Free tools to size solar panels, batteries, cables and fuses for vans, RVs, cabins and camping. Every result comes with the formula, worked examples and the assumptions behind it.
Size a system
Tick your appliances and get the battery bank, solar array, inverter and charge controller size.
Question it answers: how much solar and battery do I need?Tick your appliances and get the continuous and surge watts you need, the inverter size to buy, and the battery current at 12, 24 or 48 V.
Question it answers: what size inverter do I need?Hours of sun, days to charge, and the panel size to charge in one day, for lithium or lead-acid batteries and MPPT or PWM controllers.
Question it answers: how long will my panels take to charge the battery?Check runtime
See how many hours a portable power station runs your devices, with inverter losses and cold weather included.
Question it answers: will it run my fridge overnight?Lithium, AGM, gel or flooded banks in series and parallel, with the Peukert effect for lead-acid.
Question it answers: how long will my 100 Ah battery last?Compare lithium, AGM and lead-acid over their lifetime to find the cheapest battery per kWh you actually get.
Question it answers: which battery is really the cheapest?Home backup
Tick the appliances you must keep running and get the battery size, inverter and generator watts, fuel to store, and solar to recharge. Gallons or liters.
Question it answers: what do I need to get through a blackout?How long a generator runs on a tank of gasoline, propane or diesel, and how much fuel you need for 24 hours.
Question it answers: how long will my generator run?Lower your bill
Tick your appliances and see what each one costs per month with your state’s average rate. Find what is driving your bill.
Question it answers: what is running up my electric bill?Enter your monthly kWh to see the panels, cost and simple payback to cover your bill, with no incentives assumed.
Question it answers: how many panels would cover my bill?Van, RV and camping
Tick what you run at once and see the amps against a 30 or 50 amp RV breaker, with what to switch off.
Question it answers: will it trip the campground breaker?Hours of driving to charge a van or RV battery, and the load on your alternator.
Question it answers: how long must I drive to charge my battery?Gadgets and portable power
Farm and homestead
Solar panel watts, pump power, flow and tank size from your daily water use and well depth, in liters or gallons and meters or feet.
Question it answers: how much solar do I need to pump my water?Panel and battery size for an electric fence energizer, for the days you need it to run without sun.
Question it answers: what solar panel does my electric fence need?Wire it safely
Typical running watts, start-up surge and daily hours for about 50 common household, tool and RV appliances.
The longest cable run by amps and wire size in AWG and mm², plus fuse sizes and current ratings.
Plain-language definitions of watts, amp-hours, MPPT, depth of discharge, surge and more.
These are the main formulas we use. Each calculator page explains its own version in full, with worked examples.
| Question | Rule used | Where |
|---|---|---|
| How much energy is stored? | Wh = Ah × volts | Battery runtime |
| How much battery do I need? | Daily Wh × days without sun ÷ usable share (90% lithium, 50% lead-acid) | Van solar and battery |
| How much solar do I need? | Daily Wh ÷ (peak sun hours × 0.75) | Van solar and battery |
| What does an inverter cost me in energy? | AC loads ÷ 0.85 | Power station runtime |
| How thick must the cable be? | mm² = 2 × 0.0175 × length (m) × amps ÷ allowed volt drop | Wire gauge and fuse |
| What fuse should I use? | About 125% of the load current, never above the cable’s rating | Wire gauge and fuse |
Read more on the about page.