About RealTimeSolar
RealTimeSolar publishes solar calculators for people deciding whether to buy, and what to buy. Six of them, each covering one decision: whether panels pay for themselves, how many you need, how big a battery has to be, what charge controller to pair with it, what cable it takes, and how the whole thing fits together off-grid.
Why these are not the same as the other calculators
Most free solar calculators are lead-capture forms with arithmetic attached. These are not. Three of them deliberately disagree with the common answer, and each disagreement is the reason the page exists.
Payback is calculated for 2026, without the federal credit. The 30% residential clean energy credit expired on 31 December 2025. A great many calculators still quietly apply it, which understates payback by years. Ours does not.
Off-grid systems are sized for the worst month, not the annual average. A system sized on average sun works beautifully in July and runs out in December, because the array is roughly a third too small for the month that actually constrains it. Grid-tied systems can average, because the grid covers every shortfall. Off-grid has nothing behind it.
Cable is checked against both limits. A conductor has to carry the current without overheating (ampacity, from NEC 310.16) and deliver the power without losing too much on the way (voltage drop). Most calculators test voltage drop and stop, which on a short high-current run returns a cable that can overheat. Ours applies both and tells you which one decided.
Battery capacity is sized after depth of discharge and round-trip losses, which is why it comes out larger than the figure printed on the box. Charge controllers are sized on power for MPPT and on current for PWM, because they work differently, and the page shows how much of an array a PWM controller throws away.
Where the numbers come from
Peak sun hours are per state rather than national. Conductor resistances are from NEC Chapter 9 Table 8 and ampacities from NEC 310.16 at 75°C. Continuous-duty margins follow NEC 690.8. Where a single assumption is applied across the country — the winter design margin on the off-grid page is one — the page says so and tells you how to replace it with a real figure from NREL’s PVWatts for your own address.
What this is not
These are design aids for planning and budgeting. They are not an engineering sign-off, an electrical permit, or a substitute for a licensed installer and an inspection. Solar systems carry DC voltages and currents that will start a fire or kill you if the installation is wrong. Use the figures to understand your options and to check a quote you have been given; have the work designed and signed off by someone qualified and insured.
How the site is paid for
Some links on this site are affiliate links, including links to Amazon. If you buy through one we may earn a commission at no additional cost to you. It does not change the calculations, which are computed before any product is selected and have no knowledge of what is being advertised, and it does not change what we recommend.
Contact
Corrections are genuinely welcome, particularly on the engineering. If a figure looks wrong, write to info@realtimesolar.live and say which calculator and which number. Mistakes get fixed and the page gets updated.