12V Watermaker and Solar Panels
How to keep your battery bank balanced while making water at anchor
A 12V watermaker lets you produce fresh water at anchor without a generator — but it draws significant current from the battery while running. Solar panels are the complementary piece: energy produced during the day offsets part of the watermaker and other loads. The question is not "should I add panels?" but "do my daily consumption and charging sources balance?"
This guide helps you plan Seacraft SCW-30 and SCW-50 together with solar power. Exact panel sizing and battery capacity depend on your layout, shading and cruising area — a site survey is required for a firm plan. For power supply choice see 12V vs 230V watermaker; for 12V suitability see who needs a 12V watermaker.
Basic Formula
Current (A) × Time (hours) = Ah consumed
Example: 43 A × 3 hours ≈ 130 Ah (SCW-50)
SCW-30 and SCW-50 — How Much Current Do They Draw?
Solar planning starts with how many Ah the watermaker will consume. Values below are from Seacraft technical specs; actual draw may vary slightly with pressure, water temperature and filter condition.
| Model | Output | Power (12V) | Current (12V) | Ah in 3 hours |
|---|---|---|---|---|
| SCW-30 | 30 L/hour | ~120 W | 8–10 A | ~24–30 Ah |
| SCW-50 | 50–60 L/hour | ~550 W | ~43 A | ~130 Ah |
SCW-50 draws roughly four times more current than SCW-30. On boats with limited panel area and battery capacity, running SCW-50 for hours each day is often not sustainable. Compare both models in our SCW-30 vs SCW-50 comparison.
How to Build a Daily Energy Balance
On board, balance follows: total Ah consumed ≤ total Ah charged. The watermaker is only one part of the equation — fridge, autopilot, GPS, lighting, inverter loads and VHF share the same bank.
Charging sources usually fall into three groups:
- Solar panels — daytime, especially summer and open anchorages
- Engine alternator — high-amp charging while under way
- Marina shore power / battery charger — overnight or pre-anchor top-up
Solar alone may not cover all demand — that is realistic, not a failure. Well-positioned panels in the Aegean and Mediterranean summer can produce meaningful Ah; shaded bays, cloudy days or winter reduce output. Running the watermaker during peak solar hours leaves the bank healthier overnight.
4 Rules to Stay Balanced
- Plan daily run time from your litre requirement
- Include other loads in your Ah budget
- Estimate solar output conservatively for season and shading
- Keep alternator or marina charging as backup
How Much Does a Solar Panel Produce?
The watt rating on a panel label (e.g. 100 W, 200 W) reflects peak power under ideal conditions. Daily Ah depends on panel size, MPPT or PWM regulator efficiency, sun angle, temperature, shading and how the boat sits at anchor. The same panel performs differently in a sunny bay versus a mast-shaded cove.
As a rough guide: a well-positioned 100 W panel in summer may deliver around 40–60 Ah per day — this is a starting estimate only, not a guarantee. Two panels and an MPPT regulator increase output; bimini or boom shadow can reduce it sharply.
Example Scenarios
These examples illustrate thinking only — every boat is different.
Small sailboat — SCW-30, 2 crew
~80 L/day, watermaker ~2.5 hours, ~25 Ah draw.
200–300 Ah AGM bank + 150–200 W panels (existing or planned).
View: Low current draw makes balancing with solar and short engine runs generally easier. SCW-30 is often preferred on solar-focused setups.
Mid-size sailboat — SCW-50, 4 crew
~150 L/day, watermaker ~3 hours, ~130 Ah for watermaker alone.
400 Ah bank + fridge + autopilot pushes total daily draw higher.
View: Solar alone may not be enough; alternator charging or occasional marina top-up is usually needed. Panel area and bank size are confirmed during survey.
Catamaran — large panel area
400 W+ panel space possible on roof and bimini.
SCW-50 for 3 hours/day plus domestic fridge load.
View: More panel area pairs better with SCW-50; shading and regulator quality still matter.
Lithium battery bank
LiFePO₄ tolerates deeper discharge; usable Ah is higher than AGM for the same nominal capacity.
View: Can pair well with SCW-50; BMS, charge profiles and alternator compatibility must be checked. Battery type is planned during survey.
Battery Bank — Is Capacity Enough?
While the watermaker runs, bank voltage drops and other devices keep drawing from the same source. For AGM and gel banks, roughly 50% of total capacity is often treated as a safe usable range. Lithium systems differ and BMS limits apply.
SCW-50 at 3 hours/day (~130 Ah) alone uses a significant share of the bank. Add a typical fridge (often 40–80 Ah/day) and other loads, and banks below 400 Ah often struggle with SCW-50 plus full-time anchoring — exact thresholds vary by setup.
SCW-30 (~10 A) draws far less Ah; water production at anchor is more realistic with limited solar and battery. If daily litres are met with SCW-30, the low-current advantage may matter more than extra capacity for solar compatibility.
Tips for Using a Watermaker at Anchor
Run during daylight
Starting the unit while panels are producing leaves the bank at higher voltage overnight.
Plan run time
2–4 hours per day is usually enough and reasonable. Very long runs stress both the bank and the membrane. Use our capacity guide to set duration.
Use engine charging
The alternator is the fastest way to recover on days when solar falls short. Even short bay hops can help.
Watch voltage
Stopping before the bank drops below ~12.2 V protects battery life. A shunt-based monitor shows real Ah balance.
Don't skip flush
Fresh-water flush after production is part of membrane care. See our maintenance guide.
Survey first
Existing panels, regulators and batteries affect watermaker choice. Electrical infrastructure should be reviewed before installing a new unit.
Frequently Asked Questions
Let's Review Your Electrical Setup Together
The right watermaker for your panels and batteries — clear recommendation after survey
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