Solar & battery sizing tool
Check whether your solar array and battery bank are balanced against your daily energy needs.
Related tools
Daily energy consumption
Battery bank
Solar array
Worked example
A cruising yacht with typical liveaboard loads, 200 Ah AGM bank, 2 × 200W panels, UK summer (4 sun hours):
| Appliance | Draw | Hours/day | Wh/day |
|---|---|---|---|
| Fridge (12V compressor) | 4.5A / 54W | 12 avg | 648 Wh |
| Chart plotter | 2A / 24W | 8 | 192 Wh |
| VHF + AIS | 2A / 24W | 12 | 288 Wh |
| LED lights | 2A / 24W | 4 | 96 Wh |
| Total | 1,224 Wh |
Solar generation: 2 × 200W × 4h × 80% = 1,280 Wh/day — just covers daily use.
Usable battery: 200 Ah × 50% DoD × 12V = 1,200 Wh. Days autonomy (no sun): 1,200 ÷ 1,224 = 0.98 days — need more battery or less consumption for cloudy days.
How to use this tool
Add each electrical appliance with its wattage and daily hours of use. Then enter your battery bank size and solar array. The tool shows whether solar generation covers daily consumption, how many cloudy days your battery can sustain you, and whether you need more solar, more battery, or both.
Balance = Daily solar (Wh) − Daily consumption (Wh)
A balanced system has daily solar ≈ daily use. Add 20–30% solar margin for UK weather variability. Battery autonomy of 2–3 days is a good target for coastal cruising; liveaboards often want 3–5 days.
Frequently asked questions
How much solar do I need for a liveaboard?
Most liveaboards consume 100–200 Ah/day (1,200–2,400 Wh at 12V). At 4 UK summer peak sun hours and 80% efficiency, you need roughly 375–750 Wp of panels to break even. Most liveaboard cruisers run 400–800 Wp with 200–400 Ah of battery. In UK winter you will need shore power or a generator to supplement — budget for it.
What is "days of autonomy"?
Days of autonomy is how long your battery bank alone (with no solar input) can supply your daily consumption before reaching the depth-of-discharge limit. Two days is a practical minimum for UK coastal cruising — it means one fully overcast day won’t leave you without power. Three or more is comfortable for extended passages.
My solar almost covers my consumption — is that enough?
It depends on your cruising area and season. In the UK, even summer has cloudy days — your battery absorbs the deficit. If solar just covers daily use on a good day, you’ll slowly drain your battery on cloudy days. Aim for 120–130% of daily consumption in solar generation to build a buffer. In the Med, 100% is fine in summer.
Should I add more solar or more battery?
If your solar generation is low relative to consumption, more solar is the first fix — it addresses the root cause. More battery only extends how long you can go without sun, it doesn’t fix an under-panelled system. If you already have good solar coverage, more battery gives resilience for multi-day overcast periods.