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Sizing solar + battery for a site with an unreliable grid and a generator

A reliable grid makes solar sizing relatively simple: offset as much daytime demand as makes financial sense, add a battery if you want to shift energy into the evening. Most of the world doesn't have that luxury. If your grid is up for only part of the day and a diesel or petrol generator covers the rest, solar and battery sizing has to account for a third, usually-dominant cost: fuel.

The one thing that changes everything: when the grid is actually up

The starting point isn't a system size — it's your site's real grid pattern. WattSketch models this as a 24-hour on/off pattern per month (grid up or down, hour by hour), covering the full range from a fully reliable grid (up all 24 hours) through a fully off-grid site with a generator (up 0 hours) to the much more common case in between. A reasonable estimate from memory is a fine starting point if you don't know your pattern precisely — the sensitivity analysis below exists specifically because it's never known exactly.

Three scenarios, compared side by side

Rather than giving you one number, the hybrid-site model compares three scenarios for the same site and the same grid pattern:

1. Baseline — grid plus generator, no solar at all. What you're paying today.
2. + Solar — the same site with solar added, no battery. Solar serves load directly first; anything the generator or grid would otherwise have had to supply, it doesn't.
3. + Solar and battery — the same again, with a battery added on top.

Seeing all three side by side is the point: it shows you what solar alone actually buys you before you decide whether a battery is worth adding on top of it.

Why the battery discharges only during outages

This is the one dispatch rule different from a simple on-grid battery, and it's deliberate: generator energy almost always costs far more per effective kWh than grid energy once you count fuel, maintenance, and run-hours. So a battery charged by surplus solar is held back for the next grid-down hour, where it directly displaces expensive generator fuel — rather than being spent down against cheap grid import while the grid is still up. (A grid pattern set to on 24 hours a day collapses this back to ordinary any-time dispatch — there's no outage to save the charge for.)

Sizing the battery's power, not just its capacity

A battery needs two numbers: how much energy it can hold (kWh) and how fast it can charge or discharge (kW). WattSketch derives power from energy over a duration you set (power = energy ÷ duration, 2 hours by default), and always caps it at your system's own AC capacity — there's no separate, oversized inverter hiding behind the battery that could push out more than your solar array's own rated size.

Finding a near-optimal battery size, not just a single number

Instead of asking you to guess a battery size and check whether it's any good, the tool can sweep a whole range of sizes and reduce each one to a single comparable figure: its equivalent annual cost (today's capital cost spread evenly over the project lifetime, using the standard capital-recovery-factor formula, plus every year's fuel and maintenance, plus the present value of any future battery replacement). The chart shades every size within a small tolerance of the single cheapest one — a near-optimal range, not a falsely precise single answer, since the real optimum moves with fuel prices and your grid pattern, neither of which you can know exactly in advance. A built-in sensitivity grid reruns the whole comparison at a lower/higher fuel price and a few more/fewer hours of grid per day, so you can see how much your answer actually depends on assumptions you're not certain of.

What this does not model

The hybrid-site model always assumes 100% of demand gets served — by solar, battery, grid, or generator, in that priority order — so a battery size only ever changes cost, never how much of the load actually gets covered. Fuel consumption is modelled from a generic published fuel curve for a handful of generator size classes, not your exact generator model — see how WattSketch calculates its results for the full disclosure on every assumption.

Getting started

You'll get the most out of this if you first run a load audit from your own meter data, so the load shape behind every number here is your real demand. From there, Location picks up your site's solar resource, Configure sizes the PV array, and the "Unreliable grid & generator" card on Financials is where the three-scenario comparison and the battery sweep both live.