WattMatch
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How WattMatch Works

Every number WattMatch shows you comes from a formula. This page explains each one so you can make an informed decision — not just trust a black box.

Runtime Calculator

The runtime calculator answers: how long will this battery last running my devices? The calculation is a two-step formula:

usableWh = capacity × efficiency
estimatedHours = usableWh ÷ totalWatts

Capacity is the rated battery size in watt-hours (e.g. 1,000 Wh).

Efficiency defaults to 85%, which accounts for the DC-to-AC conversion loss inside the inverter. Real-world efficiency depends on load size, temperature, and battery age — 85% is a conservative but realistic median for modern LiFePO4 stations.

Total watts is the sum of all selected devices × their quantities. Device watt values are median figures from manufacturer specifications.

Estimated runtime is a ceiling, not a guarantee. A fridge compressor cycling on and off, a motor with high startup surge, or running a station below 20% charge can all reduce actual runtime. Add 15–20% buffer for comfort.

Recommendations

When you add devices to the Runtime Calculator, WattMatch computes recommendations for you. The engine follows a constraint-first, then score-second philosophy — a station that can actually run your devices will always beat one that's technically insufficient, no matter how impressive its specs look.

Step 1 — Feasibility filter (hard rules):

  • Stations where your total running watts exceed continuous output are excluded.
  • Stations where the peak startup surge exceeds the surge rating are excluded.

Step 2 — Fit tier classification:

  • Great fit — runtime covers your default 4-hour need with 25–100% buffer.
  • Good fit — covers the need with some headroom or minor weight trade-off.
  • Overkill — runtime is more than 2× what you need AND the unit is significantly heavier or pricier than alternatives.
  • Feasible but poor fit — technically works but runtime is below the needed threshold.

Step 3 — Scoring within each tier:

Within each fit tier, stations are ranked by a composite score blending: $/Wh (value), runtime ratio, solar input, portability (weight penalty above a use-case threshold), and output headroom above your load.

Step 4 — Picks:

  • Best overall = highest score among great-fit and good-fit stations only. Overkill stations are only shown if no great/good-fit options exist.
  • Budget pick = lowest $/Wh among great-fit and good-fit stations.
  • Best for solar = highest solar input among great-fit and good-fit stations.
A recommendation engine earns trust when it can say "this works, but it's overkill." WattMatch will never push a 3,000 Wh station for a 50W CPAP just because it scores well on raw capacity.

Surge Checker

Motor-driven devices — fridges, pumps, power tools — draw a brief burst of extra power when the motor starts. This is called startup surge (or inrush current). The station must be able to deliver this peak, or the inverter trips.

peakSurgeWatts = runningWatts (all other devices) + surgeWatts (one device starting)

Only one device starts at a time in normal use, so we compute the worst-case peak as the running draw of every other device plus the startup surge of the single device with the highest surge requirement.

Verdicts:

  • Safe — running draw is below 80% of continuous output AND surge is below 90% of station surge rating.
  • Caution — running draw is 80–100% of output OR surge is 90–100% of surge rating.
  • Overload — running draw exceeds continuous output OR surge exceeds surge rating.
Surge values in the device library are conservative (upper-range estimates). Actual surge duration is typically under 200 milliseconds — but if the station can't meet it, it will trip.

Compare Verdicts

The Compare tool scores two stations across five use cases: Camping, Home Backup, CPAP, Remote Work, and Van Life / RV. Each use case has a weighted profile — specs matter more in some contexts than others.

score = Σ (normalized_spec_score × weight) for each spec in the profile

Each spec pair is normalized to [0, 1] where 1 = better for that use case. For example, lower weight scores higher for Camping and CPAP (portability matters), while higher capacity scores higher for Home Backup.

A 5% tie threshold is applied: if the score difference between two stations is less than 5% of the total, the verdict is "Tie." Small differences in real-world performance are not meaningful, and presenting them as wins would be misleading.

All spec data comes from manufacturer-published figures. Real-world performance varies with temperature, battery age, inverter load type, and usage pattern. Use verdicts as a starting point for your research, not a final answer.

Data Sources

All product specifications on WattMatch are sourced directly from manufacturer websites, product manuals, and Amazon listings at the time of data collection. Prices are approximate and change frequently — always verify before purchasing.

Device wattage values in the calculator and surge checker use manufacturer median figures. Where a wide range is common (e.g. refrigerators), the median was chosen and noted in the device description. Surge values are conservative upper-range estimates.

WattMatch does not accept payment from manufacturers to influence ratings, verdicts, or recommendations. Some outbound purchase links are affiliate links — if you buy through them, WattMatch earns a small commission at no extra cost to you.

Runtime Calculator →Surge Checker →Compare Tool →