Solar Power Calculator V1.1

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Using the Solar Calculator Spreadsheet.

This spreadsheet allows you to type in values for all the things you can change in your solar setup, and view the likely result. Deciding on the hardware for a solar power system is extremely dependent on what you want it to do, and your budget of course. Basic facts like daily power usage are very often hazy, unknown, or variable. So whether it's a desert sunrise in in Ouazazate, Up Helly Aa in Lerwick, or just a weekend wild camping, you can design a system to suit your unique lifestyle.

You can play 'what if' with power usage, panel power, battery capacity, and the weather, to see how they change the off-grid time and the time to recover from a spell of bad weather. What if I increase my daily power usage? Can I still last indefinitely when wild camping? If not, how long can I last? What difference will an extra battery make?

Here's some notes on the numbers. If the numbers in the yellow boxes are not suitable for your situation, feel free to change them.

Daily power usage: if you don't know where to start, then compare with a standard single 100Ah wet battery, which allows 50Ah of usable charge when discharged to the recommended 50% level. Many people find that's enough for a single day, or even a weekend in the summer months. Daily power usage is the most important determining factor, but it's usually the least well defined one.

Max Percentage power of panels: A 100W panel will give 100W when horizontal at the equator at midday on midsummer day. In Europe, when it's flat on a motorhome roof you'll probably never get more than 70% of that. You can adjust this for latitude and season if you want.

Discharge Lower Limit: Depends on the battery technology. Standard flooded batteries are best kept to 50% or higher. AGMs can go to 30%, gels to 20% and lithiums to 10%, but this does vary. Follow the manufacturer's advice on this.

Equivalent hours of full sunshine: On a typical sunny autumn day in northern Europe, there's about 3 hours of full sun, then the rest of the time as the sun is lower is about equivalent to another 2 hours of full sun. So, the equivalent of 5 hours full sun per day. Depends on the season, and the latitude.

Weather Factor: This is a bodge so you can see the performance when it's cloudy. 100% is no cloud, then guess the factor if it's partly cloudy part of the day. Dull all day I'd guess is 10%. Worst case is darkness, 0%.

Weather Sustain Limit: If there's plenty of panel power, is there enough to power the loads and still store enough during the day to last through the night? If it's 100% sunny, maybe, but what if it's 50% sunny? This figure is the weather factor that will just store enough to last overnight. So you can wild camp indefinitely.

Reserve Time: If the panels can't completely refill the battery during the day, how many days can you last before the battery hits the lower discharge limit? 2 days or 2 weeks?

Recharge Time: If your battery is right down to its lower discharge limit, how long will the system take to recharge, while still powering your loads? Maybe a day or two, maybe a week, maybe it can never recharge.

Recharge Weather Factor: This allows you to see how the cloudiness affects how long it takes to recharge the battery.

Latest updates

  1. Bug fix

    Sunshine hours input variable now applies to results.