How to Calculate Your Power Budget

What a power budget is

A power budget is a simple energy audit that adds up everything an electrical system consumes in a typical day, expresses it as watt-hours (and amp-hours at your system voltage), and uses that figure to size the battery bank and the charging sources needed to keep up. In short, it is the calculation that tells you how big your batteries, solar, alternator charging and inverter need to be — before you spend money on any of them.

On a boat or an overland vehicle this matters more than it does at home, because you are running off a finite bank and replacing energy from sources that are themselves limited: sun hours, engine run time, time on a berth. Guess too small and you run flat at anchor or in camp. Guess too large and you have paid for batteries and panels you will never use. The point of the budget is to land in the middle, on purpose.

Step 1: List every load

Start with a table of every electrical load on the vessel or vehicle, no matter how small. For each one you need three things:

• Power draw in watts (W), or current in amps (A) — most appliances state one or the other.
• System voltage it runs at (12V, 24V, or 230V AC through an inverter).
• Hours per day you realistically run it.

If a device gives you amps instead of watts, multiply by its voltage: watts = volts × amps. A fridge drawing 4A at 12V is 48W.

Be honest about duty cycle. A fridge compressor might be rated at 48W but only run perhaps a third of the time in moderate weather, so its effective average is closer to 16W. A nav light runs all night; a watermaker runs an hour. Use realistic running hours, not worst-case-everything-at-once.

Step 2: Convert each load to watt-hours per day

For every line, multiply power by daily hours:

Watt-hours per day (Wh) = watts × hours per day

Watt-hours are the right working unit because they are voltage-independent. You can add a 230V AC microwave and a 12V cabin light in the same column without confusion. Sum the column and you have your total daily energy demand in Wh/day.

One adjustment matters here: anything fed through an inverter carries a conversion loss. Inverters are efficient but not perfect, so as a rough planning figure, add roughly 10–15% to AC loads to account for inverter losses. Treat that as approximate and confirm it against your inverter's stated efficiency. Watch one trap in particular: an inverter left switched on also draws a small standby (no-load) current, which can quietly dominate the daily total when it powers only small, intermittent AC loads — so either include that standby draw or plan to switch the inverter off when it is not needed.

Step 3: Convert to amp-hours at system voltage

Batteries are sold and rated in amp-hours (Ah) at a nominal voltage, so convert your daily total:

Amp-hours per day (Ah) = total Wh per day ÷ system voltage

This is exactly why higher system voltages are attractive on larger builds: the same energy at 24V is half the amps of 12V, which means thinner cable and lower losses. A 2 400Wh/day demand is 200Ah at 12V but only 100Ah at 24V.

Step 4: Add autonomy

Autonomy is how many days you want to run without any charging input — the cloudy spell at anchor, the long layover in camp. Multiply your daily Ah by the number of autonomy days you want:

Required usable capacity (Ah) = daily Ah × autonomy days

Two days of autonomy is a common, sensible target for cruising and overland use; serious off-grid liveaboards may want more.

Crucially, this is usable capacity, not nameplate capacity. You cannot safely empty a battery to zero. As approximate rules of thumb, plan on using only about 50% of a lead-acid (AGM/gel) bank's rating to preserve cycle life, and roughly 80–90% of a lithium (LFP) bank's rating. So divide your required usable capacity by that usable fraction to get the bank you actually need to buy:

Nameplate bank size (Ah) = required usable Ah ÷ usable fraction

This single step is where lithium earns its place: for the same usable budget, an LFP bank is far smaller and lighter than the lead-acid equivalent. Sizing the bank is also where the system gets genuinely safety-critical — the cable and fusing must be rated for the currents an LFP bank in particular can deliver, so size protection from the manufacturer's data and have it confirmed by a qualified installer.

Step 5: Size the charging sources

The bank stores energy; the charging sources have to replace it. Your sources are typically some combination of solar, alternator or DC-DC charging from the engine, and shore/mains charging.

For solar, do not treat nameplate watts as harvest. A realistic daily yield is the array's rated watts multiplied by your usable peak-sun-hours, with a derating allowance for heat, angle, dirt and controller losses:

Approx. daily solar Wh ≈ rated W × peak-sun-hours × derate (≈0.7–0.8)

Southern Africa is generous here — much of the region sees roughly 5–6 peak-sun-hours a day in summer, less in a Cape winter — but plan for your worst realistic season and location, not your best.

For alternator or DC-DC charging, the harvest is the charger's regulated output current multiplied by the hours the engine actually runs. For shore charging, it is the charger's output multiplied by hours on the berth.

The charging side must cover a full day's demand plus rebuild any autonomy you have spent, within the hours you genuinely have. That is why sources are specified with headroom rather than matched exactly to consumption.

A worked example: a cruising yacht / overland rig

Take a typical 12V build with these average loads:

• Fridge/freezer (duty-cycled): ~40W average → 24h → 960 Wh
• LED cabin and nav lighting: ~25W → 6h → 150 Wh
• Electronics, NMEA 2000 instruments, charging devices: ~30W → 10h → 300 Wh
• Water pump (intermittent): ~5W average → 24h → 120 Wh
• Inverter for laptop/small AC loads: ~120W → 2h → 240 Wh (+ ~15% inverter loss ≈ 276 Wh)

Daily total ≈ 1 806 Wh/day. At 12V that is about 150 Ah/day.

With 2 days autonomy, required usable capacity is about 300 Ah. Choosing LFP at ~85% usable, the nameplate bank works out to roughly 350 Ah of LFP — then rounded up to the next standard bank size so the usable figure is never short.

On the charging side, replacing ~1 800 Wh/day from solar alone, at say 5 peak-sun-hours and a 0.75 derate, needs roughly 1 800 ÷ (5 × 0.75) ≈ 480W of panel — call it a 500–600W array so you can also claw back autonomy on better days, with a DC-DC charger from the engine as backup for grey weather.

These are illustrative figures to show the method; your real numbers depend on your appliances, climate and usage. A faster way to run them — and to keep them tidy as the design evolves — is the PowerSol power-budget planner, which does the watt-hour, amp-hour, autonomy and charging maths for you.

Putting it together

A good power budget is iterative. Build the load list, see the daily total, then make decisions: a more efficient fridge or DC compressor can shrink the whole system; dropping to 24V on a bigger boat eases the cabling; trimming inverter use cuts conversion losses. Re-run the numbers until the bank, the solar and charging, and the battery bank itself all balance against how you actually use the vessel.

If you would like a second set of eyes on the numbers — or help turning a finished budget into a specified bank, charging and monitoring package — talk to PowerSol. Send us your load list (or your output from the planner) along with your typical cruising or touring pattern, and we will help you specify a system that matches the way you live aboard rather than overbuilding it.