime, efficiency losses, surge spikes, safety margins — that most people either guess or buy whatever has the most Amazon reviews.

PowerLasts — a free backup power calculator — exists to do this math in under a second. But we think you should understand what happens behind the scenes. Here is the exact decision flow, step by step, in plain English.

Step 1: Add Up Your Watts

Everything starts with your device list. You pick the devices you want to keep running during a power outage — your router, laptop, monitor, mini fridge, CPAP, whatever — and the calculator totals their wattage.

Each device draws a certain number of watts while running. A laptop might draw 50W, a monitor 30W, a mini fridge about 60W when its compressor kicks on. Add them together and you get your total simultaneous wattage — the amount of power needed to keep everything alive at the same time.

Step 2: Multiply by Your Runtime

Next, you tell the calculator how many hours you need your devices to run. This converts watts into watt-hours, which is the unit that actually describes battery capacity.

The math is straightforward. If you need 200W of devices running for 4 hours, that is 200 × 4 = 800 watt-hours of raw energy.

Watt-hours are what batteries are rated in. A power station that says “1000Wh” on the box is telling you it stores 1,000 watt-hours of energy. In a perfect world, that would run your 200W load for 5 hours. But we do not live in a perfect world, which brings us to the next step.

Step 3: Account for Efficiency Losses

Here is where most people’s napkin math goes wrong. Batteries do not deliver 100% of their rated capacity. Energy is lost in two places.

Battery discharge losses. No battery chemistry is perfectly efficient. Whether it is lithium-ion, LiFePO4, or lead-acid, some energy is lost as heat during discharge. This typically costs you about 15% of the rated capacity.

Inverter conversion losses. Your devices run on AC power (the kind that comes out of a wall outlet), but batteries store DC power. An inverter converts DC to AC, and that conversion is not free. Another ~15% is lost here.

The calculator accounts for both of these by dividing your raw watt-hours by 0.85 twice. That is not a random number — it represents the roughly 85% efficiency of each stage. Combined, you end up with about 72% of the battery’s rated capacity actually reaching your devices.

So those 800Wh you calculated in step 2? After efficiency losses, you actually need about 1,107Wh of rated battery capacity to deliver that energy. The gap is real, and it catches a lot of first-time buyers off guard. (We wrote a whole article about this if you want the deep dive.)

Step 4: Add Surge Headroom

Some devices do not draw a steady stream of power. Anything with a motor or compressor — refrigerators, air conditioners, power tools — draws a large spike when it first turns on. This startup surge can be 2 to 3 times normal running wattage.

If your device list includes anything with a motor, the calculator automatically multiplies your total by 1.3 — a 30% buffer for startup spikes. You can also manually toggle surge on if you know you have a device with inrush current that the presets did not catch.

Step 5: Add a Safety Margin

Running a battery at 100% of its rated capacity is a bad idea for two reasons. First, real-world conditions are never as clean as spec sheets suggest — temperature, battery age, and load variation all nibble at performance. Second, regularly draining a battery to its absolute floor shortens its lifespan significantly.

The calculator adds a 10% safety margin on top of everything. This means you are not buying a battery that barely covers your needs on paper. You are buying one that comfortably covers them in practice, with a little breathing room for the messy reality of actual power consumption.

Step 6: Match to Real Products

Now the calculator has a single number: the minimum usable watt-hours you need. It takes that number and runs it against a curated database of real power stations and UPS units to build your shortlist.

Here is how it filters and ranks:

Filter by type. If you chose portable (carry or wheeled) or selected a travel/camping context, only portable power stations are shown. Otherwise, both UPS units and portable power stations are in the mix.

Filter by context. Products are tagged for specific use cases — home, office, camping, travel, RV. Only products tagged for your selected context survive.

Filter by surge support. If your setup needs surge handling, products without it are removed.

Filter by capacity. Any product whose usable capacity falls short of your calculated need is eliminated.

Rank the survivors. From the products that pass all filters, the calculator picks up to three recommendations:

Best fit — the cheapest unit that is right-sized for your need (within about 3x of your required capacity, so you are not paying for way more battery than you will use). If nothing falls within that range, it falls back to the cheapest option overall. This is usually the sweet spot of price and performance.
Budget pick — the cheapest option that still meets every requirement, excluding whichever product was already picked as the best fit. This gives you a second opinion at a different price point.
Extra capacity — the cheapest unit with at least 50% more capacity than the best fit, for people who want headroom. Good if you think your needs might grow, or if you want to charge the battery less frequently.

The Full Formula

Here it is, all in one line:

(watts × hours) / 0.85 / 0.85 × surge_multiplier × 1.1

Where:

watts = total wattage of all your devices
hours = how long you need them to run
/ 0.85 = battery discharge efficiency loss
/ 0.85 = inverter conversion efficiency loss
surge_multiplier = 1.3 if any device has a motor or compressor, 1.0 otherwise
× 1.1 = 10% safety margin

Let us run a quick example. Say you have a laptop (50W), a monitor (30W), a router (15W), and a phone charger (15W). That is 110W total. You want 4 hours of runtime, and none of these devices have motors.

(110 × 4) / 0.85 / 0.85 × 1.0 × 1.1 = 670Wh

So you need a power station or UPS with at least 670 usable watt-hours. The calculator would then find you the best-fitting products at or above that threshold.

Why We Show Our Work

Most product recommendation tools are black boxes. You put in some inputs, a recommendation pops out, and you have no idea whether it is genuinely good advice or just the product with the highest commission.

The formula above is the actual logic running in the calculator — not a simplified version, not a marketing summary. You can verify every step, do the math by hand, or disagree with our assumptions and adjust accordingly.

This is exactly what the calculator does in under a second. You could do it by hand — but why would you?