Simple Solar Mathematics


How long will my batteries last when I go camping?


How large is your Deep Cycle battery.............Expressed in Amps over 20hrs

To protect your battery try to maintain a 30% safety buffer (never totally discharge your battery.

Next part is what are you running?

Typical camping needs

Waeco 45 42litre fridge

(runs at 45 watts per hour 32% running at 30 C)

Ie 45 watts x 8hrs divide by voltage 12v = 30 amps per day


2 x 13 watt fluoro about 3 hrs a day

26w x 3 divided x 12 = 6.5 amps per day

Television via an inverter

35watt x 90% efficiency for inverter say 2.5 hrs a day

35 w x loss = 39w x 2.5 divide x 12v = 8 amps per day

Radio / player

10 watts 2 to 3 hrs say 2 amps per day

Total daily load 46.5 amps per day

Divide 70 % of the battery rating by the daily load

A 100 amp battery will provide approximately 1 and days power in this situation.

How can I extend my camping trip?

Well you could take a generator but many parks do not generators to be run because of the noise and it does stress wildlife or use Solar power

A 75watt solar module will provide an average of 5.5 peak sun hours in NSW maybe 6 in Qld (This figure also can depend on location, shading, angle of mounting and direction of module).

75 watts X 5.5 hrs divide by voltage (12) = 34 amps per day

Not enough to run all your power needs

Now try "sun tracking" put the panel on an angle to maximise sun power facing North east in the morning, Due North in the middle of the day and North-West in the afternoon. You will collect possibly 7 7 hrs per day

75watt x 7 divide by voltage = (12) Almost 44 Amps

That replaces almost all your daily needs

Start with a fully charged 100-amp deep cycle battery Power Bank

So on the first day you use 46.5 amps of 70/100amps 53.5 amps

Second day replace 44 amps back to 97.5amps, Second night use 46.5 amps 51 amps

Third day replace 44 amps back to 91 amps, Third night use 46.5 amps 44.5 amps

Fourth day replace 44 amps back to 88.5amps, Fourth night use 46.5 amps 42 amps

Fifth day replace 44 amps back to 86amps, Fifth night use 46.5 amps 39.5 amps

Sixth day replace 44 amps back to 83.5amps, Sixth night use 46.5 amps 37 amps

Seventh day replace 44 amps up to 81amps, Seventh night use 46.5amps 34.5 amps

Eighth day replace 44 amps back to 78.5amps, Eighth night use 46.5 amps 32 amps

(Now very close to buffer suggest you are starting to overcycle the battery)

Ninth day replace 44 amps up to 76amps, Ninth night use 46.5amps 29.5 amps

Ten days camping on a single battery!

Pack up go home its probably raining and boy, do you need a shower!

Check electrolyte levels & put the battery onto long charge cycle say min 3.5 amps for 20 hrs ASAP.