How to Size Solar + Battery for a 24/7 Load (Free Calculator)

Solar panels are the cheapest electricity humanity has ever built — when the sun is up. The moment your load must run at 3 a.m., the question changes from "what does a panel cost?" to "what does a system cost: panels, plus storage, plus the overbuild that carries you through winter and clouds?" The free Solar + Battery Sizing Calculator answers that in one screen.

The sizing logic

A constant load needs energy 24 hours a day, but your site only delivers useful sun for some of them — measured as equivalent full sun hours (about 5 in the US Southwest, closer to 3 in Germany). During those hours the array must do two jobs at once: power the load directly and recharge the battery that will carry the night. Charging isn't free either — lithium round-trip efficiency is typically about 86%, so every night-megawatt-hour costs about 1.16 array-megawatt-hours.

Put together: PV = [load × sun-hours + load × (24 − sun-hours) ÷ RTE] ÷ sun-hours, all multiplied by an overbuild factor (1.3–1.6 is a common planning range) for weather, winter and degradation. Battery energy is the night load through RTE, plus whatever backup buffer you want.

Worked example: 100 MW, around the clock

Take a 100 MW data center at 5 sun hours, 86% RTE, a 6-hour buffer and 1.3× overbuild. The calculator sizes it at roughly a 690 MW array — about 6.9× the load — and 2.9 GWh of battery. At $0.80/W for PV and $140/kWh for storage, total capex lands near $0.96B, and spreading that over 25 years of delivered energy gives an effective capex-only cost near $44/MWh. That number — cheaper than most grid power — is why solar-plus-storage keeps winning utility bids, and why the AI buildout is chasing sunshine.

What moves the number most

Sun hours dominate: the same system in a 3-sun-hour climate needs nearly twice the array. Battery price is second — at $70/kWh the storage half of the bill drops 50%. Round-trip efficiency matters more than people expect, because it inflates both the battery and the extra array needed to fill it. The calculator's sliders make each sensitivity visible in real time, and the capex chart splits panels from batteries so you can see which lever to pull.

FAQ

Is this how professionals size systems?

It's the first-order energy balance every real study starts from. Professional designs add hourly weather simulation, degradation schedules and financing — but they rarely move the answer by more than the overbuild factor already covers.

Does it handle off-grid homes?

Yes — set the load in fractions of a megawatt (0.001 MW = 1 kW) and the same math applies.

Run your own numbers: Solar + Battery Sizing Calculator. Related: Solar Overbuild Calculator, Hyper-Scale Data Center Power Planner, and Energy Cost of Intelligence.