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Power & Off-Grid Calculators

Sizing anything electrical comes down to two questions: how much power do you need right now, and how much do you need in reserve. These four calculators answer both, in US or metric units, and show the formula behind every number.

About these power & off-grid calculators

Power sizing goes wrong in predictable ways. People buy a battery bank sized on nameplate capacity and forget that half of a lead-acid battery is unusable. They run thin cable to a shed because the current is low, and then wonder why the lights dim at the far end. They read a UPS rating as a runtime. Every one of those mistakes comes from using one number where two were needed.

The calculators here are built around that gap. The solar sizer separates the panels, which are sized on daily energy and sunshine, from the battery, which is sized on how many grey days you want to survive. The wire gauge tool runs two independent checks — heat and voltage drop — and gives you whichever demands more copper. The UPS calculator works from the battery inside the box rather than the VA number on the front, because that is what actually decides how long you have.

One of these is safety-critical. Wire sizing is governed by code: NEC 310.16 and 240.4(D) in the United States, IEC 60364-5-52 and BS 7671 in Europe and the UK. The calculator applies the standard tables and tells you which constraint decided the answer, but it cannot see your installation. Derating for grouping, ambient temperature and continuous load can all change the result, and anything hardwired needs a licensed electrician.

The other three are planning tools with real money attached. Knowing that a 1500VA UPS holds a router for hours but a gaming PC for eight minutes changes what you buy. Knowing that a home charge costs a third of a public one changes where you plug in. That is the point: a number you can act on, with its assumptions written down next to it.

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