About the Solar + Battery Sizing Calculator
Solar power is now the cheapest electricity in history, but a constant load - a data center, a factory, a village - needs power at 3 a.m. too. The honest question is not the cost of a solar panel but the cost of solar plus enough storage and overbuild to deliver around the clock. This calculator sizes that full system from first principles and prices it.
How to use it
- Enter the constant load you must serve, in megawatts.
- Set your site's equivalent full sun hours (about 5 in the US Southwest, 3 in Germany).
- Adjust battery efficiency, backup buffer, and the weather overbuild factor.
- Set current PV and battery prices.
- Read array size, battery size, total capex, and effective LCOE.
How it works
Each day the array must deliver the daytime load directly plus the night's energy through the battery, where charging losses inflate the requirement: PV(MW) = [load x sun hours + load x (24 - sun hours) / RTE] / sun hours, all multiplied by your overbuild factor. Battery energy is the night load through RTE plus your buffer hours. Capex is simply volume times unit price, and the LCOE figure spreads that capex over every megawatt-hour the system serves across its lifetime.
Worked example
A 100 MW 24/7 load at 5 sun hours, 86% RTE, 6 h buffer and 1.3x overbuild needs roughly a 690 MW array and about 2.9 GWh of battery. At $0.80/W and $140/kWh, that is on the order of $0.96B total - and an effective capex-only LCOE near $44/MWh over 25 years, which is why solar-plus-storage keeps winning bids.
Frequently asked questions
How much solar do I need to run a load 24/7?
Enough to power the load during daylight AND recharge the battery for the night - typically 3-6x the load's nameplate, depending on your site's sun hours.
What are equivalent full sun hours?
Your site's daily solar energy expressed as hours at full rated panel output. Deserts reach 6-7; northern Europe can be under 3.
Why divide battery energy by round-trip efficiency?
Some energy is lost charging and discharging (typically about 15% for lithium), so the panels must generate more than the night actually consumes.
What does the overbuild factor cover?
Cloudy days, winter sun, soiling, and degradation. 1.3-1.6 is a common planning range for high-availability loads.
Is this a substitute for professional design?
No - it is a transparent first-order sizing tool for feasibility and cost intuition, using adjustable public assumptions.